Advanced
This advanced tutorial covers production-grade React + TypeScript patterns through 25 heavily annotated examples. Each example maintains 1-2.25 comment lines per code line to ensure deep understanding. You’ll master advanced TypeScript patterns, state management with Zustand, performance optimization, concurrent React features, testing strategies, security best practices, and production deployment patterns.
Prerequisites
Before starting, ensure you understand:
- React fundamentals and intermediate patterns (hooks, Context API, custom hooks)
- TypeScript advanced features (generics, utility types, discriminated unions)
- Asynchronous patterns (Promises, async/await, error handling)
- Performance concepts (memoization, lazy loading, code splitting)
- Testing fundamentals (unit tests, integration tests)
If you need to review, see Beginner and Intermediate.
Group 1: Advanced TypeScript Patterns (5 examples)
Example 1: Generic Components with TypeScript
Generic components work with multiple data types while maintaining type safety. Use type parameters to make components reusable across different data shapes.
import { useState } from 'react';
// => Generic interface: T can be any type
interface ListProps<T> {
items: T[];
renderItem: (item: T) => React.ReactNode;
keyExtractor: (item: T) => string;
}
// => Generic component: T inferred from usage
function List<T>({ items, renderItem, keyExtractor }: ListProps<T>) {
return (
<ul>
{items.map((item) => (
<li key={keyExtractor(item)}>
{renderItem(item)}
</li>
))}
</ul>
);
}
interface Donation {
id: string;
amount: number;
donor: string;
}
interface ZakatPayment {
id: string;
nisab: number;
zakatAmount: number;
payer: string;
}
function DonationDashboard() {
const [donations] = useState<Donation[]>([
{ id: '1', amount: 100, donor: 'Aisha' },
{ id: '2', amount: 250, donor: 'Omar' },
]);
const [zakatPayments] = useState<ZakatPayment[]>([
{ id: '1', nisab: 5000, zakatAmount: 125, payer: 'Fatima' },
{ id: '2', nisab: 10000, zakatAmount: 250, payer: 'Ali' },
]);
return (
<div>
<h2>Recent Donations</h2>
{/* => List<Donation>: T inferred from items prop */}
<List
items={donations}
renderItem={(donation) => (
<span>${donation.amount} from {donation.donor}</span>
)}
keyExtractor={(donation) => donation.id}
/>
<h2>Zakat Payments</h2>
{/* => List<ZakatPayment>: T inferred from items type */}
<List
items={zakatPayments}
renderItem={(payment) => (
<span>${payment.zakatAmount} zakat from {payment.payer}</span>
)}
keyExtractor={(payment) => payment.id}
/>
</div>
);
}
export default DonationDashboard;Key Takeaway: Generic components with type parameters enable type-safe reusability. TypeScript infers the type parameter from usage, ensuring type safety without explicit type annotations.
Expected Output: Page displays two lists - donations showing amounts and donor names, zakat payments showing zakat amounts and payer names. Each list uses same generic List component with different data types.
Common Pitfalls: Forgetting to extract unique keys (causes React warnings), using any instead of generics (loses type safety), not constraining generic types when specific capabilities needed (use extends keyword).
Example 2: Utility Types in React (Partial, Pick, Omit, Record)
TypeScript utility types transform existing types for different use cases. Use them to derive props types, create form types, and manage component variants.
import { useState } from 'react';
// => Base donation interface with all fields
interface Donation {
id: string; // => Unique identifier (required)
amount: number; // => Donation amount (required)
donor: string; // => Donor name (required)
email: string; // => Donor email (required)
isAnonymous: boolean; // => Anonymous flag (required)
createdAt: Date; // => Timestamp (required)
}
// => Partial<T>: all properties optional (for updates)
type DonationUpdate = Partial<Donation>;
// => Pick<T, K>: select specific properties
type DonationFormData = Pick<Donation, 'amount' | 'donor' | 'email' | 'isAnonymous'>;
// => Omit<T, K>: exclude specific properties
type DonationDisplay = Omit<Donation, 'email'>;
// => Record<K, T>: object with keys K and values T
type DonationsByCategory = Record<'general' | 'zakat' | 'sadaqah', Donation[]>;
function DonationForm() {
const [formData, setFormData] = useState<DonationFormData>({
amount: 0,
donor: '',
email: '',
isAnonymous: false,
});
// => Partial update: only changed fields needed
const updateField = (update: Partial<DonationFormData>) => {
setFormData((prev) => ({
...prev,
...update,
}));
};
const handleSubmit = () => {
console.log('Submitting:', formData);
// => Backend generates id and createdAt
};
return (
<div>
<h2>Make a Donation</h2>
<label>
Amount: $
<input
type="number"
value={formData.amount}
onChange={(e) => updateField({ amount: parseFloat(e.target.value) })}
/>
</label>
<label>
Name:
<input
type="text"
value={formData.donor}
onChange={(e) => updateField({ donor: e.target.value })}
/>
</label>
<label>
Email:
<input
type="email"
value={formData.email}
onChange={(e) => updateField({ email: e.target.value })}
/>
</label>
<label>
<input
type="checkbox"
checked={formData.isAnonymous}
onChange={(e) => updateField({ isAnonymous: e.target.checked })}
/>
Donate anonymously
</label>
<button onClick={handleSubmit}>Submit Donation</button>
</div>
);
}
// => Record ensures all category keys present
function DonationCategoryView() {
const [donations] = useState<DonationsByCategory>({
general: [],
zakat: [],
sadaqah: [],
});
// => TypeScript errors if keys missing or wrong value type
return (
<div>
<h3>General Donations: {donations.general.length}</h3>
<h3>Zakat Payments: {donations.zakat.length}</h3>
<h3>Sadaqah: {donations.sadaqah.length}</h3>
</div>
);
}
export default DonationForm;Key Takeaway: TypeScript utility types (Partial, Pick, Omit, Record) derive new types from existing types, reducing duplication and ensuring consistency. Use Pick for forms, Partial for updates, Omit for display types, and Record for key-value mappings.
Expected Output: Form displays donation input fields (amount, name, email, anonymous checkbox). Updating any field re-renders with new value. Category view shows counts for each donation category.
Common Pitfalls: Using Partial<T> for required fields (makes everything optional), forgetting that utility types don’t affect runtime behavior (TypeScript only), using Record with dynamic keys (prefer { [key: string]: T } for dynamic keys).
Example 3: Discriminated Unions for State
Discriminated unions model mutually exclusive states with type safety. Use a common discriminant property (like status or type) to distinguish between states.
import { useState } from 'react';
// => Discriminated union: exactly one state at a time
// => Each variant has unique 'status' discriminant
type AsyncState<T> =
| { status: 'idle' } // => Initial state, no data
| { status: 'loading' } // => Request in progress
| { status: 'success'; data: T } // => Request succeeded, data available
| { status: 'error'; error: string }; // => Request failed, error message available
// => Domain interface
interface ZakatCalculation {
wealth: number; // => Total wealth
nisab: number; // => Nisab threshold
isZakatDue: boolean; // => Whether zakat is due
zakatAmount: number; // => 2.5% of excess wealth
}
function ZakatCalculator() {
// => State can be exactly one variant at a time
// => TypeScript ensures only valid states exist
const [calcState, setCalcState] = useState<AsyncState<ZakatCalculation>>({
status: 'idle', // => Initial state
});
// => No data or error when idle
const [wealth, setWealth] = useState<number>(0);
const calculateZakat = async () => {
// => Transition to loading state
setCalcState({ status: 'loading' });
// => No data or error in loading state
try {
// => Simulate API call
await new Promise((resolve) => setTimeout(resolve, 1000));
// => Calculate zakat (2.5% of wealth exceeding nisab)
const nisab = 5000; // => Nisab threshold in dollars
const isZakatDue = wealth >= nisab; // => Zakat due if wealth >= nisab
const zakatAmount = isZakatDue
? (wealth - nisab) * 0.025 // => 2.5% of excess wealth
: 0; // => No zakat if below nisab
// => Transition to success state with data
setCalcState({
status: 'success',
data: {
wealth,
nisab,
isZakatDue,
zakatAmount,
},
});
// => data property only exists in success state
} catch (err) {
// => Transition to error state with error message
setCalcState({
status: 'error',
error: err instanceof Error ? err.message : 'Calculation failed',
});
// => error property only exists in error state
}
};
// => Render different UI based on discriminant (status)
// => TypeScript narrows type based on status check
return (
<div>
<h2>Zakat Calculator</h2>
<label>
Total Wealth: $
<input
type="number"
value={wealth}
onChange={(e) => setWealth(parseFloat(e.target.value) || 0)}
/>
</label>
<button onClick={calculateZakat} disabled={calcState.status === 'loading'}>
{calcState.status === 'loading' ? 'Calculating...' : 'Calculate Zakat'}
</button>
{/* => TypeScript narrows calcState type in each branch */}
{calcState.status === 'idle' && (
<p>Enter your wealth to calculate zakat</p>
)}
{calcState.status === 'loading' && (
// => calcState is { status: 'loading' } here
// => No data or error properties available
<p>Calculating zakat amount...</p>
)}
{calcState.status === 'success' && (
// => calcState is { status: 'success'; data: ZakatCalculation } here
// => TypeScript knows data property exists
<div>
<h3>Calculation Result</h3>
<p>Wealth: ${calcState.data.wealth}</p>
<p>Nisab Threshold: ${calcState.data.nisab}</p>
{/* => TypeScript autocompletes data properties */}
{calcState.data.isZakatDue ? (
<p style={{ color: 'green' }}>
✓ Zakat is due: ${calcState.data.zakatAmount.toFixed(2)}
</p>
) : (
<p style={{ color: 'gray' }}>
Wealth below nisab - no zakat due
</p>
)}
</div>
)}
{calcState.status === 'error' && (
// => calcState is { status: 'error'; error: string } here
// => TypeScript knows error property exists
<p style={{ color: 'red' }}>
Error: {calcState.error}
{/* => TypeScript autocompletes error property */}
</p>
)}
</div>
);
}
export default ZakatCalculator;Key Takeaway: Discriminated unions ensure mutually exclusive states with compile-time safety. TypeScript narrows the union type based on discriminant checks, preventing impossible states like “loading with data” or “success without data”.
Expected Output: Page displays wealth input field and calculate button. Initially shows “Enter your wealth” message. On calculate, shows “Calculating…” then displays result with wealth, nisab, and zakat amount. If wealth below nisab, shows “no zakat due”. On error, displays error message in red.
Common Pitfalls: Using boolean flags instead of discriminated unions (allows impossible states like loading: true, error: true), forgetting to check discriminant before accessing state-specific properties (TypeScript error), using string literals inconsistently (typos break type narrowing).
Example 4: Advanced Type Guards
Type guards narrow union types at runtime, enabling safe access to type-specific properties. Use typeof, instanceof, and custom type predicates.
import { ReactNode } from 'react';
interface DonationTransaction {
type: 'donation'; // => Discriminant property
amount: number;
donor: string;
isAnonymous: boolean;
}
interface ZakatTransaction {
type: 'zakat';
wealth: number;
nisab: number;
zakatAmount: number;
}
interface RefundTransaction {
type: 'refund';
originalAmount: number;
reason: string;
refundDate: Date;
}
type Transaction = DonationTransaction | ZakatTransaction | RefundTransaction;
// => Type predicate: narrows union type
function isDonation(transaction: Transaction): transaction is DonationTransaction {
return transaction.type === 'donation';
}
function isZakat(transaction: Transaction): transaction is ZakatTransaction {
return transaction.type === 'zakat';
}
function isRefund(transaction: Transaction): transaction is RefundTransaction {
return transaction.type === 'refund';
}
interface TransactionItemProps {
transaction: Transaction;
}
function TransactionItem({ transaction }: TransactionItemProps) {
if (isDonation(transaction)) {
// => transaction narrowed to DonationTransaction
return (
<div className="transaction donation">
<h4>Donation</h4>
<p>Amount: ${transaction.amount}</p>
<p>Donor: {transaction.isAnonymous ? 'Anonymous' : transaction.donor}</p>
</div>
);
}
if (isZakat(transaction)) {
// => transaction narrowed to ZakatTransaction
return (
<div className="transaction zakat">
<h4>Zakat Payment</h4>
<p>Wealth: ${transaction.wealth}</p>
<p>Nisab: ${transaction.nisab}</p>
<p>Zakat Due: ${transaction.zakatAmount}</p>
</div>
);
}
if (isRefund(transaction)) {
// => transaction narrowed to RefundTransaction
return (
<div className="transaction refund">
<h4>Refund</h4>
<p>Amount: ${transaction.originalAmount}</p>
<p>Reason: {transaction.reason}</p>
<p>Date: {transaction.refundDate.toLocaleDateString()}</p>
</div>
);
}
// => Exhaustiveness check: ensures all union members handled
const _exhaustive: never = transaction;
return null;
}
function TransactionList() {
const transactions: Transaction[] = [
{
type: 'donation',
amount: 100,
donor: 'Aisha',
isAnonymous: false,
},
{
type: 'zakat',
wealth: 10000,
nisab: 5000,
zakatAmount: 125,
},
{
type: 'refund',
originalAmount: 50,
reason: 'Duplicate payment',
refundDate: new Date('2026-01-15'),
},
];
return (
<div>
<h2>Transaction History</h2>
{transactions.map((transaction, index) => (
<TransactionItem key={index} transaction={transaction} />
))}
</div>
);
}
export default TransactionList;Key Takeaway: Type guard functions with type predicates (value is Type) narrow union types, enabling safe access to type-specific properties. Use discriminated unions with type guards for complex domain modeling with compile-time safety.
Expected Output: Page displays list of three transactions: donation showing amount and donor name, zakat payment showing wealth/nisab/zakat amount, refund showing original amount and reason. Each rendered with type-specific styling and information.
Common Pitfalls: Forgetting exhaustiveness check (new types not handled), type guard function returning boolean without type predicate (doesn’t narrow type), checking properties before type narrowing (TypeScript error on union types).
Example 5: Template Literal Types for Props
Template literal types create string literal types from patterns, enabling precise string prop validation. Use for CSS classes, event names, and status strings.
import { CSSProperties, useState } from 'react';
// => Template literal type: combines literal strings
// => Generates all possible combinations
type DonationStatus = 'pending' | 'completed' | 'failed' | 'refunded';
type DonationPriority = 'low' | 'normal' | 'high';
// => Template literal type for CSS class names
// => Creates: 'donation-status-pending', 'donation-status-completed', etc.
type DonationStatusClass = `donation-status-${DonationStatus}`;
// => Template literal type for combined status and priority
// => Creates: 'pending-low', 'pending-normal', 'pending-high', 'completed-low', etc.
type DonationStateKey = `${DonationStatus}-${DonationPriority}`;
// => Template literal type for event handler prop names
// => Creates: 'onStatusPending', 'onStatusCompleted', 'onStatusFailed', 'onStatusRefunded'
type StatusEventHandler = `onStatus${Capitalize<DonationStatus>}`;
// => Props interface using template literal types
interface DonationCardProps {
status: DonationStatus; // => One of four valid statuses
priority: DonationPriority; // => One of three priorities
amount: number; // => Donation amount
className?: DonationStatusClass; // => CSS class must match pattern
onStatusPending?: () => void; // => Event handler for pending status
onStatusCompleted?: () => void; // => Event handler for completed status
onStatusFailed?: () => void; // => Event handler for failed status
onStatusRefunded?: () => void; // => Event handler for refunded status
}
function DonationCard({
status,
priority,
amount,
className,
onStatusPending,
onStatusCompleted,
onStatusFailed,
onStatusRefunded,
}: DonationCardProps) {
// => Get status-specific color
// => Object keys are DonationStatus literals
const statusColors: Record<DonationStatus, string> = {
pending: '#FFA500', // => Orange for pending
completed: '#28A745', // => Green for completed
failed: '#DC3545', // => Red for failed
refunded: '#6C757D', // => Gray for refunded
};
// => Get priority badge
const priorityLabels: Record<DonationPriority, string> = {
low: '⬇️ Low',
normal: '➡️ Normal',
high: '⬆️ High',
};
// => Handle status-specific actions
const handleAction = () => {
// => Call appropriate event handler based on status
if (status === 'pending' && onStatusPending) {
onStatusPending(); // => Call pending handler
} else if (status === 'completed' && onStatusCompleted) {
onStatusCompleted(); // => Call completed handler
} else if (status === 'failed' && onStatusFailed) {
onStatusFailed(); // => Call failed handler
} else if (status === 'refunded' && onStatusRefunded) {
onStatusRefunded(); // => Call refunded handler
}
};
// => Card style based on status
const cardStyle: CSSProperties = {
border: `2px solid ${statusColors[status]}`,
borderRadius: '8px',
padding: '16px',
marginBottom: '12px',
backgroundColor: '#FFFFFF',
};
return (
<div className={className} style={cardStyle}>
{/* => Display status badge with color */}
<div
style={{
display: 'inline-block',
padding: '4px 8px',
borderRadius: '4px',
backgroundColor: statusColors[status],
color: '#FFFFFF',
fontWeight: 'bold',
marginRight: '8px',
}}
>
{status.toUpperCase()}
</div>
{/* => Display priority badge */}
<div style={{ display: 'inline-block' }}>
{priorityLabels[priority]}
</div>
{/* => Display amount */}
<div style={{ marginTop: '12px', fontSize: '24px', fontWeight: 'bold' }}>
${amount}
</div>
{/* => Action button (only for non-completed status) */}
{status !== 'completed' && (
<button
onClick={handleAction}
style={{
marginTop: '12px',
padding: '8px 16px',
borderRadius: '4px',
border: 'none',
backgroundColor: statusColors[status],
color: '#FFFFFF',
cursor: 'pointer',
}}
>
Handle {status}
</button>
)}
</div>
);
}
function DonationDashboard() {
const [donations, setDonations] = useState([
{ id: 1, status: 'pending' as DonationStatus, priority: 'high' as DonationPriority, amount: 100 },
{ id: 2, status: 'completed' as DonationStatus, priority: 'normal' as DonationPriority, amount: 250 },
{ id: 3, status: 'failed' as DonationStatus, priority: 'low' as DonationPriority, amount: 75 },
]);
return (
<div>
<h2>Donation Dashboard</h2>
{donations.map((donation) => (
<DonationCard
key={donation.id}
status={donation.status}
priority={donation.priority}
amount={donation.amount}
className={`donation-status-${donation.status}` as DonationStatusClass}
// => TypeScript validates className matches template pattern
onStatusPending={() => console.log('Pending donation clicked')}
onStatusCompleted={() => console.log('Completed donation clicked')}
onStatusFailed={() => console.log('Failed donation clicked')}
onStatusRefunded={() => console.log('Refunded donation clicked')}
/>
))}
</div>
);
}
export default DonationDashboard;Key Takeaway: Template literal types create precise string types from patterns, enabling compile-time validation of CSS classes, event names, and status strings. Combine with Record types for exhaustive mapping and with Capitalize/Uppercase utilities for naming conventions.
Expected Output: Page displays three donation cards with colored borders and status badges. Pending donation has orange border with “PENDING” badge and high priority icon. Completed donation has green border with “COMPLETED” badge (no action button). Failed donation has red border with “FAILED” badge and low priority icon. Each card shows donation amount.
Common Pitfalls: Using plain strings instead of template literals (loses type safety), forgetting to use as assertion for dynamic template strings (TypeScript can’t infer), overusing template literal types for simple cases (use union types for short lists).
Group 2: Advanced State Management with Zustand (5 examples)
Example 6: Zustand Store Setup
Zustand provides lightweight global state management without boilerplate. Create stores with TypeScript for type-safe state access.
// store.ts
import { create } from 'zustand';
// => State interface defines store shape
interface DonationState {
// => State properties
donations: Array<{
id: string;
amount: number;
donor: string;
timestamp: Date;
}>;
totalAmount: number; // => Derived state (sum of all donations)
isLoading: boolean; // => Loading flag for async operations
// => Actions (functions to modify state)
addDonation: (amount: number, donor: string) => void;
removeDonation: (id: string) => void;
clearDonations: () => void;
setLoading: (loading: boolean) => void;
}
// => Create Zustand store with type parameter
// => create<T> ensures store matches interface T
export const useDonationStore = create<DonationState>((set, get) => ({
// => Initial state
donations: [], // => Empty array initially
totalAmount: 0, // => Zero total initially
isLoading: false, // => Not loading initially
// => Action: add new donation
// => set() updates state immutably
addDonation: (amount, donor) => {
const newDonation = {
id: Date.now().toString(), // => Generate unique ID
amount,
donor,
timestamp: new Date(), // => Current timestamp
};
set((state) => ({
// => Create new state object
// => Never mutate state directly
donations: [...state.donations, newDonation],
// => Spread existing donations, append new one
totalAmount: state.totalAmount + amount,
// => Update total by adding new amount
}));
},
// => Action: remove donation by ID
removeDonation: (id) => {
set((state) => {
// => Find donation to remove
const donation = state.donations.find((d) => d.id === id);
if (!donation) return state; // => Return unchanged state if not found
return {
donations: state.donations.filter((d) => d.id !== id),
// => Filter out donation with matching ID
totalAmount: state.totalAmount - donation.amount,
// => Subtract removed donation amount
};
});
},
// => Action: clear all donations
clearDonations: () => {
set({
donations: [], // => Reset to empty array
totalAmount: 0, // => Reset total to zero
});
},
// => Action: set loading state
setLoading: (loading) => {
set({ isLoading: loading }); // => Update single property
},
}));
// Component.tsx
import { useDonationStore } from './store';
function DonationTracker() {
// => Subscribe to store state
// => Component re-renders when selected state changes
const donations = useDonationStore((state) => state.donations);
const totalAmount = useDonationStore((state) => state.totalAmount);
const addDonation = useDonationStore((state) => state.addDonation);
const removeDonation = useDonationStore((state) => state.removeDonation);
const clearDonations = useDonationStore((state) => state.clearDonations);
// => Select specific slices - component only re-renders when these change
const handleQuickDonate = (amount: number) => {
// => Call store action directly
addDonation(amount, 'Anonymous'); // => Updates global state
// => All components subscribed to donations re-render
};
return (
<div>
<h2>Donation Tracker</h2>
{/* => Display total from global state */}
<div style={{ fontSize: '32px', fontWeight: 'bold', marginBottom: '16px' }}>
Total: ${totalAmount}
</div>
{/* => Quick donate buttons */}
<div>
<button onClick={() => handleQuickDonate(10)}>Donate \$10</button>
<button onClick={() => handleQuickDonate(25)}>Donate \$25</button>
<button onClick={() => handleQuickDonate(50)}>Donate \$50</button>
<button onClick={() => clearDonations()}>Clear All</button>
</div>
{/* => Display donations from global state */}
<ul>
{donations.map((donation) => (
<li key={donation.id}>
${donation.amount} from {donation.donor}
{' - '}
{donation.timestamp.toLocaleTimeString()}
{' '}
<button onClick={() => removeDonation(donation.id)}>Remove</button>
</li>
))}
</ul>
</div>
);
}
export default DonationTracker;Key Takeaway: Zustand provides lightweight global state with minimal boilerplate. Define state interface, create store with actions, subscribe to slices in components. Components automatically re-render when selected state changes.
Expected Output: Page displays total donation amount (initially $0), quick donate buttons ($10, $25, $50, Clear All), and list of donations with timestamps and remove buttons. Clicking donate buttons adds donations to list and updates total. Remove buttons delete individual donations. Clear All resets everything.
Common Pitfalls: Mutating state directly instead of using set() (breaks reactivity), selecting entire store in component (causes unnecessary re-renders), forgetting to type store interface (loses type safety), not using selector functions (performance issues with large stores).
Example 7: Zustand with TypeScript and Slices
Slices organize large stores into logical modules. Combine slices for separation of concerns while maintaining single store.
// donationSlice.ts
import { StateCreator } from 'zustand';
// => StateCreator: Zustand type for slice creators
// => Donation slice interface
export interface DonationSlice {
donations: Array<{
id: string;
amount: number;
donor: string;
}>;
donationTotal: number; // => Total donation amount
addDonation: (amount: number, donor: string) => void;
removeDonation: (id: string) => void;
}
// => Slice creator with StateCreator typing
// => First generic: combined store type
// => Last generic: this slice's type
export const createDonationSlice: StateCreator<
DonationSlice & ZakatSlice,
[],
[],
DonationSlice
> = (set) => ({
// => set: Zustand function to update state
donations: [],
// => donations: empty array initially
donationTotal: 0,
// => donationTotal: 0 initially
addDonation: (amount, donor) => {
set((state) => ({
// => state: current store state (all slices)
donations: [
...state.donations,
{ id: Date.now().toString(), amount, donor },
// => id: timestamp ensures uniqueness
],
donationTotal: state.donationTotal + amount,
// => Add amount to running total
}));
},
removeDonation: (id) => {
set((state) => {
const donation = state.donations.find((d) => d.id === id);
// => Find donation to calculate total adjustment
if (!donation) return state;
// => No change if donation not found
return {
donations: state.donations.filter((d) => d.id !== id),
// => Remove donation from array
donationTotal: state.donationTotal - donation.amount,
// => Subtract amount from total
};
});
},
});
// zakatSlice.ts
import { StateCreator } from 'zustand';
// => Zakat slice interface
export interface ZakatSlice {
zakatPayments: Array<{
id: string;
wealth: number;
zakatAmount: number;
payer: string;
}>;
zakatTotal: number; // => Total zakat collected
addZakatPayment: (wealth: number, zakatAmount: number, payer: string) => void;
removeZakatPayment: (id: string) => void;
}
// => Zakat slice creator (same pattern as donation)
export const createZakatSlice: StateCreator<
DonationSlice & ZakatSlice,
[],
[],
ZakatSlice
> = (set) => ({
zakatPayments: [],
// => zakatPayments: empty array initially
zakatTotal: 0,
// => zakatTotal: 0 initially
addZakatPayment: (wealth, zakatAmount, payer) => {
set((state) => ({
zakatPayments: [
...state.zakatPayments,
{ id: Date.now().toString(), wealth, zakatAmount, payer },
// => Create new payment with timestamp id
],
zakatTotal: state.zakatTotal + zakatAmount,
// => Add zakatAmount to running total
}));
},
removeZakatPayment: (id) => {
set((state) => {
const payment = state.zakatPayments.find((p) => p.id === id);
// => Find payment to calculate total adjustment
if (!payment) return state;
// => No change if payment not found
return {
zakatPayments: state.zakatPayments.filter((p) => p.id !== id),
// => Remove payment from array
zakatTotal: state.zakatTotal - payment.zakatAmount,
// => Subtract zakatAmount from total
};
});
},
});
// store.ts
import { create } from 'zustand';
// => create: Zustand store factory function
import { DonationSlice, createDonationSlice } from './donationSlice';
import { ZakatSlice, createZakatSlice } from './zakatSlice';
// => Combined store type: intersection of both slices
type FinancialStore = DonationSlice & ZakatSlice;
// => Create store with both slices merged
// => (...a): Zustand's set, get, api parameters
export const useFinancialStore = create<FinancialStore>()((...a) => ({
...createDonationSlice(...a),
// => Spreads donation state and actions into store
...createZakatSlice(...a),
// => Spreads zakat state and actions into store
}));
// => Result: single hook accessing all state/actions
// Component.tsx
import { useFinancialStore } from './store';
function FinancialDashboard() {
// => Select donation slice state with selector functions
const donations = useFinancialStore((state) => state.donations);
// => donations: reactive, updates on donation changes
const donationTotal = useFinancialStore((state) => state.donationTotal);
// => donationTotal: reactive, updates on total changes
const addDonation = useFinancialStore((state) => state.addDonation);
// => addDonation: stable function reference
// => Select zakat slice state
const zakatPayments = useFinancialStore((state) => state.zakatPayments);
// => zakatPayments: reactive, updates on payment changes
const zakatTotal = useFinancialStore((state) => state.zakatTotal);
// => zakatTotal: reactive, updates on total changes
const addZakatPayment = useFinancialStore((state) => state.addZakatPayment);
// => addZakatPayment: stable function reference
// => Derived state: computed from both slices
const grandTotal = donationTotal + zakatTotal;
// => grandTotal: recalculates when either total changes
return (
<div>
<h2>Financial Dashboard</h2>
{/* => Grand total from both slices */}
<div style={{ fontSize: '32px', fontWeight: 'bold', marginBottom: '16px' }}>
Grand Total: ${grandTotal}
</div>
{/* => Donation section */}
<div>
<h3>Donations: ${donationTotal}</h3>
<button onClick={() => addDonation(100, 'Aisha')}>
Add \$100 Donation
</button>
<ul>
{donations.map((d) => (
<li key={d.id}>
${d.amount} from {d.donor}
</li>
))}
</ul>
</div>
{/* => Zakat section */}
<div>
<h3>Zakat: ${zakatTotal}</h3>
<button onClick={() => addZakatPayment(10000, 125, 'Omar')}>
Add \$125 Zakat Payment
</button>
<ul>
{zakatPayments.map((p) => (
<li key={p.id}>
${p.zakatAmount} from {p.payer} (wealth: ${p.wealth})
</li>
))}
</ul>
</div>
</div>
);
}
export default FinancialDashboard;Key Takeaway: Organize large Zustand stores into slices for separation of concerns. Each slice defines its own state and actions. Combine slices into single store with proper TypeScript typing. Components select from any slice seamlessly.
Expected Output: Page displays grand total (sum of donations and zakat), two sections with individual totals. Donation section shows list and “Add $100 Donation” button. Zakat section shows list and “Add $125 Zakat Payment” button. Clicking buttons adds items to respective lists and updates totals.
Common Pitfalls: Incorrect StateCreator typing (breaks type inference), slices modifying other slice’s state (breaks separation), not combining slices properly (missing state or actions), over-slicing small stores (unnecessary complexity).
Example 8: Zustand Middleware (Persist, Devtools)
Zustand middleware enhances stores with persistence, devtools integration, and logging. Chain middleware for powerful debugging and state management.
import { create } from 'zustand';
// => create: Zustand store factory
import { persist, createJSONStorage } from 'zustand/middleware';
// => persist: saves state to storage
// => createJSONStorage: configures localStorage/sessionStorage
import { devtools } from 'zustand/middleware';
// => devtools: integrates with Redux DevTools
// => State interface
interface DonationState {
donations: Array<{
id: string;
amount: number;
donor: string;
timestamp: string; // => String for JSON serialization
}>;
totalAmount: number;
addDonation: (amount: number, donor: string) => void;
removeDonation: (id: string) => void;
clearDonations: () => void;
}
// => Middleware chain: devtools wraps persist wraps store
// => Execution order: devtools() → persist() → store
export const useDonationStore = create<DonationState>()(
devtools(
// => devtools: Redux DevTools integration
// => Enables time-travel debugging
persist(
// => persist: localStorage persistence
// => Auto-rehydrates state on mount
(set) => ({
// => set: Zustand state updater function
donations: [],
// => donations: [] initially (or rehydrated from localStorage)
totalAmount: 0,
// => totalAmount: 0 initially (or rehydrated)
// => Add donation action
addDonation: (amount, donor) => {
set(
(state) => {
// => state: current store state
const newDonation = {
id: Date.now().toString(),
// => id: timestamp string (unique)
amount,
donor,
timestamp: new Date().toISOString(),
// => ISO string: JSON-serializable for localStorage
};
return {
donations: [...state.donations, newDonation],
// => Immutable array update
totalAmount: state.totalAmount + amount,
// => Add to running total
};
},
false,
// => false: merge state (don't replace entirely)
'donation/add'
// => Action name visible in Redux DevTools
);
},
// => Remove donation action
removeDonation: (id) => {
set(
(state) => {
const donation = state.donations.find((d) => d.id === id);
// => Find donation to calculate total adjustment
if (!donation) return state;
// => No change if not found
return {
donations: state.donations.filter((d) => d.id !== id),
// => Remove from array
totalAmount: state.totalAmount - donation.amount,
// => Subtract from total
};
},
false,
'donation/remove'
// => Devtools action name
);
},
// => Clear all donations action
clearDonations: () => {
set(
{
donations: [],
// => Reset to empty array
totalAmount: 0,
// => Reset total to 0
},
false,
'donation/clear'
// => Devtools action name
);
},
}),
{
name: 'donation-storage',
// => localStorage key: localStorage['donation-storage']
storage: createJSONStorage(() => localStorage),
// => Storage engine: localStorage (can be sessionStorage)
partialize: (state) => ({
donations: state.donations,
totalAmount: state.totalAmount,
// => Only persist data, exclude functions
}),
// => Optional custom serialization:
// serialize: (state) => JSON.stringify(state),
// deserialize: (str) => JSON.parse(str),
}
),
{
name: 'DonationStore',
// => Store name shown in Redux DevTools
}
)
);
// Component.tsx
import { useEffect } from 'react';
import { useDonationStore } from './store';
function PersistedDonationTracker() {
// => Selector: extract donations from store
const donations = useDonationStore((state) => state.donations);
// => Re-renders when donations change
const totalAmount = useDonationStore((state) => state.totalAmount);
// => Re-renders when totalAmount changes
const addDonation = useDonationStore((state) => state.addDonation);
// => Stable reference (doesn't cause re-renders)
const removeDonation = useDonationStore((state) => state.removeDonation);
const clearDonations = useDonationStore((state) => state.clearDonations);
// => Effect: log rehydration
useEffect(() => {
console.log('Store rehydrated from localStorage');
// => Runs after persist middleware loads saved state
console.log('Loaded donations:', donations);
// => Shows donations from previous session
}, []);
// => [] dependency: runs once on mount
return (
<div>
<h2>Persisted Donation Tracker</h2>
<p style={{ color: 'gray', fontSize: '14px' }}>
State persists across page reloads. Open Redux DevTools to inspect actions.
</p>
{/* => Display total */}
<div style={{ fontSize: '32px', fontWeight: 'bold', marginBottom: '16px' }}>
Total: ${totalAmount}
</div>
{/* => Action buttons */}
<div>
<button onClick={() => addDonation(10, 'Anonymous')}>
Donate \$10
</button>
<button onClick={() => addDonation(25, 'Anonymous')}>
Donate \$25
</button>
<button onClick={() => clearDonations()}>
Clear All
</button>
</div>
{/* => Donation list */}
<ul>
{donations.map((donation) => (
<li key={donation.id}>
${donation.amount} from {donation.donor}
{' - '}
{new Date(donation.timestamp).toLocaleString()}
{/* => Parse ISO string back to Date for display */}
{' '}
<button onClick={() => removeDonation(donation.id)}>Remove</button>
</li>
))}
</ul>
{/* => Instructions for testing */}
<div style={{ marginTop: '24px', padding: '12px', backgroundColor: '#F0F0F0' }}>
<h4>Testing Tips:</h4>
<ol>
<li>Add donations and reload page - state persists</li>
<li>Open Redux DevTools (F12 → Redux tab)</li>
<li>See actions logged as "donation/add", "donation/remove", etc.</li>
<li>Use time-travel debugging to step through actions</li>
<li>Check localStorage['donation-storage'] in devtools</li>
</ol>
</div>
</div>
);
}
export default PersistedDonationTracker;Key Takeaway: Zustand middleware adds powerful features: persist() saves state to localStorage for cross-session persistence, devtools() integrates Redux DevTools for time-travel debugging and action logging. Chain middleware with proper ordering: devtools(persist(store)).
Expected Output: Page displays donation tracker with total, action buttons, and donation list. State persists across page reloads (stored in localStorage). Redux DevTools shows actions as “donation/add”, “donation/remove”, “donation/clear”. Instructions explain how to test persistence and devtools integration.
Common Pitfalls: Wrong middleware order (persist should wrap store, devtools should wrap persist), forgetting action names in set() calls (shows as undefined in devtools), persisting non-serializable values (Dates, functions cause errors), not partializing state (persists unnecessary data).
Example 9: Server State vs Client State Separation
Separate server state (API data) from client state (UI state) for clarity. Use Zustand for client state, React Query for server state.
// clientStore.ts - Client state (UI state)
import { create } from 'zustand';
// => Client state: UI concerns only
// => Things user controls that don't persist to server
interface ClientState {
sidebarOpen: boolean; // => Sidebar visibility (UI state)
selectedTab: 'donations' | 'zakat' | 'reports'; // => Active tab (UI state)
sortOrder: 'asc' | 'desc'; // => Sort direction (UI state)
filterAmount: number | null; // => Filter threshold (UI state)
// => Client actions
toggleSidebar: () => void;
setSelectedTab: (tab: 'donations' | 'zakat' | 'reports') => void;
setSortOrder: (order: 'asc' | 'desc') => void;
setFilterAmount: (amount: number | null) => void;
}
// => Client store: manages UI state only
export const useClientStore = create<ClientState>((set) => ({
// => Initial client state
sidebarOpen: true,
selectedTab: 'donations',
sortOrder: 'desc',
filterAmount: null,
// => Client actions: update UI state only
toggleSidebar: () => set((state) => ({ sidebarOpen: !state.sidebarOpen })),
setSelectedTab: (tab) => set({ selectedTab: tab }),
setSortOrder: (order) => set({ sortOrder: order }),
setFilterAmount: (amount) => set({ filterAmount: amount }),
}));
// api.ts - Server state (API calls)
// => Server state managed by React Query, not Zustand
// => Data from API, cached and synchronized
// => Domain interface
interface Donation {
id: string;
amount: number;
donor: string;
timestamp: string;
}
// => API functions for server state
export const donationApi = {
// => Fetch donations from server
fetchDonations: async (): Promise<Donation[]> => {
// => Simulate API call
await new Promise((resolve) => setTimeout(resolve, 1000));
// => In production: const response = await fetch('/api/donations');
// => In production: return response.json();
// => Mock data for demonstration
return [
{ id: '1', amount: 100, donor: 'Aisha', timestamp: '2026-01-29T10:00:00Z' },
{ id: '2', amount: 250, donor: 'Omar', timestamp: '2026-01-29T11:00:00Z' },
{ id: '3', amount: 75, donor: 'Fatima', timestamp: '2026-01-29T12:00:00Z' },
];
},
// => Create new donation on server
createDonation: async (amount: number, donor: string): Promise<Donation> => {
await new Promise((resolve) => setTimeout(resolve, 500));
// => In production: POST to /api/donations
return {
id: Date.now().toString(),
amount,
donor,
timestamp: new Date().toISOString(),
};
},
};
// Component.tsx
import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
import { useClientStore } from './clientStore';
import { donationApi, Donation } from './api';
function DonationDashboard() {
// => Client state from Zustand (UI concerns)
const sidebarOpen = useClientStore((state) => state.sidebarOpen);
const selectedTab = useClientStore((state) => state.selectedTab);
const sortOrder = useClientStore((state) => state.sortOrder);
const filterAmount = useClientStore((state) => state.filterAmount);
const toggleSidebar = useClientStore((state) => state.toggleSidebar);
const setSortOrder = useClientStore((state) => state.setSortOrder);
const setFilterAmount = useClientStore((state) => state.setFilterAmount);
// => Server state from React Query (API data)
const queryClient = useQueryClient();
// => Query: fetch donations from server
const {
data: donations, // => Server data
isLoading, // => Loading state (server)
error, // => Error state (server)
} = useQuery({
queryKey: ['donations'], // => Cache key
queryFn: donationApi.fetchDonations, // => Fetch function
staleTime: 5000, // => Data fresh for 5 seconds
// => React Query handles caching, refetching, deduplication
});
// => Mutation: create new donation on server
const createMutation = useMutation({
mutationFn: ({ amount, donor }: { amount: number; donor: string }) =>
donationApi.createDonation(amount, donor),
onSuccess: () => {
// => Invalidate query to refetch updated data
queryClient.invalidateQueries({ queryKey: ['donations'] });
// => Triggers automatic refetch
},
});
// => Derived state: filter and sort based on client state
const filteredDonations = donations
? donations
.filter((d) =>
filterAmount === null ? true : d.amount >= filterAmount
)
// => Filter by client state (filterAmount)
.sort((a, b) =>
sortOrder === 'asc'
? a.amount - b.amount
: b.amount - a.amount
)
// => Sort by client state (sortOrder)
: [];
// => Handle create donation
const handleCreateDonation = () => {
createMutation.mutate({ amount: 100, donor: 'Anonymous' });
// => Triggers server mutation
// => onSuccess refetches donations automatically
};
return (
<div style={{ display: 'flex' }}>
{/* => Sidebar (client state) */}
{sidebarOpen && (
<div style={{ width: '200px', padding: '16px', backgroundColor: '#F5F5F5' }}>
<h3>Filters</h3>
{/* => Sort order (client state) */}
<div>
<label>
Sort:
<select
value={sortOrder}
onChange={(e) => setSortOrder(e.target.value as 'asc' | 'desc')}
>
<option value="desc">Highest First</option>
<option value="asc">Lowest First</option>
</select>
</label>
</div>
{/* => Filter amount (client state) */}
<div>
<label>
Min Amount:
<input
type="number"
value={filterAmount ?? ''}
onChange={(e) =>
setFilterAmount(e.target.value ? parseFloat(e.target.value) : null)
}
/>
</label>
</div>
</div>
)}
{/* => Main content */}
<div style={{ flex: 1, padding: '16px' }}>
<button onClick={toggleSidebar}>
{/* => Toggle sidebar (client state) */}
{sidebarOpen ? 'Hide' : 'Show'} Filters
</button>
<h2>Donations</h2>
{/* => Server state: loading */}
{isLoading && <p>Loading donations...</p>}
{/* => Server state: error */}
{error && <p style={{ color: 'red' }}>Error loading donations</p>}
{/* => Server state: success */}
{donations && (
<>
<p>Total donations: {filteredDonations.length}</p>
<button
onClick={handleCreateDonation}
disabled={createMutation.isPending}
>
{/* => Mutation state from React Query */}
{createMutation.isPending ? 'Adding...' : 'Add \$100 Donation'}
</button>
<ul>
{filteredDonations.map((donation) => (
<li key={donation.id}>
${donation.amount} from {donation.donor}
{' - '}
{new Date(donation.timestamp).toLocaleString()}
</li>
))}
</ul>
</>
)}
</div>
</div>
);
}
export default DonationDashboard;Key Takeaway: Separate client state (UI concerns, managed by Zustand) from server state (API data, managed by React Query). Client state controls UI elements like sidebar visibility and sort order. Server state handles API data with automatic caching, refetching, and synchronization. Never duplicate server data in Zustand.
Expected Output: Page displays sidebar with sort order dropdown and minimum amount filter, main area with donation list. Sidebar toggle button shows/hides filters. Sort order and filter update list immediately (client state). “Add $100 Donation” button triggers server mutation, shows “Adding…” during request, then refetches and updates list automatically (server state).
Common Pitfalls: Storing server data in Zustand (causes synchronization issues), not invalidating queries after mutations (stale data), using client state for data that should come from server (loses single source of truth), over-fetching by not leveraging React Query’s caching.
Example 10: State Machine Pattern with XState
State machines model complex workflows with explicit states and transitions. Use XState for finite state machines with TypeScript support.
import { createMachine, assign } from 'xstate';
import { useMachine } from '@xstate/react';
// => Domain interface
interface DonationContext {
amount: number; // => Donation amount
donor: string; // => Donor name
email: string; // => Donor email
confirmationCode: string; // => Confirmation code from server
error: string; // => Error message
}
// => Event types for state machine
type DonationEvent =
| { type: 'ENTER_DETAILS'; amount: number; donor: string; email: string }
| { type: 'CONFIRM' }
| { type: 'SUBMIT' }
| { type: 'SUCCESS'; confirmationCode: string }
| { type: 'FAILURE'; error: string }
| { type: 'RETRY' }
| { type: 'RESET' };
// => Define state machine
// => States: idle → entering → confirming → processing → success/failure
const donationMachine = createMachine({
// => Machine configuration
id: 'donation',
initial: 'idle', // => Start in idle state
context: {
// => Initial context values
amount: 0,
donor: '',
email: '',
confirmationCode: '',
error: '',
} as DonationContext,
schema: {
context: {} as DonationContext,
events: {} as DonationEvent,
},
states: {
// => State: idle (waiting for input)
idle: {
on: {
ENTER_DETAILS: {
// => Transition to entering state when ENTER_DETAILS event
target: 'entering',
actions: assign({
// => Update context with event data
amount: (_, event) => event.amount,
donor: (_, event) => event.donor,
email: (_, event) => event.email,
}),
// => assign() creates new context immutably
},
},
},
// => State: entering (collecting details)
entering: {
on: {
CONFIRM: 'confirming', // => Transition to confirming
RESET: 'idle', // => Transition back to idle
},
},
// => State: confirming (user reviews details)
confirming: {
on: {
SUBMIT: 'processing', // => Transition to processing
ENTER_DETAILS: {
// => Go back to entering with new details
target: 'entering',
actions: assign({
amount: (_, event) => event.amount,
donor: (_, event) => event.donor,
email: (_, event) => event.email,
}),
},
RESET: 'idle',
},
},
// => State: processing (submitting to server)
processing: {
// => invoke: run async operation
invoke: {
id: 'submitDonation',
src: async (context) => {
// => Simulate API call
await new Promise((resolve) => setTimeout(resolve, 2000));
// => Simulate random success/failure
if (Math.random() > 0.3) {
return { confirmationCode: `CONF-${Date.now()}` };
} else {
throw new Error('Payment gateway timeout');
}
},
onDone: {
// => Success: transition to success state
target: 'success',
actions: assign({
confirmationCode: (_, event) => event.data.confirmationCode,
}),
},
onError: {
// => Error: transition to failure state
target: 'failure',
actions: assign({
error: (_, event) =>
event.data instanceof Error ? event.data.message : 'Unknown error',
}),
},
},
},
// => State: success (donation completed)
success: {
on: {
RESET: {
// => Reset to idle, clear context
target: 'idle',
actions: assign({
amount: 0,
donor: '',
email: '',
confirmationCode: '',
error: '',
}),
},
},
},
// => State: failure (donation failed)
failure: {
on: {
RETRY: 'processing', // => Retry processing
RESET: {
target: 'idle',
actions: assign({
amount: 0,
donor: '',
email: '',
confirmationCode: '',
error: '',
}),
},
},
},
},
});
function DonationStateMachine() {
// => useMachine hook: provides current state and send function
const [state, send] = useMachine(donationMachine);
// => state.value is current state name ('idle', 'entering', etc.)
// => state.context is current context data
// => send() dispatches events to machine
// => Local form state (temporary, not in machine)
const [tempAmount, setTempAmount] = useState(0);
const [tempDonor, setTempDonor] = useState('');
const [tempEmail, setTempEmail] = useState('');
// => Check current state
const isIdle = state.matches('idle');
const isEntering = state.matches('entering');
const isConfirming = state.matches('confirming');
const isProcessing = state.matches('processing');
const isSuccess = state.matches('success');
const isFailure = state.matches('failure');
return (
<div>
<h2>Donation State Machine</h2>
{/* => Current state indicator */}
<div style={{ padding: '8px', backgroundColor: '#F0F0F0', marginBottom: '16px' }}>
Current State: <strong>{String(state.value)}</strong>
</div>
{/* => State: idle */}
{isIdle && (
<div>
<p>Ready to accept new donation</p>
<button
onClick={() =>
send({
type: 'ENTER_DETAILS',
amount: 100,
donor: 'Anonymous',
email: 'anon@example.com',
})
}
>
Quick Donate \$100
</button>
</div>
)}
{/* => State: entering */}
{isEntering && (
<div>
<h3>Enter Donation Details</h3>
<label>
Amount: $
<input
type="number"
value={tempAmount}
onChange={(e) => setTempAmount(parseFloat(e.target.value) || 0)}
/>
</label>
<label>
Name:
<input
type="text"
value={tempDonor}
onChange={(e) => setTempDonor(e.target.value)}
/>
</label>
<label>
Email:
<input
type="email"
value={tempEmail}
onChange={(e) => setTempEmail(e.target.value)}
/>
</label>
<div>
<button
onClick={() =>
send({
type: 'ENTER_DETAILS',
amount: tempAmount,
donor: tempDonor,
email: tempEmail,
})
}
>
Update Details
</button>
<button onClick={() => send({ type: 'CONFIRM' })}>
Review Donation
</button>
<button onClick={() => send({ type: 'RESET' })}>Cancel</button>
</div>
</div>
)}
{/* => State: confirming */}
{isConfirming && (
<div>
<h3>Confirm Donation</h3>
<p>Amount: ${state.context.amount}</p>
<p>Donor: {state.context.donor}</p>
<p>Email: {state.context.email}</p>
<div>
<button onClick={() => send({ type: 'SUBMIT' })}>
Submit Donation
</button>
<button
onClick={() =>
send({
type: 'ENTER_DETAILS',
amount: state.context.amount,
donor: state.context.donor,
email: state.context.email,
})
}
>
Edit Details
</button>
<button onClick={() => send({ type: 'RESET' })}>Cancel</button>
</div>
</div>
)}
{/* => State: processing */}
{isProcessing && (
<div>
<h3>Processing Donation...</h3>
<p>Please wait while we process your donation of ${state.context.amount}</p>
<div className="spinner">⏳ Processing...</div>
</div>
)}
{/* => State: success */}
{isSuccess && (
<div>
<h3 style={{ color: 'green' }}>✓ Donation Successful!</h3>
<p>Amount: ${state.context.amount}</p>
<p>Confirmation Code: {state.context.confirmationCode}</p>
<button onClick={() => send({ type: 'RESET' })}>
Make Another Donation
</button>
</div>
)}
{/* => State: failure */}
{isFailure && (
<div>
<h3 style={{ color: 'red' }}>✗ Donation Failed</h3>
<p>Error: {state.context.error}</p>
<div>
<button onClick={() => send({ type: 'RETRY' })}>
Retry Payment
</button>
<button onClick={() => send({ type: 'RESET' })}>
Start Over
</button>
</div>
</div>
)}
</div>
);
}
export default DonationStateMachine;Key Takeaway: State machines model complex workflows with explicit states and allowed transitions. XState provides TypeScript-safe state machines with context for data, guards for conditional transitions, and invoke for async operations. Prevents impossible states and makes workflows predictable.
Expected Output: Page shows current state indicator. In idle state, displays “Quick Donate $100” button. Entering state shows form fields and “Review Donation” button. Confirming state displays entered details with “Submit Donation” and “Edit Details” buttons. Processing state shows loading spinner. Success state displays confirmation code. Failure state shows error message with “Retry Payment” button.
Common Pitfalls: Not defining exhaustive states (allows undefined states), forgetting to assign context in actions (context doesn’t update), using state machine for simple boolean states (over-engineering), not typing events properly (loses TypeScript safety).
Group 3: Performance Optimization (5 examples)
Example 11: Code Splitting with React.lazy and Suspense
Code splitting reduces initial bundle size by loading components on-demand. Use React.lazy for dynamic imports and Suspense for loading states.
import { Suspense, lazy, useState } from 'react';
// => React.lazy() creates separate bundles (loaded on-demand)
const DonationForm = lazy(() => import('./DonationForm'));
// => import() returns Promise<{ default: Component }>
const ZakatCalculator = lazy(() => import('./ZakatCalculator'));
const ReportsPanel = lazy(() => import('./ReportsPanel'));
// => Loading fallback displayed during component load
function LoadingSpinner() {
return (
<div style={{ padding: '20px', textAlign: 'center' }}>
<div className="spinner">⏳ Loading...</div>
</div>
);
}
function Dashboard() {
const [activeTab, setActiveTab] = useState<'donate' | 'zakat' | 'reports'>('donate');
// => Tab state determines which lazy component to render
return (
<div>
<h2>Financial Dashboard</h2>
<div style={{ marginBottom: '20px' }}>
<button
onClick={() => setActiveTab('donate')}
style={{ fontWeight: activeTab === 'donate' ? 'bold' : 'normal' }}
>
Donations
</button>
<button
onClick={() => setActiveTab('zakat')}
style={{ fontWeight: activeTab === 'zakat' ? 'bold' : 'normal' }}
>
Zakat Calculator
</button>
<button
onClick={() => setActiveTab('reports')}
style={{ fontWeight: activeTab === 'reports' ? 'bold' : 'normal' }}
>
Reports
</button>
</div>
{/* => Suspense boundary catches lazy loading, shows fallback */}
<Suspense fallback={<LoadingSpinner />}>
{activeTab === 'donate' && <DonationForm />}
{/* => Bundle loads on first render, then cached */}
{activeTab === 'zakat' && <ZakatCalculator />}
{activeTab === 'reports' && <ReportsPanel />}
</Suspense>
</div>
);
}
export default Dashboard;
// DonationForm.tsx (separate file, code-split)
function DonationForm() {
const [amount, setAmount] = useState(0);
return (
<div>
<h3>Make a Donation</h3>
<label>
Amount: $
<input
type="number"
value={amount}
onChange={(e) => setAmount(parseFloat(e.target.value) || 0)}
/>
</label>
<button>Submit Donation</button>
</div>
);
}
export default DonationForm;
// ZakatCalculator.tsx (separate file, code-split)
function ZakatCalculator() {
const [wealth, setWealth] = useState(0);
return (
<div>
<h3>Calculate Zakat</h3>
<label>
Total Wealth: $
<input
type="number"
value={wealth}
onChange={(e) => setWealth(parseFloat(e.target.value) || 0)}
/>
</label>
<button>Calculate</button>
</div>
);
}
export default ZakatCalculator;
// ReportsPanel.tsx (separate file, code-split)
function ReportsPanel() {
return (
<div>
<h3>Financial Reports</h3>
<p>Monthly donation summary...</p>
<p>Zakat calculations...</p>
</div>
);
}
export default ReportsPanel;Key Takeaway: React.lazy() enables code splitting by dynamically importing components. Wrap lazy components in Suspense boundaries with fallback UI for loading states. Each lazy component creates separate bundle loaded on-demand, reducing initial load time.
Expected Output: Page displays tab navigation (Donations, Zakat Calculator, Reports) with active tab bolded. On mount, only Donations tab component loads. Clicking other tabs shows “⏳ Loading…” briefly, then displays respective component. Network tab shows separate JavaScript bundles loading per tab. Initial page load faster due to smaller main bundle.
Common Pitfalls: Forgetting Suspense boundary (error: lazy component not wrapped), nested Suspense without proper fallbacks (cascading loading states confuse users), over-splitting small components (more network requests than performance gain), using lazy() for components needed on initial render (adds unnecessary delay).
Example 12: Route-Based Code Splitting
Route-based code splitting loads route components on-demand, reducing initial bundle size significantly. Combine React Router with React.lazy for automatic route-level splitting.
import { Suspense, lazy } from 'react';
import { BrowserRouter, Routes, Route, Link, Navigate } from 'react-router-dom';
// => Lazy-load route components (separate bundles per route)
const HomePage = lazy(() => import('./routes/HomePage'));
// => Chunk: HomePage.abc123.js
const DonationPage = lazy(() => import('./routes/DonationPage'));
const ZakatPage = lazy(() => import('./routes/ZakatPage'));
const ReportsPage = lazy(() => import('./routes/ReportsPage'));
const NotFoundPage = lazy(() => import('./routes/NotFoundPage'));
// => Route loading fallback
function RouteLoadingFallback() {
return (
<div style={{ padding: '40px', textAlign: 'center' }}>
<div style={{ fontSize: '48px' }}>⏳</div>
<p>Loading page...</p>
</div>
);
}
// => Layout component (NOT lazy - always visible)
function Layout({ children }: { children: React.ReactNode }) {
return (
<div>
<nav style={{ padding: '16px', backgroundColor: '#F5F5F5', marginBottom: '20px' }}>
<Link to="/" style={{ marginRight: '16px' }}>Home</Link>
<Link to="/donate" style={{ marginRight: '16px' }}>Donate</Link>
<Link to="/zakat" style={{ marginRight: '16px' }}>Zakat</Link>
<Link to="/reports" style={{ marginRight: '16px' }}>Reports</Link>
</nav>
{children}
</div>
);
}
function App() {
return (
<BrowserRouter>
<Layout>
{/* => Suspense catches all route lazy loading */}
<Suspense fallback={<RouteLoadingFallback />}>
<Routes>
<Route path="/" element={<HomePage />} />
{/* => Bundle loads when route first accessed */}
<Route path="/donate" element={<DonationPage />} />
<Route path="/zakat" element={<ZakatPage />} />
<Route path="/reports" element={<ReportsPage />} />
<Route path="/404" element={<NotFoundPage />} />
<Route path="*" element={<Navigate to="/404" replace />} />
</Routes>
</Suspense>
</Layout>
</BrowserRouter>
);
}
export default App;
// routes/HomePage.tsx (separate file, code-split)
function HomePage() {
return (
<div style={{ padding: '20px' }}>
<h1>Welcome to Financial Platform</h1>
<p>Manage your donations, calculate zakat, and view financial reports.</p>
</div>
);
}
export default HomePage;
// routes/DonationPage.tsx (separate file, code-split)
import { useState } from 'react';
function DonationPage() {
const [amount, setAmount] = useState(0);
const [donor, setDonor] = useState('');
return (
<div style={{ padding: '20px' }}>
<h1>Make a Donation</h1>
<label>
Amount: $
<input
type="number"
value={amount}
onChange={(e) => setAmount(parseFloat(e.target.value) || 0)}
/>
</label>
<label>
Name:
<input
type="text"
value={donor}
onChange={(e) => setDonor(e.target.value)}
/>
</label>
<button>Submit Donation</button>
</div>
);
}
export default DonationPage;
// routes/ZakatPage.tsx (separate file, code-split)
import { useState } from 'react';
function ZakatPage() {
const [wealth, setWealth] = useState(0);
const [zakatAmount, setZakatAmount] = useState(0);
const calculateZakat = () => {
const nisab = 5000; // => Minimum wealth threshold
if (wealth >= nisab) {
setZakatAmount((wealth - nisab) * 0.025);
// => 2.5% of wealth above nisab
} else {
setZakatAmount(0);
}
};
return (
<div style={{ padding: '20px' }}>
<h1>Zakat Calculator</h1>
<label>
Total Wealth: $
<input
type="number"
value={wealth}
onChange={(e) => setWealth(parseFloat(e.target.value) || 0)}
/>
</label>
<button onClick={calculateZakat}>Calculate Zakat</button>
{zakatAmount > 0 && (
<p style={{ fontSize: '24px', fontWeight: 'bold', color: 'green' }}>
Zakat Due: ${zakatAmount.toFixed(2)}
</p>
)}
</div>
);
}
export default ZakatPage;
// routes/ReportsPage.tsx (separate file, code-split)
function ReportsPage() {
return (
<div style={{ padding: '20px' }}>
<h1>Financial Reports</h1>
<div>
<h2>Monthly Summary</h2>
<p>Total donations: \$2,500</p>
<p>Total zakat: \$1,250</p>
</div>
</div>
);
}
export default ReportsPage;
// routes/NotFoundPage.tsx (separate file, code-split)
function NotFoundPage() {
return (
<div style={{ padding: '20px', textAlign: 'center' }}>
<h1>404 - Page Not Found</h1>
<p>The page you're looking for doesn't exist.</p>
</div>
);
}
export default NotFoundPage;Key Takeaway: Route-based code splitting loads route components on-demand, significantly reducing initial bundle size. Combine React Router with React.lazy and Suspense for automatic route-level splitting. Navigation links trigger lazy loading when clicked. Layout components (nav, footer) stay in main bundle for instant visibility.
Expected Output: Page displays navigation bar immediately (Home, Donate, Zakat, Reports links). On initial load, only HomePage bundle loads. Clicking “Donate” shows “Loading page…” briefly, then displays donation form. Clicking other links loads respective bundles on-demand. Browser network tab shows separate chunks loading per route (HomePage.abc123.js, DonationPage.def456.js, etc.).
Common Pitfalls: Lazy-loading layout components (causes flash of unstyled content), no Suspense fallback at route level (white screen during load), prefetching all routes on mount (negates code splitting benefits), lazy-loading components used on every route (adds delay without benefit).
Example 13: React.memo and Memoization Strategies
React.memo prevents unnecessary re-renders of child components. Use for expensive components that receive stable props.
import { memo, useState, useCallback, useMemo } from 'react';
// => Domain interface
interface Donation {
id: string;
amount: number;
donor: string;
}
// => Props interface for memoized component
interface DonationItemProps {
donation: Donation;
onRemove: (id: string) => void;
}
// => Memoized component: only re-renders if props change
// => React.memo wraps component, performs shallow prop comparison
// => Prevents re-render when parent re-renders but props unchanged
const DonationItem = memo(({ donation, onRemove }: DonationItemProps) => {
console.log(`Rendering DonationItem ${donation.id}`);
// => Log to track re-renders (remove in production)
// => With memo: only logs when donation or onRemove changes
// => Without memo: logs on every parent re-render
return (
<li style={{ padding: '8px', marginBottom: '4px', backgroundColor: '#F9F9F9' }}>
${donation.amount} from {donation.donor}
{' '}
<button onClick={() => onRemove(donation.id)}>Remove</button>
{/* => onRemove must be stable (useCallback) to prevent re-renders */}
</li>
);
});
// => React.memo performs shallow comparison of props
// => Re-renders only if donation object reference changes
// => or onRemove function reference changes
// => Custom comparison function (optional)
// => Provides more control over when component re-renders
const DonationItemWithCustomCompare = memo(
({ donation, onRemove }: DonationItemProps) => {
return (
<li>
${donation.amount} from {donation.donor}
{' '}
<button onClick={() => onRemove(donation.id)}>Remove</button>
</li>
);
},
(prevProps, nextProps) => {
// => Custom comparison function
// => Return true to SKIP re-render (props considered equal)
// => Return false to RE-RENDER (props considered different)
// => Only re-render if donation amount or id changes
// => Ignore donor name changes
return (
prevProps.donation.id === nextProps.donation.id &&
prevProps.donation.amount === nextProps.donation.amount
// => onRemove assumed stable, not compared
);
// => More control than default shallow comparison
}
);
function DonationList() {
const [donations, setDonations] = useState<Donation[]>([
{ id: '1', amount: 100, donor: 'Aisha' },
{ id: '2', amount: 250, donor: 'Omar' },
{ id: '3', amount: 75, donor: 'Fatima' },
]);
const [filterAmount, setFilterAmount] = useState(0);
// => State change causes parent re-render
// => Without memo: all DonationItem children re-render
// => With memo: only affected DonationItems re-render
// => useCallback: memoize function reference
// => Returns same function reference across re-renders if deps unchanged
// => Prevents breaking React.memo optimization
const handleRemove = useCallback((id: string) => {
setDonations((prev) => prev.filter((d) => d.id !== id));
// => setDonations always stable (React guarantees)
}, []);
// => Empty deps: handleRemove reference never changes
// => Without useCallback: new function created on every render
// => New function reference breaks React.memo optimization
// => useMemo: memoize computed value
// => Recomputes only when dependencies change
const filteredDonations = useMemo(() => {
console.log('Filtering donations...');
// => Log to track recomputation
// => Only logs when donations or filterAmount changes
return donations.filter((d) => d.amount >= filterAmount);
}, [donations, filterAmount]);
// => Recomputes only when donations or filterAmount changes
// => Without useMemo: filters on every render (even unrelated state changes)
// => Expensive computation example
const totalAmount = useMemo(() => {
console.log('Calculating total...');
// => Expensive operation (simulated)
return donations.reduce((sum, d) => sum + d.amount, 0);
}, [donations]);
// => Recomputes only when donations changes
// => Without useMemo: recalculates on every render
// => Local state (unrelated to donations)
const [count, setCount] = useState(0);
// => Changing count triggers re-render
// => With memo: DonationItems don't re-render (props unchanged)
// => With useMemo: filteredDonations not recomputed (deps unchanged)
// => With useCallback: handleRemove reference unchanged
return (
<div>
<h2>Donation List with Memoization</h2>
{/* => Display memoized total */}
<div style={{ fontSize: '24px', fontWeight: 'bold', marginBottom: '16px' }}>
Total: ${totalAmount}
{/* => Recomputed only when donations changes */}
</div>
{/* => Filter input */}
<div style={{ marginBottom: '16px' }}>
<label>
Min Amount: $
<input
type="number"
value={filterAmount}
onChange={(e) => setFilterAmount(parseFloat(e.target.value) || 0)}
/>
</label>
{/* => Changing filter triggers re-render */}
{/* => useMemo recomputes filteredDonations */}
{/* => DonationItems with changed visibility re-render */}
</div>
{/* => Unrelated state (for testing memoization) */}
<div style={{ marginBottom: '16px' }}>
<button onClick={() => setCount(count + 1)}>
Increment Count: {count}
</button>
{/* => Changing count triggers re-render */}
{/* => With memo: DonationItems don't re-render */}
{/* => Without memo: all DonationItems re-render unnecessarily */}
</div>
{/* => Donation list */}
<ul>
{filteredDonations.map((donation) => (
<DonationItem
key={donation.id}
donation={donation}
onRemove={handleRemove}
// => handleRemove stable (useCallback)
// => donation object from filteredDonations (useMemo)
// => DonationItem memo prevents unnecessary re-renders
/>
))}
</ul>
{/* => Performance tips */}
<div style={{ marginTop: '24px', padding: '12px', backgroundColor: '#F0F0F0' }}>
<h4>Optimization Notes:</h4>
<ol>
<li>Click "Increment Count" - DonationItems don't re-render (memo works)</li>
<li>Check console logs - filtering only happens when filter/donations change</li>
<li>Total calculation only happens when donations change</li>
<li>handleRemove function reference stable across re-renders</li>
</ol>
</div>
</div>
);
}
export default DonationList;Key Takeaway: React.memo prevents re-renders when props unchanged (shallow comparison). useCallback memoizes function references to prevent breaking memo optimization. useMemo memoizes computed values to avoid expensive recalculations. Use all three together for maximum performance: memo for components, useCallback for passed functions, useMemo for expensive computations.
Expected Output: Page displays donation list with total, minimum amount filter, and “Increment Count” button. On initial render, console shows “Rendering DonationItem 1/2/3”, “Filtering donations…”, “Calculating total…”. Clicking “Increment Count” only increments counter - no DonationItem re-renders or filtering/calculation logs. Changing filter shows “Filtering donations…” log and updates list. Adding/removing donations shows all logs.
Common Pitfalls: Using React.memo without useCallback for passed functions (breaks optimization), memoizing everything (premature optimization, harder to maintain), not checking if optimization helps (profile first), comparing complex objects in memo without custom comparator (objects always different by reference).
Example 14: Virtual Scrolling for Large Lists
Virtual scrolling renders only visible items in large lists, dramatically improving performance. Use react-window for efficient large list rendering.
import { FixedSizeList } from 'react-window';
import { useMemo } from 'react';
interface Donation {
id: string;
amount: number;
donor: string;
timestamp: string;
}
// => Generate 10,000 donations for performance testing
function generateDonations(count: number): Donation[] {
const donations: Donation[] = [];
for (let i = 0; i < count; i++) {
donations.push({
id: `donation-${i}`,
amount: Math.floor(Math.random() * 500) + 10,
// => Random \$10-\$510
donor: `Donor ${i + 1}`,
timestamp: new Date(
Date.now() - Math.random() * 365 * 24 * 60 * 60 * 1000
).toISOString(),
// => Random date within last year
});
}
return donations;
}
function VirtualizedDonationList() {
// => Memoize: prevent regenerating 10,000 items on each render
const donations = useMemo(() => generateDonations(10000), []);
// => Row renderer: called only for visible rows
const Row = ({ index, style }: { index: number; style: React.CSSProperties }) => {
const donation = donations[index];
// => react-window manages which indices to render
return (
<div style={{
...style,
// => style: positioning from react-window (absolute)
padding: '12px',
borderBottom: '1px solid #E0E0E0',
display: 'flex',
justifyContent: 'space-between',
alignItems: 'center',
}}>
<div>
<strong>${donation.amount}</strong> from {donation.donor}
</div>
<div style={{ color: '#666', fontSize: '14px' }}>
{new Date(donation.timestamp).toLocaleDateString()}
</div>
</div>
);
};
// => Only ~20 Row instances in DOM (reused during scroll)
return (
<div>
<h2>Virtualized Donation List</h2>
<p style={{ color: '#666', marginBottom: '16px' }}>
Showing 10,000 donations using virtual scrolling. Only visible items rendered.
</p>
{/* => FixedSizeList: renders only visible portion */}
<FixedSizeList
height={600}
// => 600px container: ~12 rows visible at 50px each
itemCount={donations.length}
// => Total items: 10,000
itemSize={50}
// => Fixed 50px per row
width="100%"
overscanCount={5}
// => Render 5 extra rows above/below (prevents scroll flash)
>
{Row}
</FixedSizeList>
{/* => Performance comparison */}
<div style={{ marginTop: '24px', padding: '16px', backgroundColor: '#F0F0F0' }}>
<h3>Performance Comparison</h3>
<table style={{ width: '100%', textAlign: 'left' }}>
<thead>
<tr>
<th>Metric</th>
<th>Without Virtualization</th>
<th>With Virtualization</th>
</tr>
</thead>
<tbody>
<tr>
<td>DOM Nodes</td>
<td>10,000</td>
<td>~20 (visible + buffer)</td>
</tr>
<tr>
<td>Initial Render</td>
<td>~500ms</td>
<td>~20ms</td>
</tr>
<tr>
<td>Scroll Performance</td>
<td>Janky</td>
<td>Smooth 60fps</td>
</tr>
<tr>
<td>Memory Usage</td>
<td>~50MB</td>
<td>~5MB</td>
</tr>
</tbody>
</table>
{/* => Approximate numbers for demonstration */}
</div>
{/* => Usage notes */}
<div style={{ marginTop: '16px', padding: '12px', backgroundColor: '#FFF8E1' }}>
<h4>When to Use Virtual Scrolling:</h4>
<ul>
<li>Lists with 100+ items</li>
<li>Infinite scroll / pagination alternatives</li>
<li>Tables with many rows</li>
<li>Chat message histories</li>
<li>File explorers with large directories</li>
</ul>
<h4>When NOT to Use:</h4>
<ul>
<li>Lists under 50 items (unnecessary complexity)</li>
<li>Dynamic row heights (use VariableSizeList)</li>
<li>Lists that need full-text search (entire dataset not in DOM)</li>
</ul>
</div>
</div>
);
}
export default VirtualizedDonationList;Key Takeaway: Virtual scrolling renders only visible list items, dramatically improving performance for large lists. react-window provides FixedSizeList for fixed-height rows and VariableSizeList for dynamic heights. Only ~20 DOM nodes exist regardless of list size. Use for lists with 100+ items to prevent browser lag.
Expected Output: Page displays heading and scrollable list of 10,000 donations. Scrolling is smooth at 60fps. Browser DevTools shows only ~20 list item DOM nodes despite 10,000 total items. Performance comparison table shows dramatic improvements. Usage notes explain when to use virtualization.
Common Pitfalls: Wrong itemSize (causes misaligned items), dynamic row heights with FixedSizeList (use VariableSizeList), not memoizing row renderer (causes excessive re-renders), forgetting overscanCount (white flash during fast scrolling), using for small lists (unnecessary complexity).
Example 15: Web Workers for Heavy Computations
Web Workers run JavaScript in background threads, preventing UI blocking during heavy computations. Use for CPU-intensive tasks like data processing, encryption, or complex calculations.
import { useState, useEffect, useRef } from 'react';
// worker.ts (separate file, loaded as Web Worker)
// => Web Worker code runs in separate thread
// => No access to DOM or React state
// => Communicates via messages
// => Self-contained worker code
const workerCode = `
// => Worker receives messages via onmessage
self.onmessage = function(e) {
const { type, data } = e.data;
if (type === 'CALCULATE_ZAKAT') {
// => CPU-intensive zakat calculation
// => Processes large dataset without blocking UI
const { donations } = data;
const results = donations.map((donation) => {
// => Simulate expensive computation
// => In production: complex financial calculations
let sum = 0;
for (let i = 0; i < 1000000; i++) {
sum += Math.sqrt(donation.amount);
}
const nisab = 5000;
const wealth = donation.amount;
const isZakatDue = wealth >= nisab;
const zakatAmount = isZakatDue ? (wealth - nisab) * 0.025 : 0;
return {
id: donation.id,
donor: donation.donor,
amount: donation.amount,
zakatAmount,
isZakatDue,
};
});
// => Send results back to main thread
self.postMessage({
type: 'CALCULATION_COMPLETE',
data: results,
});
}
};
`;
// => Domain interfaces
interface Donation {
id: string;
amount: number;
donor: string;
}
interface ZakatResult {
id: string;
donor: string;
amount: number;
zakatAmount: number;
isZakatDue: boolean;
}
function ZakatCalculatorWithWorker() {
const [donations] = useState<Donation[]>([
// => Sample donations for processing
{ id: '1', amount: 10000, donor: 'Aisha' },
{ id: '2', amount: 3000, donor: 'Omar' },
{ id: '3', amount: 7500, donor: 'Fatima' },
{ id: '4', amount: 15000, donor: 'Ali' },
{ id: '5', amount: 4500, donor: 'Zahra' },
]);
const [results, setResults] = useState<ZakatResult[]>([]);
const [isCalculating, setIsCalculating] = useState(false);
const [calculationTime, setCalculationTime] = useState(0);
// => Worker reference persists across re-renders
const workerRef = useRef<Worker | null>(null);
// => Initialize worker on mount
useEffect(() => {
// => Create blob from worker code string
// => Blob contains worker JavaScript code
const blob = new Blob([workerCode], { type: 'application/javascript' });
// => Create object URL from blob
const workerUrl = URL.createObjectURL(blob);
// => Create Worker from blob URL
// => Worker runs in separate thread
workerRef.current = new Worker(workerUrl);
// => Worker ready to receive messages
// => Set up message handler
// => Receives messages from worker
workerRef.current.onmessage = (e) => {
const { type, data } = e.data;
if (type === 'CALCULATION_COMPLETE') {
// => Worker finished calculation
// => Update state with results
setResults(data);
setIsCalculating(false);
setCalculationTime(performance.now() - startTimeRef.current);
// => UI never blocked during calculation
}
};
// => Cleanup: terminate worker on unmount
return () => {
workerRef.current?.terminate();
// => Free worker thread resources
URL.revokeObjectURL(workerUrl);
// => Clean up blob URL
};
}, []);
// => Empty deps: worker created once on mount
// => Track calculation start time
const startTimeRef = useRef(0);
// => Trigger calculation in worker
const handleCalculate = () => {
if (!workerRef.current) return;
// => Record start time
startTimeRef.current = performance.now();
// => Set calculating state
setIsCalculating(true);
// => Send message to worker
// => Worker receives in onmessage handler
workerRef.current.postMessage({
type: 'CALCULATE_ZAKAT',
data: { donations },
});
// => Main thread continues running
// => UI remains responsive
// => Worker processes in background thread
};
// => Calculate without worker (blocks UI)
// => For comparison/demonstration
const handleCalculateMainThread = () => {
setIsCalculating(true);
const startTime = performance.now();
// => Heavy computation on main thread
// => UI freezes during this operation
const results = donations.map((donation) => {
// => Simulate expensive computation
let sum = 0;
for (let i = 0; i < 1000000; i++) {
sum += Math.sqrt(donation.amount);
}
// => UI blocked during loop
const nisab = 5000;
const wealth = donation.amount;
const isZakatDue = wealth >= nisab;
const zakatAmount = isZakatDue ? (wealth - nisab) * 0.025 : 0;
return {
id: donation.id,
donor: donation.donor,
amount: donation.amount,
zakatAmount,
isZakatDue,
};
});
// => Update state after computation
setResults(results);
setIsCalculating(false);
setCalculationTime(performance.now() - startTime);
// => UI unfrozen after setState
};
return (
<div>
<h2>Zakat Calculator with Web Worker</h2>
{/* => Donation list */}
<div style={{ marginBottom: '16px' }}>
<h3>Donations:</h3>
<ul>
{donations.map((d) => (
<li key={d.id}>
${d.amount} from {d.donor}
</li>
))}
</ul>
</div>
{/* => Calculation controls */}
<div style={{ marginBottom: '16px' }}>
<button
onClick={handleCalculate}
disabled={isCalculating}
style={{
padding: '12px 24px',
marginRight: '8px',
backgroundColor: '#4CAF50',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: isCalculating ? 'not-allowed' : 'pointer',
}}
>
{isCalculating ? 'Calculating (Worker)...' : 'Calculate (Worker)'}
</button>
{/* => Worker calculation: UI stays responsive */}
<button
onClick={handleCalculateMainThread}
disabled={isCalculating}
style={{
padding: '12px 24px',
backgroundColor: '#FF9800',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: isCalculating ? 'not-allowed' : 'pointer',
}}
>
{isCalculating ? 'Calculating (Main Thread)...' : 'Calculate (Main Thread)'}
</button>
{/* => Main thread calculation: UI freezes */}
</div>
{/* => Calculation time */}
{calculationTime > 0 && (
<div style={{ marginBottom: '16px', padding: '12px', backgroundColor: '#E3F2FD' }}>
<strong>Calculation Time:</strong> {calculationTime.toFixed(2)}ms
</div>
)}
{/* => Results */}
{results.length > 0 && (
<div>
<h3>Zakat Results:</h3>
<table style={{ width: '100%', borderCollapse: 'collapse' }}>
<thead>
<tr style={{ backgroundColor: '#F5F5F5' }}>
<th style={{ padding: '12px', textAlign: 'left', border: '1px solid #ddd' }}>
Donor
</th>
<th style={{ padding: '12px', textAlign: 'left', border: '1px solid #ddd' }}>
Amount
</th>
<th style={{ padding: '12px', textAlign: 'left', border: '1px solid #ddd' }}>
Zakat Due
</th>
<th style={{ padding: '12px', textAlign: 'left', border: '1px solid #ddd' }}>
Status
</th>
</tr>
</thead>
<tbody>
{results.map((result) => (
<tr key={result.id}>
<td style={{ padding: '12px', border: '1px solid #ddd' }}>
{result.donor}
</td>
<td style={{ padding: '12px', border: '1px solid #ddd' }}>
${result.amount}
</td>
<td style={{ padding: '12px', border: '1px solid #ddd' }}>
${result.zakatAmount.toFixed(2)}
</td>
<td
style={{
padding: '12px',
border: '1px solid #ddd',
color: result.isZakatDue ? 'green' : 'gray',
fontWeight: result.isZakatDue ? 'bold' : 'normal',
}}
>
{result.isZakatDue ? '✓ Zakat Due' : 'Below Nisab'}
</td>
</tr>
))}
</tbody>
</table>
</div>
)}
{/* => Usage notes */}
<div style={{ marginTop: '24px', padding: '16px', backgroundColor: '#FFF8E1' }}>
<h4>Testing Web Worker Performance:</h4>
<ol>
<li>
Click "Calculate (Worker)" - UI stays responsive, you can scroll/click during calculation
</li>
<li>
Click "Calculate (Main Thread)" - UI freezes, page becomes unresponsive during calculation
</li>
<li>
Compare calculation times - both similar duration, but worker doesn't block UI
</li>
</ol>
<h4>When to Use Web Workers:</h4>
<ul>
<li>Heavy data processing (filtering, sorting large datasets)</li>
<li>Encryption/decryption operations</li>
<li>Image/video processing</li>
<li>Complex mathematical calculations</li>
<li>Parsing large JSON/XML files</li>
</ul>
<h4>Limitations:</h4>
<ul>
<li>No DOM access (can't manipulate React components)</li>
<li>No React state access (communicate via messages only)</li>
<li>Setup overhead (~1ms per message)</li>
<li>Data copied between threads (not suitable for huge objects)</li>
</ul>
</div>
</div>
);
}
export default ZakatCalculatorWithWorker;Key Takeaway: Web Workers run JavaScript in background threads, preventing UI blocking during heavy computations. Main thread stays responsive while worker processes data. Use for CPU-intensive tasks like data processing, encryption, or complex calculations. Communicate via postMessage/onmessage. Workers have no DOM or React access.
Expected Output: Page displays donation list, two calculation buttons (Worker and Main Thread), and results table. Clicking “Calculate (Worker)” shows “Calculating…” but UI remains responsive (can scroll, click). Clicking “Calculate (Main Thread)” freezes UI until calculation completes. Both display calculation time and zakat results table showing donor names, amounts, zakat due, and status with color coding.
Common Pitfalls: Trying to access DOM in worker (error: DOM not available), passing non-serializable data (functions, DOM nodes cause errors), creating workers in render (memory leak), not terminating workers on unmount (resource leak), using workers for quick operations (overhead > benefit).
Group 4: Concurrent Features and Advanced Patterns (5 examples)
Example 16: Suspense for Data Fetching
Suspense handles async data loading declaratively. Components “suspend” while loading, Suspense boundary shows fallback UI.
import { Suspense } from 'react';
// => Suspense: declarative loading state boundary
// => Resource interface: Suspense-compatible data fetcher
// => read() must throw promise (pending) or return data (success)
interface Resource<T> {
read(): T;
// => read(): throws promise | throws error | returns T
}
// => wrapPromise: converts Promise to Suspense Resource
function wrapPromise<T>(promise: Promise<T>): Resource<T> {
let status: 'pending' | 'success' | 'error' = 'pending';
// => status: tracks promise lifecycle
let result: T;
// => result: stores resolved data
let error: any;
// => error: stores rejection reason
// => Execute promise immediately (not lazy)
const suspender = promise.then(
(data) => {
status = 'success';
// => Update status on resolution
result = data;
// => Cache resolved data
},
(err) => {
status = 'error';
// => Update status on rejection
error = err;
// => Cache error
}
);
return {
read(): T {
// => Pending: throw promise to trigger Suspense
if (status === 'pending') {
throw suspender;
// => Suspense catches, shows fallback
}
// => Error: throw to trigger Error Boundary
if (status === 'error') {
throw error;
// => Error Boundary handles
}
// => Success: return cached data
return result;
// => Component renders with data
},
};
}
interface Donation {
id: string;
amount: number;
donor: string;
timestamp: string;
}
// => fetchDonations: returns Resource (not Promise)
function fetchDonations(): Resource<Donation[]> {
const promise = new Promise<Donation[]>((resolve) => {
setTimeout(() => {
// => Simulate 2-second API delay
resolve([
{ id: '1', amount: 100, donor: 'Aisha', timestamp: '2026-01-29T10:00:00Z' },
{ id: '2', amount: 250, donor: 'Omar', timestamp: '2026-01-29T11:00:00Z' },
{ id: '3', amount: 75, donor: 'Fatima', timestamp: '2026-01-29T12:00:00Z' },
]);
// => Resolve with donation array
}, 2000);
});
return wrapPromise(promise);
// => Convert Promise to Suspense Resource
}
// => Create resource immediately (module scope)
// => Starts fetch before component mounts (early fetch)
const donationResource = fetchDonations();
// => Component reading from Suspense resource
function DonationList() {
// => read(): throws promise (pending) or returns data (success)
const donations = donationResource.read();
// => If pending: throws → Suspense shows fallback
// => If success: returns → component renders
// => Component "suspends" while loading
return (
<div>
<h3>Recent Donations</h3>
<ul>
{donations.map((donation) => (
<li key={donation.id}>
${donation.amount} from {donation.donor}
{' - '}
{new Date(donation.timestamp).toLocaleString()}
{/* => Parse ISO timestamp to locale string */}
</li>
))}
</ul>
<p>Total: ${donations.reduce((sum, d) => sum + d.amount, 0)}</p>
{/* => Compute total from donation amounts */}
</div>
);
}
function SuspenseDemo() {
return (
<div>
<h2>Suspense for Data Fetching</h2>
{/* => Suspense: declarative loading boundary */}
<Suspense
fallback={
// => fallback: shown while DonationList suspended
<div style={{ padding: '20px', textAlign: 'center' }}>
<div style={{ fontSize: '32px' }}>⏳</div>
<p>Loading donations...</p>
</div>
}
>
{/* => DonationList calls read() → throws if pending */}
{/* => Suspense catches thrown promise → shows fallback */}
{/* => When promise resolves → re-renders DonationList */}
<DonationList />
</Suspense>
</div>
);
}
export default SuspenseDemo;Key Takeaway: Suspense handles async data loading declaratively. Components “suspend” (throw promise) while loading data. Suspense boundary catches suspension and shows fallback UI. When data ready, component renders normally. Enables “render-as-you-fetch” pattern for better UX.
Expected Output: Page displays heading, then shows loading fallback (⏳ with “Loading donations…” text) for 2 seconds. After data loads, displays donation list with three donations and total amount.
Common Pitfalls: Creating resource inside component (causes infinite suspension), no Suspense boundary (error: suspended component not wrapped), fetching on render instead of early (loses performance benefit), using with legacy data fetching libraries (not Suspense-compatible).
Example 17: startTransition for Non-Urgent Updates
startTransition marks state updates as low-priority, keeping UI responsive during expensive updates. Use for non-urgent updates like filtering or search results.
import { useState, startTransition, useDeferredValue } from 'react';
interface Donation {
id: string;
amount: number;
donor: string;
category: string;
}
// => Generate 5000 donations (large enough to cause filter lag)
function generateDonations(count: number): Donation[] {
const categories = ['general', 'zakat', 'sadaqah', 'emergency'];
const donations: Donation[] = [];
for (let i = 0; i < count; i++) {
donations.push({
id: `donation-${i}`,
amount: Math.floor(Math.random() * 500) + 10,
donor: `Donor ${i + 1}`,
category: categories[Math.floor(Math.random() * categories.length)],
});
}
return donations;
}
function TransitionDemo() {
const [donations] = useState(() => generateDonations(5000));
// => 5000 donations: filtering is expensive
const [query, setQuery] = useState('');
// => query: urgent update (input value)
const [filteredDonations, setFilteredDonations] = useState(donations);
// => filteredDonations: non-urgent update
const [isPending, startTransition] = useState(false);
// => isPending: true while transition in progress
// => WITHOUT startTransition: blocks UI
const handleSearchWithoutTransition = (value: string) => {
setQuery(value);
// => Urgent: update immediately
const filtered = donations.filter(
(d) =>
d.donor.toLowerCase().includes(value.toLowerCase()) ||
d.category.toLowerCase().includes(value.toLowerCase())
);
// => Expensive: filtering 5000 items blocks UI
setFilteredDonations(filtered);
// => Blocks until complete, causes input lag
};
// => WITH startTransition: keeps input responsive
const handleSearchWithTransition = (value: string) => {
setQuery(value);
// => Urgent: updates immediately
// => Mark filtered update as non-urgent
startTransition(() => {
const filtered = donations.filter(
(d) =>
d.donor.toLowerCase().includes(value.toLowerCase()) ||
d.category.toLowerCase().includes(value.toLowerCase())
);
setFilteredDonations(filtered);
// => Non-urgent: React can interrupt for urgent updates
});
// => Input stays responsive during filtering
};
return (
<div>
<h2>startTransition Demo</h2>
<div style={{ marginBottom: '16px' }}>
<label>
Search donations (try typing quickly):
<input
type="text"
value={query}
onChange={(e) => handleSearchWithTransition(e.target.value)}
// => Using transition: input stays responsive
style={{
padding: '8px',
fontSize: '16px',
width: '100%',
marginTop: '8px',
}}
placeholder="Search by donor or category..."
/>
</label>
</div>
{/* => Pending indicator during transition */}
{isPending && (
<div style={{ padding: '8px', backgroundColor: '#FFF8E1', marginBottom: '16px' }}>
⏳ Filtering results...
</div>
)}
<div style={{ marginBottom: '16px' }}>
<strong>Results:</strong> {filteredDonations.length} of {donations.length} donations
</div>
{/* => Show first 100 results */}
<div style={{ maxHeight: '400px', overflowY: 'auto', border: '1px solid #ddd' }}>
{filteredDonations.slice(0, 100).map((donation) => (
<div
key={donation.id}
style={{
padding: '12px',
borderBottom: '1px solid #f0f0f0',
}}
>
<strong>${donation.amount}</strong> from {donation.donor}
{' - '}
<span style={{ color: '#666' }}>{donation.category}</span>
</div>
))}
{/* => Rendering happens during transition (non-blocking) */}
</div>
{/* => Comparison */}
<div style={{ marginTop: '24px', padding: '16px', backgroundColor: '#F0F0F0' }}>
<h3>startTransition Benefits:</h3>
<table style={{ width: '100%', textAlign: 'left' }}>
<thead>
<tr>
<th>Aspect</th>
<th>Without startTransition</th>
<th>With startTransition</th>
</tr>
</thead>
<tbody>
<tr>
<td>Input Responsiveness</td>
<td>Laggy (blocks on keystroke)</td>
<td>Smooth (never blocks)</td>
</tr>
<tr>
<td>Results Update</td>
<td>Immediate but janky</td>
<td>Slightly delayed but smooth</td>
</tr>
<tr>
<td>User Experience</td>
<td>Frustrating during typing</td>
<td>Responsive, professional feel</td>
</tr>
<tr>
<td>Priority</td>
<td>All updates equal priority</td>
<td>Input urgent, filtering deferred</td>
</tr>
</tbody>
</table>
</div>
{/* => Usage notes */}
<div style={{ marginTop: '16px', padding: '12px', backgroundColor: '#FFF8E1' }}>
<h4>When to Use startTransition:</h4>
<ul>
<li>Filtering/searching large lists (keeps input responsive)</li>
<li>Tab switching with heavy rendering (current tab responsive)</li>
<li>Non-urgent updates during user interaction (form input, clicks)</li>
<li>Updating derived state from expensive computations</li>
</ul>
<h4>When NOT to Use:</h4>
<ul>
<li>Input field controlled values (must update immediately)</li>
<li>Critical UI feedback (loading states, error messages)</li>
<li>Quick updates (no performance benefit)</li>
</ul>
</div>
</div>
);
}
export default TransitionDemo;Key Takeaway: startTransition marks state updates as non-urgent, preventing them from blocking urgent updates like input. React can interrupt transitions for higher-priority work. Keeps UI responsive during expensive updates. Use for filtering, search results, tab switching, and derived state updates.
Expected Output: Page displays search input, results count, and donation list. Typing in search input stays smooth and responsive even with 5000 donations being filtered. Pending indicator shows “⏳ Filtering results…” during transition. Results update slightly delayed but input never lags. Comparison table explains benefits.
Common Pitfalls: Wrapping controlled input values in transitions (input lag), using for critical updates (delays important feedback), overusing transitions (unnecessary complexity), not showing pending state (users confused by delay).
Example 18: useDeferredValue for Expensive Renders
useDeferredValue defers expensive rendering based on state value. React shows stale value during expensive updates, keeping UI responsive.
import { useState, useDeferredValue, memo } from 'react';
interface Donation {
id: string;
amount: number;
donor: string;
}
function generateDonations(count: number): Donation[] {
return Array.from({ length: count }, (_, i) => ({
id: `donation-${i}`,
amount: Math.floor(Math.random() * 500) + 10,
donor: `Donor ${i + 1}`,
}));
}
// => Expensive component: memoized to skip renders when props unchanged
const ExpensiveDonationList = memo(
({ donations, query }: { donations: Donation[]; query: string }) => {
// => Simulate expensive render (100ms block)
const startTime = performance.now();
while (performance.now() - startTime < 100) {
// => In production: complex calculations, heavy rendering
}
const filtered = donations.filter((d) =>
d.donor.toLowerCase().includes(query.toLowerCase())
);
return (
<div>
<p>Showing {filtered.length} of {donations.length} donations</p>
<div style={{ maxHeight: '300px', overflowY: 'auto', border: '1px solid #ddd' }}>
{filtered.map((donation) => (
<div
key={donation.id}
style={{ padding: '8px', borderBottom: '1px solid #f0f0f0' }}
>
${donation.amount} from {donation.donor}
</div>
))}
</div>
</div>
);
}
);
function DeferredValueDemo() {
const [donations] = useState(() => generateDonations(1000));
const [query, setQuery] = useState('');
// => query: urgent, updates immediately
// => deferredQuery: can lag behind query during expensive renders
const deferredQuery = useDeferredValue(query);
// => Keeps input responsive
const isStale = query !== deferredQuery;
// => true when deferredQuery hasn't caught up to query
return (
<div>
<h2>useDeferredValue Demo</h2>
<div style={{ marginBottom: '16px' }}>
<label>
Search donations (try typing quickly):
<input
type="text"
value={query}
onChange={(e) => setQuery(e.target.value)}
// => query updates immediately (input responsive)
style={{
padding: '8px',
fontSize: '16px',
width: '100%',
marginTop: '8px',
}}
placeholder="Search by donor name..."
/>
</label>
</div>
{/* => Show indicator when deferred value lagging */}
{isStale && (
<div style={{ padding: '8px', backgroundColor: '#FFF8E1', marginBottom: '16px' }}>
⏳ Updating results...
</div>
)}
{/* => Dim list during update */}
<div style={{ opacity: isStale ? 0.6 : 1, transition: 'opacity 0.2s' }}>
<ExpensiveDonationList donations={donations} query={deferredQuery} />
{/* => Uses deferredQuery (can be stale), not query */}
</div>
{/* => Comparison */}
<div style={{ marginTop: '24px', padding: '16px', backgroundColor: '#F0F0F0' }}>
<h3>useDeferredValue vs useMemo:</h3>
<table style={{ width: '100%', textAlign: 'left' }}>
<thead>
<tr>
<th>Aspect</th>
<th>useMemo</th>
<th>useDeferredValue</th>
</tr>
</thead>
<tbody>
<tr>
<td>Purpose</td>
<td>Memoize computed value</td>
<td>Defer expensive renders</td>
</tr>
<tr>
<td>When Recomputes</td>
<td>Dependencies change</td>
<td>Value changes + React idle</td>
</tr>
<tr>
<td>Input Responsiveness</td>
<td>Can block if computation slow</td>
<td>Never blocks (defers update)</td>
</tr>
<tr>
<td>Use Case</td>
<td>Expensive calculations</td>
<td>Expensive rendering</td>
</tr>
</tbody>
</table>
</div>
{/* => Usage notes */}
<div style={{ marginTop: '16px', padding: '12px', backgroundColor: '#E3F2FD' }}>
<h4>How useDeferredValue Works:</h4>
<ol>
<li>User types in input → query state updates immediately</li>
<li>React attempts to render with new deferredQuery</li>
<li>Render is expensive (takes time) → React continues with stale value</li>
<li>Input stays responsive, shows stale results temporarily</li>
<li>When React idle → completes render with new value</li>
</ol>
<h4>When to Use useDeferredValue:</h4>
<ul>
<li>Expensive list rendering based on search/filter</li>
<li>Charts/visualizations updating from input</li>
<li>Any slow UI update during user interaction</li>
</ul>
<h4>Benefits:</h4>
<ul>
<li>Input never lags (always responsive)</li>
<li>No manual debouncing needed</li>
<li>React automatically optimizes timing</li>
<li>Better UX than blocking updates</li>
</ul>
</div>
</div>
);
}
export default DeferredValueDemo;Key Takeaway: useDeferredValue defers expensive renders by allowing React to show stale values during expensive updates. Input stays responsive while expensive component updates in background. Automatically handles timing - no manual debouncing needed. Use for expensive rendering based on user input.
Expected Output: Page displays search input and donation list. Typing in input stays smooth and responsive. During expensive renders, “⏳ Updating results…” appears and list slightly dims. Results update after short delay. Comparison table explains difference from useMemo. Usage notes explain mechanism and benefits.
Common Pitfalls: Using for cheap renders (unnecessary overhead), not showing stale state indicator (users confused by delay), using with controlled input values directly (input lag), combining with useMemo unnecessarily (redundant optimization).
Example 19: Error Boundaries with Retry Logic
Error boundaries catch React component errors and show fallback UI. Add retry logic to recover from transient failures.
import { Component, ReactNode, useState } from 'react';
// => Error boundary props interface
interface ErrorBoundaryProps {
children: ReactNode;
fallback?: (error: Error, retry: () => void) => ReactNode;
}
// => Error boundary state interface
interface ErrorBoundaryState {
hasError: boolean;
error: Error | null;
errorInfo: React.ErrorInfo | null;
errorCount: number; // => Track retry attempts
}
// => Error boundary class component
// => Must be class component (no hooks equivalent yet)
class ErrorBoundary extends Component<ErrorBoundaryProps, ErrorBoundaryState> {
constructor(props: ErrorBoundaryProps) {
super(props);
// => Initial state: no error
this.state = {
hasError: false,
error: null,
errorInfo: null,
errorCount: 0,
};
}
// => Static method called when child component throws error
// => Returns new state to render fallback UI
static getDerivedStateFromError(error: Error): Partial<ErrorBoundaryState> {
// => Update state to trigger fallback render
return {
hasError: true,
error,
};
}
// => Lifecycle method called after error caught
// => Used for error logging, analytics
componentDidCatch(error: Error, errorInfo: React.ErrorInfo) {
// => Log error details
console.error('Error caught by boundary:', error);
console.error('Component stack:', errorInfo.componentStack);
// => In production: send to error tracking service (Sentry, LogRocket, etc.)
// => Update state with error details
this.setState({
errorInfo,
errorCount: this.state.errorCount + 1,
});
}
// => Retry handler
// => Resets error state, attempts to render children again
handleRetry = () => {
console.log('Retrying render...');
// => Reset error state
this.setState({
hasError: false,
error: null,
errorInfo: null,
// => Keep errorCount to track retry attempts
});
// => Children re-render when state updates
};
render() {
if (this.state.hasError && this.state.error) {
// => Error occurred, render fallback UI
if (this.props.fallback) {
// => Use custom fallback if provided
return this.props.fallback(this.state.error, this.handleRetry);
}
// => Default fallback UI
return (
<div style={{ padding: '24px', backgroundColor: '#FFEBEE', borderRadius: '8px' }}>
<h2 style={{ color: '#C62828' }}>⚠️ Something went wrong</h2>
<details style={{ marginTop: '16px', cursor: 'pointer' }}>
<summary style={{ fontWeight: 'bold' }}>Error details</summary>
<pre style={{ marginTop: '8px', fontSize: '12px', overflow: 'auto' }}>
{this.state.error.toString()}
{this.state.errorInfo?.componentStack}
</pre>
</details>
<div style={{ marginTop: '16px' }}>
<button
onClick={this.handleRetry}
style={{
padding: '12px 24px',
backgroundColor: '#2196F3',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Retry
</button>
</div>
<p style={{ marginTop: '16px', color: '#666' }}>
Retry attempts: {this.state.errorCount}
</p>
</div>
);
}
// => No error, render children normally
return this.props.children;
}
}
// => Buggy component for demonstration
// => Throws error when shouldFail is true
function BuggyDonationForm({ shouldFail }: { shouldFail: boolean }) {
if (shouldFail) {
// => Simulate runtime error
throw new Error('Failed to load donation form: API connection timeout');
// => Error boundary catches this error
}
return (
<div style={{ padding: '16px', border: '1px solid #4CAF50', borderRadius: '8px' }}>
<h3>Donation Form</h3>
<form>
<label>
Amount: $
<input type="number" placeholder="100" />
</label>
<button type="submit">Submit Donation</button>
</form>
</div>
);
}
function ErrorBoundaryDemo() {
const [shouldFail, setShouldFail] = useState(false);
const [key, setKey] = useState(0);
// => Key forces unmount/remount of BuggyDonationForm
// => Toggle error state
const handleToggleError = () => {
setShouldFail(!shouldFail);
// => Next render: BuggyDonationForm throws or succeeds
};
// => Force retry by remounting component
const handleForceRetry = () => {
setKey((prev) => prev + 1);
// => Changing key forces React to unmount and remount
// => Fresh component instance, error boundary resets
};
return (
<div>
<h2>Error Boundary with Retry Logic</h2>
{/* => Controls */}
<div style={{ marginBottom: '24px' }}>
<button
onClick={handleToggleError}
style={{
padding: '12px 24px',
marginRight: '8px',
backgroundColor: shouldFail ? '#4CAF50' : '#F44336',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
{shouldFail ? 'Fix Component' : 'Break Component'}
</button>
<button
onClick={handleForceRetry}
style={{
padding: '12px 24px',
backgroundColor: '#2196F3',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Force Retry (Remount)
</button>
</div>
{/* => Error boundary wraps potentially buggy component */}
<ErrorBoundary
key={key}
// => Key prop forces boundary to reset on remount
fallback={(error, retry) => (
// => Custom fallback UI
<div
style={{
padding: '24px',
backgroundColor: '#FFEBEE',
borderRadius: '8px',
border: '2px solid #C62828',
}}
>
<h3 style={{ color: '#C62828' }}>⚠️ Donation Form Error</h3>
<p style={{ marginTop: '12px' }}>
We encountered an error loading the donation form. This could be due to:
</p>
<ul style={{ marginTop: '8px' }}>
<li>Network connection issues</li>
<li>Server timeout</li>
<li>Temporary service unavailability</li>
</ul>
<details style={{ marginTop: '16px', cursor: 'pointer' }}>
<summary style={{ fontWeight: 'bold', color: '#666' }}>
Technical details
</summary>
<pre
style={{
marginTop: '8px',
fontSize: '12px',
overflow: 'auto',
backgroundColor: '#FFF',
padding: '12px',
borderRadius: '4px',
}}
>
{error.message}
</pre>
</details>
<button
onClick={retry}
style={{
marginTop: '16px',
padding: '12px 24px',
backgroundColor: '#4CAF50',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Retry Loading Form
</button>
<p style={{ marginTop: '12px', color: '#666', fontSize: '14px' }}>
If problem persists, please contact support.
</p>
</div>
)}
>
{/* => Component that might throw error */}
<BuggyDonationForm shouldFail={shouldFail} />
{/* => Error boundary catches errors from children */}
</ErrorBoundary>
{/* => Usage notes */}
<div style={{ marginTop: '24px', padding: '16px', backgroundColor: '#E3F2FD' }}>
<h3>Error Boundary Features:</h3>
<ul>
<li>Catches errors in child component tree</li>
<li>Prevents entire app crash</li>
<li>Shows user-friendly fallback UI</li>
<li>Provides retry mechanism for recovery</li>
<li>Logs errors for debugging/monitoring</li>
</ul>
<h3>What Error Boundaries Catch:</h3>
<ul>
<li>Render errors (component lifecycle)</li>
<li>Constructor errors</li>
<li>useEffect errors (some cases)</li>
</ul>
<h3>What Error Boundaries DON'T Catch:</h3>
<ul>
<li>Event handler errors (use try-catch)</li>
<li>Async code errors (use .catch())</li>
<li>Server-side rendering errors</li>
<li>Errors in error boundary itself</li>
</ul>
</div>
</div>
);
}
export default ErrorBoundaryDemo;Key Takeaway: Error boundaries catch component errors and show fallback UI, preventing entire app crash. Must be class components (no hooks equivalent yet). Add retry logic to recover from transient failures. Use custom fallback UI for user-friendly error messages. Log errors to monitoring services for debugging.
Expected Output: Page displays two buttons (“Break Component” and “Force Retry”) and donation form. Clicking “Break Component” causes form to throw error. Error boundary catches error and displays red error fallback UI with error message, technical details (collapsible), and “Retry Loading Form” button. Clicking “Fix Component” or retry button recovers form. Retry attempts counter tracks recovery attempts.
Common Pitfalls: Not adding error boundaries (entire app crashes on error), error boundary too broad (catches errors it shouldn’t), no retry mechanism (users stuck on error screen), not logging errors (can’t debug production issues), using for event handler errors (error boundaries don’t catch those).
Example 20: Render-as-You-Fetch Pattern
Render-as-you-fetch starts data fetching early (before render), enabling faster page loads. Combine with Suspense for optimal loading UX.
import { Suspense, useState, lazy } from 'react';
// => Resource interface for Suspense-compatible fetching
interface Resource<T> {
read(): T;
}
// => Wrap promise for Suspense
function wrapPromise<T>(promise: Promise<T>): Resource<T> {
let status: 'pending' | 'success' | 'error' = 'pending';
let result: T;
let error: any;
const suspender = promise.then(
(data) => {
status = 'success';
result = data;
},
(err) => {
status = 'error';
error = err;
}
);
return {
read(): T {
if (status === 'pending') throw suspender;
if (status === 'error') throw error;
return result;
},
};
}
// => Domain interfaces
interface DonationSummary {
totalAmount: number;
donationCount: number;
topDonor: string;
}
interface Donation {
id: string;
amount: number;
donor: string;
}
// => API functions
// => Return resources immediately (don't wait)
function fetchDonationSummary(): Resource<DonationSummary> {
const promise = new Promise<DonationSummary>((resolve) => {
setTimeout(() => {
// => Simulate 2-second API call
resolve({
totalAmount: 12500,
donationCount: 45,
topDonor: 'Aisha',
});
}, 2000);
});
return wrapPromise(promise);
// => Returns resource immediately, starts fetching in background
}
function fetchDonations(): Resource<Donation[]> {
const promise = new Promise<Donation[]>((resolve) => {
setTimeout(() => {
// => Simulate 1.5-second API call
resolve([
{ id: '1', amount: 100, donor: 'Aisha' },
{ id: '2', amount: 250, donor: 'Omar' },
{ id: '3', amount: 75, donor: 'Fatima' },
]);
}, 1500);
});
return wrapPromise(promise);
// => Returns resource immediately, starts fetching in background
}
// => Summary component (reads from resource)
function DonationSummary({ resource }: { resource: Resource<DonationSummary> }) {
const summary = resource.read();
// => Suspends if data not ready
// => Returns data when ready
return (
<div style={{ padding: '16px', backgroundColor: '#E3F2FD', borderRadius: '8px' }}>
<h3>Donation Summary</h3>
<p><strong>Total Amount:</strong> ${summary.totalAmount}</p>
<p><strong>Total Donations:</strong> {summary.donationCount}</p>
<p><strong>Top Donor:</strong> {summary.topDonor}</p>
</div>
);
}
// => List component (reads from resource)
function DonationList({ resource }: { resource: Resource<Donation[]> }) {
const donations = resource.read();
// => Suspends if data not ready
// => Returns data when ready
return (
<div style={{ marginTop: '16px' }}>
<h3>Recent Donations</h3>
<ul>
{donations.map((donation) => (
<li key={donation.id}>
${donation.amount} from {donation.donor}
</li>
))}
</ul>
</div>
);
}
function RenderAsFetchDemo() {
const [showDashboard, setShowDashboard] = useState(false);
// => ANTI-PATTERN: Fetch-on-render
// => Resources created AFTER render starts
// => Sequential loading (slow)
const handleFetchOnRender = () => {
setShowDashboard(true);
// => Component renders first
// => Then creates resources (starts fetching)
// => Then waits for data
// => SLOW: render → fetch → wait
};
// => PATTERN: Render-as-you-fetch
// => Resources created BEFORE render starts
// => Parallel loading (fast)
const [resources, setResources] = useState<{
summary: Resource<DonationSummary> | null;
donations: Resource<Donation[]> | null;
}>({ summary: null, donations: null });
const handleRenderAsFetch = () => {
// => Start fetching IMMEDIATELY (before render)
// => Both requests start in parallel
const summaryResource = fetchDonationSummary();
const donationsResource = fetchDonations();
// => Fetching started, resources created
// => Store resources
setResources({
summary: summaryResource,
donations: donationsResource,
});
// => Show dashboard
setShowDashboard(true);
// => Render starts with fetching already in progress
// => FAST: fetch → render → wait (parallel)
};
return (
<div>
<h2>Render-as-You-Fetch Pattern</h2>
{/* => Action buttons */}
{!showDashboard && (
<div style={{ marginBottom: '24px' }}>
<button
onClick={handleRenderAsFetch}
style={{
padding: '12px 24px',
marginRight: '8px',
backgroundColor: '#4CAF50',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Load Dashboard (Render-as-Fetch)
</button>
<p style={{ marginTop: '8px', color: '#666' }}>
✓ Starts fetching before render (parallel, fast)
</p>
</div>
)}
{/* => Dashboard with render-as-fetch pattern */}
{showDashboard && resources.summary && resources.donations && (
<div>
{/* => Summary suspends while loading */}
<Suspense
fallback={
<div style={{ padding: '16px', backgroundColor: '#F5F5F5' }}>
Loading summary...
</div>
}
>
<DonationSummary resource={resources.summary} />
{/* => Reads from resource (suspends if not ready) */}
</Suspense>
{/* => List suspends independently */}
<Suspense
fallback={
<div style={{ marginTop: '16px', padding: '16px', backgroundColor: '#F5F5F5' }}>
Loading donations...
</div>
}
>
<DonationList resource={resources.donations} />
{/* => Reads from resource (suspends if not ready) */}
</Suspense>
<button
onClick={() => {
setShowDashboard(false);
setResources({ summary: null, donations: null });
}}
style={{
marginTop: '24px',
padding: '8px 16px',
backgroundColor: '#666',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Reset Demo
</button>
</div>
)}
{/* => Pattern comparison */}
<div style={{ marginTop: '24px', padding: '16px', backgroundColor: '#F0F0F0' }}>
<h3>Pattern Comparison:</h3>
<table style={{ width: '100%', textAlign: 'left' }}>
<thead>
<tr>
<th>Pattern</th>
<th>Execution Order</th>
<th>Total Time</th>
<th>UX</th>
</tr>
</thead>
<tbody>
<tr>
<td><strong>Fetch-on-Render</strong></td>
<td>
1. Render component<br />
2. Start fetch 1<br />
3. Wait for fetch 1<br />
4. Start fetch 2<br />
5. Wait for fetch 2
</td>
<td>2s + 1.5s = 3.5s</td>
<td>Sequential, slow</td>
</tr>
<tr style={{ backgroundColor: '#C8E6C9' }}>
<td><strong>Render-as-Fetch</strong></td>
<td>
1. Start fetch 1 & 2 (parallel)<br />
2. Render component<br />
3. Wait for both fetches
</td>
<td>max(2s, 1.5s) = 2s</td>
<td>Parallel, fast ✓</td>
</tr>
</tbody>
</table>
</div>
{/* => Usage notes */}
<div style={{ marginTop: '16px', padding: '12px', backgroundColor: '#E3F2FD' }}>
<h4>Render-as-You-Fetch Benefits:</h4>
<ul>
<li><strong>Parallel fetching:</strong> Multiple requests start simultaneously</li>
<li><strong>Early fetching:</strong> Requests start before component renders</li>
<li><strong>Faster page loads:</strong> Total time = slowest request (not sum)</li>
<li><strong>Better UX:</strong> Content appears as soon as ready</li>
</ul>
<h4>Implementation Steps:</h4>
<ol>
<li>Create resource functions that return immediately</li>
<li>Call resource functions BEFORE showing component</li>
<li>Pass resources to components via props</li>
<li>Components read() from resources (suspend if not ready)</li>
<li>Suspense boundaries handle loading states</li>
</ol>
<h4>Best Practices:</h4>
<ul>
<li>Start fetching as early as possible (event handlers, route changes)</li>
<li>Use separate Suspense boundaries for independent data</li>
<li>Show progressive loading (parts of UI load independently)</li>
<li>Combine with React Router for route-level fetching</li>
</ul>
</div>
</div>
);
}
export default RenderAsFetchDemo;Key Takeaway: Render-as-you-fetch starts data fetching before component renders, enabling parallel requests and faster page loads. Create resources immediately, pass to components, components read() and suspend if not ready. Combines with Suspense for progressive loading. Much faster than fetch-on-render (sequential) pattern.
Expected Output: Page displays “Load Dashboard (Render-as-Fetch)” button with explanation. Clicking button shows two loading fallbacks: “Loading summary…” and “Loading donations…”. Summary loads after 2 seconds (blue box with total amount, count, top donor). Donations load after 1.5 seconds (list of 3 donations). Both load in parallel (total time 2s, not 3.5s). Pattern comparison table explains timing difference. Reset button clears dashboard.
Common Pitfalls: Creating resources inside components (fetch-on-render, slow), not using Suspense (errors on suspended reads), sequential resource creation (loses parallel benefit), recreating resources on every render (refetch on every render).
Group 5: Testing, Security, and Production (5 examples)
Example 21: Vitest with React Testing Library
Vitest provides fast unit testing for React components. React Testing Library enables user-centric testing focusing on behavior, not implementation.
// DonationForm.tsx - Component to test
import { useState } from 'react';
export interface DonationFormProps {
onSubmit: (amount: number, donor: string) => void;
}
export function DonationForm({ onSubmit }: DonationFormProps) {
const [amount, setAmount] = useState(0);
const [donor, setDonor] = useState('');
const [error, setError] = useState('');
const handleSubmit = (e: React.FormEvent) => {
e.preventDefault();
// => Validation
if (amount <= 0) {
setError('Amount must be greater than \$0');
return;
}
if (donor.trim() === '') {
setError('Donor name is required');
return;
}
// => Clear error and submit
setError('');
onSubmit(amount, donor);
// => Reset form
setAmount(0);
setDonor('');
};
return (
<form onSubmit={handleSubmit} data-testid="donation-form">
<div>
<label htmlFor="amount">Amount: $</label>
<input
id="amount"
type="number"
value={amount}
onChange={(e) => setAmount(parseFloat(e.target.value) || 0)}
data-testid="amount-input"
/>
</div>
<div>
<label htmlFor="donor">Donor Name:</label>
<input
id="donor"
type="text"
value={donor}
onChange={(e) => setDonor(e.target.value)}
data-testid="donor-input"
/>
</div>
{error && (
<div role="alert" style={{ color: 'red' }}>
{error}
</div>
)}
<button type="submit" data-testid="submit-button">
Submit Donation
</button>
</form>
);
}
// DonationForm.test.tsx - Vitest + React Testing Library tests
import { describe, it, expect, vi } from 'vitest';
import { render, screen, fireEvent, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { DonationForm } from './DonationForm';
// => Test suite for DonationForm component
describe('DonationForm', () => {
// => Test: renders form fields
it('renders amount input, donor input, and submit button', () => {
// => Arrange: render component with mock onSubmit
const mockOnSubmit = vi.fn();
// => vi.fn() creates mock function for testing
render(<DonationForm onSubmit={mockOnSubmit} />);
// => Assert: form fields present in DOM
// => screen queries simulate user perspective
expect(screen.getByLabelText(/amount/i)).toBeInTheDocument();
// => getByLabelText finds input by associated label
expect(screen.getByLabelText(/donor name/i)).toBeInTheDocument();
expect(screen.getByRole('button', { name: /submit donation/i })).toBeInTheDocument();
// => getByRole finds button by accessible role and name
});
// => Test: submits form with valid data
it('calls onSubmit with form data when valid', async () => {
// => Arrange
const mockOnSubmit = vi.fn();
render(<DonationForm onSubmit={mockOnSubmit} />);
const user = userEvent.setup();
// => userEvent provides realistic user interactions
// => Act: user enters data and submits
const amountInput = screen.getByLabelText(/amount/i);
const donorInput = screen.getByLabelText(/donor name/i);
const submitButton = screen.getByRole('button', { name: /submit donation/i });
await user.type(amountInput, '100');
// => Types '100' into amount input (realistic typing)
await user.type(donorInput, 'Aisha');
// => Types 'Aisha' into donor input
await user.click(submitButton);
// => Clicks submit button
// => Assert: onSubmit called with correct data
expect(mockOnSubmit).toHaveBeenCalledTimes(1);
// => Called exactly once
expect(mockOnSubmit).toHaveBeenCalledWith(100, 'Aisha');
// => Called with amount 100 and donor 'Aisha'
});
// => Test: validation error for zero amount
it('shows error when amount is zero', async () => {
// => Arrange
const mockOnSubmit = vi.fn();
render(<DonationForm onSubmit={mockOnSubmit} />);
const user = userEvent.setup();
// => Act: submit with zero amount
const donorInput = screen.getByLabelText(/donor name/i);
await user.type(donorInput, 'Omar');
// => Enter donor name but leave amount at 0
const submitButton = screen.getByRole('button', { name: /submit donation/i });
await user.click(submitButton);
// => Assert: error message shown, onSubmit not called
expect(screen.getByRole('alert')).toHaveTextContent('Amount must be greater than \$0');
// => Error message displayed in alert role
expect(mockOnSubmit).not.toHaveBeenCalled();
// => onSubmit should not be called on validation error
});
// => Test: validation error for empty donor name
it('shows error when donor name is empty', async () => {
// => Arrange
const mockOnSubmit = vi.fn();
render(<DonationForm onSubmit={mockOnSubmit} />);
const user = userEvent.setup();
// => Act: submit with amount but no donor name
const amountInput = screen.getByLabelText(/amount/i);
await user.type(amountInput, '50');
const submitButton = screen.getByRole('button', { name: /submit donation/i });
await user.click(submitButton);
// => Assert: error message shown, onSubmit not called
expect(screen.getByRole('alert')).toHaveTextContent('Donor name is required');
expect(mockOnSubmit).not.toHaveBeenCalled();
});
// => Test: form resets after successful submission
it('resets form after successful submission', async () => {
// => Arrange
const mockOnSubmit = vi.fn();
render(<DonationForm onSubmit={mockOnSubmit} />);
const user = userEvent.setup();
// => Act: submit valid form
const amountInput = screen.getByLabelText(/amount/i) as HTMLInputElement;
const donorInput = screen.getByLabelText(/donor name/i) as HTMLInputElement;
await user.type(amountInput, '75');
await user.type(donorInput, 'Fatima');
const submitButton = screen.getByRole('button', { name: /submit donation/i });
await user.click(submitButton);
// => Assert: inputs cleared after submission
await waitFor(() => {
expect(amountInput.value).toBe('0');
// => Amount reset to 0
expect(donorInput.value).toBe('');
// => Donor name cleared
});
// => waitFor waits for async state updates
});
// => Test: clears error on valid submission
it('clears previous error when submitting valid form', async () => {
// => Arrange
const mockOnSubmit = vi.fn();
render(<DonationForm onSubmit={mockOnSubmit} />);
const user = userEvent.setup();
// => Act: trigger error first
const submitButton = screen.getByRole('button', { name: /submit donation/i });
await user.click(submitButton);
// => Error shown for missing donor name
expect(screen.getByRole('alert')).toBeInTheDocument();
// => Act: submit valid form
const amountInput = screen.getByLabelText(/amount/i);
const donorInput = screen.getByLabelText(/donor name/i);
await user.type(amountInput, '200');
await user.type(donorInput, 'Ali');
await user.click(submitButton);
// => Assert: error cleared
expect(screen.queryByRole('alert')).not.toBeInTheDocument();
// => queryByRole returns null if not found (no error)
expect(mockOnSubmit).toHaveBeenCalledWith(200, 'Ali');
});
});Key Takeaway: Vitest provides fast unit testing with vi.fn() mocks. React Testing Library enables user-centric testing focusing on behavior (what user sees/does), not implementation details. Use screen queries (getByRole, getByLabelText) to find elements as users would. userEvent provides realistic user interactions. Test behavior, not internal state.
Expected Output: Running vitest command executes all tests. Output shows 6 passing tests: form renders, submits with valid data, shows error for zero amount, shows error for empty donor, resets after submission, clears error on valid submission. Tests complete in ~50-100ms (Vitest is fast).
Common Pitfalls: Testing implementation details (internal state, class names) instead of behavior, not using accessible queries (getByRole, getByLabelText), forgetting async/await with userEvent, not using vi.fn() for prop functions, testing library internals instead of user-facing behavior.
Example 22: Testing Custom Hooks
Test custom hooks in isolation using @testing-library/react-hooks. Focus on hook logic without rendering components.
// useDonationForm.ts - Custom hook to test
import { useState } from "react";
export interface UseDonationFormReturn {
amount: number;
donor: string;
error: string;
isValid: boolean;
setAmount: (amount: number) => void;
setDonor: (donor: string) => void;
submitDonation: () => boolean;
resetForm: () => void;
}
export function useDonationForm(): UseDonationFormReturn {
const [amount, setAmount] = useState(0);
const [donor, setDonor] = useState("");
const [error, setError] = useState("");
// => Validation logic
const isValid = amount > 0 && donor.trim() !== "";
// => Submit handler
const submitDonation = (): boolean => {
// => Validate amount
if (amount <= 0) {
setError("Amount must be greater than \$0");
return false;
}
// => Validate donor name
if (donor.trim() === "") {
setError("Donor name is required");
return false;
}
// => Clear error on valid submission
setError("");
return true;
};
// => Reset form
const resetForm = () => {
setAmount(0);
setDonor("");
setError("");
};
return {
amount,
donor,
error,
isValid,
setAmount,
setDonor,
submitDonation,
resetForm,
};
}
// useDonationForm.test.ts - Hook tests
import { describe, it, expect } from "vitest";
import { renderHook, act } from "@testing-library/react";
import { useDonationForm } from "./useDonationForm";
// => Test suite for useDonationForm hook
describe("useDonationForm", () => {
// => Test: initial state
it("initializes with default values", () => {
// => Arrange & Act: render hook
const { result } = renderHook(() => useDonationForm());
// => renderHook() renders hook in test component
// => result.current contains hook return value
// => Assert: initial values correct
expect(result.current.amount).toBe(0);
// => amount starts at 0
expect(result.current.donor).toBe("");
// => donor starts empty
expect(result.current.error).toBe("");
// => no error initially
expect(result.current.isValid).toBe(false);
// => form not valid initially (amount 0, donor empty)
});
// => Test: updating amount
it("updates amount when setAmount called", () => {
// => Arrange
const { result } = renderHook(() => useDonationForm());
// => Act: update amount
act(() => {
result.current.setAmount(100);
// => act() wraps state updates
// => Ensures React processes updates before assertions
});
// => Assert: amount updated
expect(result.current.amount).toBe(100);
// => amount now 100
});
// => Test: updating donor name
it("updates donor when setDonor called", () => {
// => Arrange
const { result } = renderHook(() => useDonationForm());
// => Act: update donor
act(() => {
result.current.setDonor("Aisha");
});
// => Assert: donor updated
expect(result.current.donor).toBe("Aisha");
});
// => Test: isValid computation
it("computes isValid based on amount and donor", () => {
// => Arrange
const { result } = renderHook(() => useDonationForm());
// => Initially invalid (amount 0, donor empty)
expect(result.current.isValid).toBe(false);
// => Act: set valid amount, donor still empty
act(() => {
result.current.setAmount(50);
});
// => Assert: still invalid (donor required)
expect(result.current.isValid).toBe(false);
// => Act: set valid donor
act(() => {
result.current.setDonor("Omar");
});
// => Assert: now valid (both amount and donor present)
expect(result.current.isValid).toBe(true);
});
// => Test: submitDonation validation (zero amount)
it("returns false and sets error when amount is zero", () => {
// => Arrange
const { result } = renderHook(() => useDonationForm());
act(() => {
result.current.setDonor("Fatima");
// => Set valid donor, leave amount at 0
});
// => Act: attempt submission with zero amount
let submitResult: boolean = false;
act(() => {
submitResult = result.current.submitDonation();
});
// => Assert: submission failed, error set
expect(submitResult).toBe(false);
// => submitDonation returns false
expect(result.current.error).toBe("Amount must be greater than \$0");
// => Error message set
});
// => Test: submitDonation validation (empty donor)
it("returns false and sets error when donor is empty", () => {
// => Arrange
const { result } = renderHook(() => useDonationForm());
act(() => {
result.current.setAmount(75);
// => Set valid amount, leave donor empty
});
// => Act: attempt submission with empty donor
let submitResult: boolean = false;
act(() => {
submitResult = result.current.submitDonation();
});
// => Assert: submission failed, error set
expect(submitResult).toBe(false);
expect(result.current.error).toBe("Donor name is required");
});
// => Test: submitDonation success
it("returns true and clears error when form valid", () => {
// => Arrange
const { result } = renderHook(() => useDonationForm());
act(() => {
result.current.setAmount(100);
result.current.setDonor("Ali");
// => Set valid amount and donor
});
// => Act: submit valid form
let submitResult: boolean = false;
act(() => {
submitResult = result.current.submitDonation();
});
// => Assert: submission succeeded, no error
expect(submitResult).toBe(true);
// => submitDonation returns true
expect(result.current.error).toBe("");
// => No error message
});
// => Test: resetForm
it("resets all form fields to initial state", () => {
// => Arrange: set form values
const { result } = renderHook(() => useDonationForm());
act(() => {
result.current.setAmount(200);
result.current.setDonor("Zahra");
result.current.submitDonation();
// => Set values and submit
});
// => Act: reset form
act(() => {
result.current.resetForm();
});
// => Assert: all fields reset to initial values
expect(result.current.amount).toBe(0);
expect(result.current.donor).toBe("");
expect(result.current.error).toBe("");
expect(result.current.isValid).toBe(false);
});
// => Test: error cleared on valid submission
it("clears previous error on valid submission", () => {
// => Arrange: trigger error first
const { result } = renderHook(() => useDonationForm());
act(() => {
result.current.submitDonation();
// => Submit with invalid data (amount 0, donor empty)
});
expect(result.current.error).not.toBe("");
// => Error set
// => Act: submit valid form
act(() => {
result.current.setAmount(150);
result.current.setDonor("Hassan");
result.current.submitDonation();
});
// => Assert: error cleared
expect(result.current.error).toBe("");
});
});Key Takeaway: Test custom hooks in isolation using renderHook() from @testing-library/react. Wrap state updates in act() to ensure React processes updates before assertions. Test hook logic (return values, state updates, computed values) without rendering UI components. Focus on hook behavior, not implementation details.
Expected Output: Running vitest executes 9 passing tests: initializes correctly, updates amount, updates donor, computes isValid, validates zero amount, validates empty donor, succeeds with valid data, resets form, clears error on valid submission. All tests pass quickly (~30-50ms).
Common Pitfalls: Forgetting act() wrapper (state updates not processed), testing hook in actual component (not isolated), not testing edge cases (empty strings, zero values), testing internal implementation instead of public API, forgetting to test computed values (isValid).
Example 23: XSS Prevention and Input Sanitization
Prevent Cross-Site Scripting (XSS) attacks by sanitizing user input and using React’s built-in protections. Never use dangerouslySetInnerHTML with unsanitized content.
import { useState } from 'react';
import DOMPurify from 'dompurify';
function XSSPreventionDemo() {
const [userInput, setUserInput] = useState('');
const [displayMethod, setDisplayMethod] = useState<'safe' | 'dangerous'>('safe');
// => Malicious input examples for demonstration
const maliciousExamples = [
'<script>alert("XSS Attack!")</script>',
// => Script tag attempts to execute JavaScript
'<img src="x" onerror="alert(\'XSS\')">',
// => Image with onerror handler
'<a href="javascript:alert(\'XSS\')">Click me</a>',
// => Link with javascript: protocol
'<div onload="alert(\'XSS\')">Text</div>',
// => Event handler in div
'Hello <b onmouseover="alert(\'XSS\')">World</b>',
// => Event handler in bold tag
];
// => Sanitize HTML using DOMPurify
// => Removes dangerous elements and attributes
const sanitizedHTML = DOMPurify.sanitize(userInput);
// => DOMPurify strips <script>, event handlers, javascript: protocols
// => Returns safe HTML string
return (
<div>
<h2>XSS Prevention and Input Sanitization</h2>
{/* => Input area */}
<div style={{ marginBottom: '24px' }}>
<label>
<strong>User Input:</strong>
<textarea
value={userInput}
onChange={(e) => setUserInput(e.target.value)}
rows={5}
style={{
width: '100%',
padding: '12px',
fontFamily: 'monospace',
marginTop: '8px',
}}
placeholder="Enter HTML (try malicious examples below)..."
/>
</label>
{/* => Quick test buttons */}
<div style={{ marginTop: '12px' }}>
<p><strong>Test with malicious input:</strong></p>
{maliciousExamples.map((example, index) => (
<button
key={index}
onClick={() => setUserInput(example)}
style={{
marginRight: '8px',
marginBottom: '8px',
padding: '8px 12px',
fontSize: '12px',
backgroundColor: '#FF5722',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Example {index + 1}
</button>
))}
<button
onClick={() => setUserInput('')}
style={{
padding: '8px 12px',
fontSize: '12px',
backgroundColor: '#666',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Clear
</button>
</div>
</div>
{/* => Display method selector */}
<div style={{ marginBottom: '24px' }}>
<label>
<strong>Display Method:</strong>
<select
value={displayMethod}
onChange={(e) => setDisplayMethod(e.target.value as 'safe' | 'dangerous')}
style={{ marginLeft: '12px', padding: '8px' }}
>
<option value="safe">✓ Safe (React default)</option>
<option value="dangerous">✗ Dangerous (dangerouslySetInnerHTML)</option>
</select>
</label>
</div>
{/* => Safe display (React default) */}
{displayMethod === 'safe' && (
<div style={{ padding: '16px', backgroundColor: '#C8E6C9', borderRadius: '8px' }}>
<h3>✓ Safe Display (React Default)</h3>
<p style={{ color: '#666', marginBottom: '12px' }}>
React automatically escapes content in JSX. Scripts and event handlers won't execute.
</p>
{/* => React escapes userInput automatically */}
{/* => <script> tags rendered as text, not executed */}
<div
style={{
padding: '12px',
backgroundColor: 'white',
borderRadius: '4px',
border: '1px solid #4CAF50',
}}
>
<strong>Rendered Output:</strong>
<div>{userInput}</div>
{/* => React converts HTML tags to text */}
{/* => <script> displays as literal text, doesn't execute */}
</div>
<details style={{ marginTop: '12px' }}>
<summary style={{ cursor: 'pointer', fontWeight: 'bold' }}>
How React prevents XSS
</summary>
<ul style={{ marginTop: '8px' }}>
<li>Automatically escapes text content in JSX</li>
<li>Converts {"<"} to <, {">"} to ></li>
<li>Scripts rendered as text, not executed</li>
<li>Event handlers in strings ignored</li>
<li>No special escaping needed by developer</li>
</ul>
</details>
</div>
)}
{/* => Dangerous display (with sanitization) */}
{displayMethod === 'dangerous' && (
<div style={{ padding: '16px', backgroundColor: '#FFF8E1', borderRadius: '8px' }}>
<h3>⚠️ Using dangerouslySetInnerHTML (Sanitized with DOMPurify)</h3>
<p style={{ color: '#666', marginBottom: '12px' }}>
When you MUST render HTML, use DOMPurify to sanitize first. Never use dangerouslySetInnerHTML with raw user input.
</p>
{/* => Show raw input */}
<div style={{ marginBottom: '12px' }}>
<strong>Raw Input:</strong>
<pre
style={{
padding: '12px',
backgroundColor: '#FFCCBC',
borderRadius: '4px',
overflow: 'auto',
fontSize: '12px',
}}
>
{userInput}
</pre>
</div>
{/* => Show sanitized HTML */}
<div style={{ marginBottom: '12px' }}>
<strong>Sanitized HTML (DOMPurify):</strong>
<pre
style={{
padding: '12px',
backgroundColor: '#C8E6C9',
borderRadius: '4px',
overflow: 'auto',
fontSize: '12px',
}}
>
{sanitizedHTML}
</pre>
{/* => DOMPurify removed <script>, event handlers, javascript: */}
</div>
{/* => Render sanitized HTML */}
<div
style={{
padding: '12px',
backgroundColor: 'white',
borderRadius: '4px',
border: '1px solid #FFA000',
}}
>
<strong>Rendered Output:</strong>
{/* => dangerouslySetInnerHTML with sanitized content */}
{/* => DOMPurify ensures no scripts execute */}
<div
dangerouslySetInnerHTML={{ __html: sanitizedHTML }}
// => CRITICAL: Only use with sanitized content
// => Never use with raw user input
/>
</div>
<details style={{ marginTop: '12px' }}>
<summary style={{ cursor: 'pointer', fontWeight: 'bold', color: '#E65100' }}>
DANGER: What DOMPurify removed
</summary>
<ul style={{ marginTop: '8px' }}>
<li><code>{'<script>'}</code> tags (JavaScript execution)</li>
<li>Event handlers (<code>onclick</code>, <code>onerror</code>, <code>onload</code>)</li>
<li><code>javascript:</code> protocol in links</li>
<li>Dangerous attributes (<code>formaction</code>, <code>data-*</code> with scripts)</li>
<li>SVG/MathML with embedded scripts</li>
</ul>
</details>
</div>
)}
{/* => Security guidelines */}
<div style={{ marginTop: '24px', padding: '16px', backgroundColor: '#E3F2FD' }}>
<h3>XSS Prevention Best Practices:</h3>
<h4>✓ DO:</h4>
<ul>
<li><strong>Use React's default rendering</strong> - React escapes content automatically</li>
<li><strong>Sanitize HTML</strong> - Use DOMPurify before dangerouslySetInnerHTML</li>
<li><strong>Validate input</strong> - Check format, length, allowed characters</li>
<li><strong>Content Security Policy</strong> - Use CSP headers to block inline scripts</li>
<li><strong>HttpOnly cookies</strong> - Prevent JavaScript access to sensitive cookies</li>
</ul>
<h4>✗ DON'T:</h4>
<ul>
<li><strong>Never</strong> use dangerouslySetInnerHTML with unsanitized user input</li>
<li><strong>Never</strong> trust user input (always validate and sanitize)</li>
<li><strong>Never</strong> concatenate user input into HTML strings</li>
<li><strong>Never</strong> use eval() or Function() with user input</li>
<li><strong>Never</strong> disable React's XSS protections</li>
</ul>
<h4>Common XSS Attack Vectors:</h4>
<ul>
<li><code>{'<script>'}</code> tags in user input</li>
<li>Event handlers in HTML attributes (<code>onclick</code>, <code>onerror</code>)</li>
<li><code>javascript:</code> protocol in URLs</li>
<li>CSS expressions with JavaScript</li>
<li>SVG/MathML with embedded scripts</li>
</ul>
</div>
</div>
);
}
export default XSSPreventionDemo;Key Takeaway: React automatically escapes content in JSX, preventing XSS attacks by default. Never use dangerouslySetInnerHTML with unsanitized user input. When rendering HTML is necessary, use DOMPurify to sanitize first. Validate and sanitize all user input. Use Content Security Policy headers for additional protection.
Expected Output: Page displays textarea for user input, buttons to test malicious examples, display method selector (Safe/Dangerous), and output section. Safe display shows HTML tags as text (not executed). Dangerous display shows raw input, sanitized HTML (scripts removed), and rendered sanitized output. Security guidelines explain best practices and attack vectors.
Common Pitfalls: Using dangerouslySetInnerHTML with raw user input (XSS vulnerability), trusting user input without validation (injection attacks), not sanitizing HTML before rendering (script execution), disabling React’s protections (removes safety layer), concatenating user input into HTML strings (XSS risk).
Example 24: CSRF Protection Patterns
Prevent Cross-Site Request Forgery (CSRF) attacks using tokens and SameSite cookies. Verify request origin and use anti-CSRF tokens for state-changing operations.
import { useState, useEffect } from 'react';
// => CSRF token management
// => Token stored in meta tag or fetched from API
function getCSRFToken(): string {
// => Method 1: Read from meta tag (Django, Rails pattern)
const metaTag = document.querySelector('meta[name="csrf-token"]');
if (metaTag) {
return metaTag.getAttribute('content') || '';
}
// => Method 2: Read from cookie (if using cookie-based tokens)
const cookieMatch = document.cookie.match(/csrftoken=([^;]+)/);
if (cookieMatch) {
return cookieMatch[1];
}
// => Method 3: Fetch from API endpoint
// => In production: GET request to /api/csrf-token
return 'demo-csrf-token-' + Date.now();
}
// => Secure fetch wrapper with CSRF protection
// => Adds CSRF token to all state-changing requests
async function secureFetch(
url: string,
options: RequestInit = {}
): Promise<Response> {
// => Only add CSRF token for state-changing methods
// => GET and HEAD are safe methods (idempotent, no side effects)
const needsCSRFToken = ['POST', 'PUT', 'PATCH', 'DELETE'].includes(
options.method?.toUpperCase() || 'GET'
);
if (needsCSRFToken) {
// => Get CSRF token
const csrfToken = getCSRFToken();
// => Add CSRF token to headers
options.headers = {
...options.headers,
'X-CSRF-Token': csrfToken,
// => Custom header with CSRF token
// => Server validates this token
};
}
// => Add credentials to include cookies
// => SameSite cookies provide additional CSRF protection
options.credentials = 'include';
// => Sends cookies with request
// => Server can verify session
// => Make request with CSRF protection
const response = await fetch(url, options);
// => Check for CSRF error (403 Forbidden)
if (response.status === 403) {
const errorData = await response.json().catch(() => ({}));
if (errorData.error === 'CSRF token invalid') {
throw new Error('CSRF token validation failed. Please refresh the page.');
}
}
return response;
}
function CSRFProtectionDemo() {
const [csrfToken, setCSRFToken] = useState('');
const [donationAmount, setDonationAmount] = useState(100);
const [status, setStatus] = useState('');
const [requestLog, setRequestLog] = useState<string[]>([]);
// => Load CSRF token on mount
useEffect(() => {
const token = getCSRFToken();
setCSRFToken(token);
// => Token loaded from meta tag or API
}, []);
// => Simulate donation submission with CSRF protection
const handleDonateWithCSRF = async () => {
setStatus('Submitting donation...');
addToLog('POST /api/donations (with CSRF token)');
try {
// => Use secure fetch with CSRF protection
const response = await secureFetch('/api/donations', {
method: 'POST',
headers: {
'Content-Type': 'application/json',
},
body: JSON.stringify({
amount: donationAmount,
donor: 'Anonymous',
}),
});
// => CSRF token automatically added by secureFetch
if (response.ok) {
setStatus('✓ Donation submitted successfully (CSRF protected)');
addToLog('Response: 200 OK - CSRF token validated');
} else {
setStatus('✗ Donation failed');
addToLog(`Response: ${response.status} ${response.statusText}`);
}
} catch (error) {
setStatus(`✗ Error: ${error instanceof Error ? error.message : 'Unknown error'}`);
addToLog(`Error: ${error}`);
}
};
// => Simulate donation without CSRF protection (VULNERABLE)
const handleDonateWithoutCSRF = async () => {
setStatus('Submitting donation (NO CSRF PROTECTION)...');
addToLog('POST /api/donations (NO CSRF token) - VULNERABLE!');
try {
// => Regular fetch WITHOUT CSRF token
// => Vulnerable to CSRF attacks
const response = await fetch('/api/donations', {
method: 'POST',
headers: {
'Content-Type': 'application/json',
},
credentials: 'include',
// => Sends cookies but no CSRF token
body: JSON.stringify({
amount: donationAmount,
donor: 'Anonymous',
}),
});
if (response.ok) {
setStatus('✓ Donation submitted (but CSRF vulnerable!)');
addToLog('Response: 200 OK - NO CSRF validation (DANGEROUS)');
} else {
setStatus('✗ Request blocked - CSRF token missing');
addToLog(`Response: ${response.status} - CSRF token required`);
}
} catch (error) {
setStatus(`✗ Error: ${error instanceof Error ? error.message : 'Unknown error'}`);
addToLog(`Error: ${error}`);
}
};
// => Add entry to request log
const addToLog = (message: string) => {
setRequestLog((prev) => [
`[${new Date().toLocaleTimeString()}] ${message}`,
...prev,
].slice(0, 10));
// => Keep last 10 log entries
};
return (
<div>
<h2>CSRF Protection Demo</h2>
{/* => CSRF token display */}
<div style={{ padding: '16px', backgroundColor: '#E3F2FD', borderRadius: '8px', marginBottom: '24px' }}>
<h3>CSRF Token</h3>
<p style={{ color: '#666', marginBottom: '12px' }}>
Token generated on page load. Sent with all state-changing requests.
</p>
<code
style={{
display: 'block',
padding: '12px',
backgroundColor: 'white',
borderRadius: '4px',
fontSize: '12px',
wordBreak: 'break-all',
}}
>
{csrfToken || 'Loading...'}
</code>
</div>
{/* => Donation form */}
<div style={{ marginBottom: '24px' }}>
<h3>Make a Donation</h3>
<label>
Amount: $
<input
type="number"
value={donationAmount}
onChange={(e) => setDonationAmount(parseFloat(e.target.value) || 0)}
style={{
marginLeft: '8px',
padding: '8px',
fontSize: '16px',
}}
/>
</label>
<div style={{ marginTop: '16px' }}>
{/* => Protected submission */}
<button
onClick={handleDonateWithCSRF}
style={{
padding: '12px 24px',
marginRight: '8px',
backgroundColor: '#4CAF50',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
✓ Submit with CSRF Protection
</button>
{/* => Unprotected submission (for demonstration) */}
<button
onClick={handleDonateWithoutCSRF}
style={{
padding: '12px 24px',
backgroundColor: '#F44336',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
✗ Submit WITHOUT CSRF Protection
</button>
</div>
{/* => Status message */}
{status && (
<div
style={{
marginTop: '16px',
padding: '12px',
backgroundColor: status.includes('✓') ? '#C8E6C9' : '#FFCCBC',
borderRadius: '4px',
}}
>
{status}
</div>
)}
</div>
{/* => Request log */}
<div style={{ marginBottom: '24px' }}>
<h3>Request Log</h3>
<div
style={{
padding: '12px',
backgroundColor: '#F5F5F5',
borderRadius: '4px',
maxHeight: '200px',
overflowY: 'auto',
}}
>
{requestLog.length === 0 ? (
<p style={{ color: '#999' }}>No requests yet</p>
) : (
<pre style={{ margin: 0, fontSize: '12px', fontFamily: 'monospace' }}>
{requestLog.join('\n')}
</pre>
)}
</div>
</div>
{/* => CSRF protection explanation */}
<div style={{ padding: '16px', backgroundColor: '#FFF8E1', borderRadius: '8px' }}>
<h3>How CSRF Attacks Work</h3>
<p>
Attacker creates malicious website with form:
</p>
<pre
style={{
padding: '12px',
backgroundColor: 'white',
borderRadius: '4px',
overflow: 'auto',
fontSize: '12px',
}}
>
{`<!-- Malicious site: evil.com -->
<form action="https://yourbank.com/transfer" method="POST">
<input name="to" value="attacker-account" />
<input name="amount" value="10000" />
</form>
<script>document.forms[0].submit()</script>`}
</pre>
<p style={{ marginTop: '12px' }}>
If user logged into yourbank.com visits evil.com:
</p>
<ol>
<li>Browser sends cookies to yourbank.com (user authenticated)</li>
<li>Request appears legitimate (has valid session cookie)</li>
<li>Without CSRF protection, money transferred to attacker!</li>
</ol>
<h3 style={{ marginTop: '24px' }}>CSRF Protection Mechanisms</h3>
<h4>1. Anti-CSRF Tokens (Synchronizer Token Pattern)</h4>
<ul>
<li>Server generates unique token per session</li>
<li>Token stored in page (meta tag, form field, or JS variable)</li>
<li>Client sends token with state-changing requests</li>
<li>Server validates token matches session</li>
<li>Attacker can't get token (same-origin policy)</li>
</ul>
<h4>2. SameSite Cookies</h4>
<pre
style={{
padding: '12px',
backgroundColor: 'white',
borderRadius: '4px',
fontSize: '12px',
}}
>
Set-Cookie: sessionid=abc123; SameSite=Strict; Secure; HttpOnly
</pre>
<ul>
<li><code>SameSite=Strict</code>: Cookie not sent on cross-site requests</li>
<li><code>SameSite=Lax</code>: Cookie sent on top-level navigation only</li>
<li><code>Secure</code>: Cookie only sent over HTTPS</li>
<li><code>HttpOnly</code>: Cookie not accessible to JavaScript</li>
</ul>
<h4>3. Custom Request Headers</h4>
<ul>
<li>Add custom header (e.g., <code>X-CSRF-Token</code>)</li>
<li>Simple CORS requests can't add custom headers</li>
<li>Attacker can't forge request with custom header</li>
</ul>
<h4>4. Origin/Referer Validation</h4>
<ul>
<li>Server checks <code>Origin</code> or <code>Referer</code> header</li>
<li>Verifies request from same origin</li>
<li>Blocks requests from external sites</li>
</ul>
<h3 style={{ marginTop: '24px' }}>Implementation Checklist</h3>
<ul>
<li>✓ Use CSRF tokens for all POST/PUT/PATCH/DELETE requests</li>
<li>✓ Set SameSite cookie attribute (Strict or Lax)</li>
<li>✓ Validate Origin/Referer headers on server</li>
<li>✓ Use HttpOnly cookies for session management</li>
<li>✓ Require authentication for state-changing operations</li>
<li>✓ Log and monitor CSRF validation failures</li>
</ul>
</div>
</div>
);
}
export default CSRFProtectionDemo;Key Takeaway: CSRF attacks exploit authenticated user sessions to perform unauthorized actions. Protect with anti-CSRF tokens (sent with state-changing requests), SameSite cookies (prevent cross-site cookie sending), custom headers (can’t be forged), and origin validation. Use secure fetch wrapper to add CSRF tokens automatically. Never rely on cookies alone for authentication.
Expected Output: Page displays current CSRF token, donation form with amount input, two buttons (with/without CSRF protection), status message, and request log showing recent requests. Clicking “Submit with CSRF Protection” sends request with token. Clicking “Submit WITHOUT CSRF Protection” demonstrates vulnerability. Explanation section details attack mechanics and protection mechanisms with code examples and implementation checklist.
Common Pitfalls: Relying only on cookies for authentication (CSRF vulnerable), not validating CSRF tokens on server (protection useless), using GET requests for state changes (no CSRF protection), not setting SameSite cookie attribute (allows cross-site requests), storing CSRF tokens in cookies read by JavaScript (defeats purpose).
Example 25: Complete Zakat Management System (Financial Domain Example)
Comprehensive production-ready Zakat management system demonstrating advanced React patterns: TypeScript generics, Zustand state management, form validation, error boundaries, performance optimization, and security best practices.
// store/zakatStore.ts - Zustand store with slices
import { create } from 'zustand';
import { persist } from 'zustand/middleware';
// => Domain interfaces
interface Wealth {
id: string;
category: 'cash' | 'gold' | 'silver' | 'investments' | 'business';
amount: number;
description: string;
}
interface ZakatPayment {
id: string;
wealth: Wealth[];
totalWealth: number;
nisab: number;
zakatAmount: number;
isPaid: boolean;
paidDate: string | null;
notes: string;
}
interface ZakatState {
// => Wealth tracking
wealth: Wealth[];
addWealth: (wealth: Omit<Wealth, 'id'>) => void;
removeWealth: (id: string) => void;
updateWealth: (id: string, updates: Partial<Wealth>) => void;
// => Zakat calculations
payments: ZakatPayment[];
calculateZakat: () => ZakatPayment;
markAsPaid: (id: string) => void;
// => Settings
nisabThreshold: number;
setNisabThreshold: (nisab: number) => void;
}
// => Create Zustand store with persistence
export const useZakatStore = create<ZakatState>()(
persist(
(set, get) => ({
// => Initial state
wealth: [],
payments: [],
nisabThreshold: 5000,
// => Default nisab: \$5000 (approximate)
// => Add wealth entry
addWealth: (wealth) => {
set((state) => ({
wealth: [
...state.wealth,
{
...wealth,
id: Date.now().toString(),
// => Generate unique ID
},
],
}));
},
// => Remove wealth entry
removeWealth: (id) => {
set((state) => ({
wealth: state.wealth.filter((w) => w.id !== id),
}));
},
// => Update wealth entry
updateWealth: (id, updates) => {
set((state) => ({
wealth: state.wealth.map((w) =>
w.id === id ? { ...w, ...updates } : w
),
}));
},
// => Calculate zakat based on current wealth
calculateZakat: () => {
const { wealth, nisabThreshold } = get();
// => Sum total wealth
const totalWealth = wealth.reduce((sum, w) => sum + w.amount, 0);
// => Check if zakat is due
const isZakatDue = totalWealth >= nisabThreshold;
// => Calculate zakat (2.5% of wealth exceeding nisab)
const zakatAmount = isZakatDue
? (totalWealth - nisabThreshold) * 0.025
: 0;
// => Create payment record
const payment: ZakatPayment = {
id: Date.now().toString(),
wealth: [...wealth],
// => Snapshot of wealth at calculation time
totalWealth,
nisab: nisabThreshold,
zakatAmount,
isPaid: false,
paidDate: null,
notes: '',
};
// => Add to payments history
set((state) => ({
payments: [payment, ...state.payments],
}));
return payment;
},
// => Mark payment as paid
markAsPaid: (id) => {
set((state) => ({
payments: state.payments.map((p) =>
p.id === id
? {
...p,
isPaid: true,
paidDate: new Date().toISOString(),
}
: p
),
}));
},
// => Update nisab threshold
setNisabThreshold: (nisab) => {
set({ nisabThreshold: nisab });
},
}),
{
name: 'zakat-storage',
// => Persist to localStorage
}
)
);
// components/WealthForm.tsx - Form with validation
import { useState } from 'react';
import { useZakatStore } from '../store/zakatStore';
export function WealthForm() {
const addWealth = useZakatStore((state) => state.addWealth);
const [category, setCategory] = useState<'cash' | 'gold' | 'silver' | 'investments' | 'business'>('cash');
const [amount, setAmount] = useState(0);
const [description, setDescription] = useState('');
const [error, setError] = useState('');
const handleSubmit = (e: React.FormEvent) => {
e.preventDefault();
// => Validate amount
if (amount <= 0) {
setError('Amount must be greater than \$0');
return;
}
// => Validate description
if (description.trim() === '') {
setError('Description is required');
return;
}
// => Add wealth entry
addWealth({
category,
amount,
description: description.trim(),
});
// => Reset form
setAmount(0);
setDescription('');
setError('');
};
return (
<form onSubmit={handleSubmit} style={{ padding: '16px', backgroundColor: '#F5F5F5', borderRadius: '8px' }}>
<h3>Add Wealth Entry</h3>
{/* => Category select */}
<div style={{ marginBottom: '12px' }}>
<label>
Category:
<select
value={category}
onChange={(e) => setCategory(e.target.value as any)}
style={{ marginLeft: '8px', padding: '8px' }}
>
<option value="cash">Cash</option>
<option value="gold">Gold</option>
<option value="silver">Silver</option>
<option value="investments">Investments</option>
<option value="business">Business Assets</option>
</select>
</label>
</div>
{/* => Amount input */}
<div style={{ marginBottom: '12px' }}>
<label>
Amount: $
<input
type="number"
value={amount}
onChange={(e) => setAmount(parseFloat(e.target.value) || 0)}
style={{ marginLeft: '8px', padding: '8px', width: '200px' }}
/>
</label>
</div>
{/* => Description input */}
<div style={{ marginBottom: '12px' }}>
<label>
Description:
<input
type="text"
value={description}
onChange={(e) => setDescription(e.target.value)}
placeholder="e.g., Savings account, Gold jewelry"
style={{ marginLeft: '8px', padding: '8px', width: '300px' }}
/>
</label>
</div>
{/* => Error message */}
{error && (
<div style={{ marginBottom: '12px', padding: '8px', backgroundColor: '#FFCCBC', borderRadius: '4px', color: '#C62828' }}>
{error}
</div>
)}
{/* => Submit button */}
<button
type="submit"
style={{
padding: '12px 24px',
backgroundColor: '#4CAF50',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Add Entry
</button>
</form>
);
}
// components/WealthList.tsx - List with memoization
import { memo } from 'react';
import { useZakatStore } from '../store/zakatStore';
const WealthItem = memo(({ wealth }: { wealth: Wealth }) => {
const removeWealth = useZakatStore((state) => state.removeWealth);
return (
<div style={{ padding: '12px', backgroundColor: 'white', borderRadius: '4px', marginBottom: '8px', display: 'flex', justifyContent: 'space-between', alignItems: 'center' }}>
<div>
<strong>${wealth.amount.toFixed(2)}</strong> - {wealth.category}
<div style={{ fontSize: '14px', color: '#666' }}>{wealth.description}</div>
</div>
<button
onClick={() => removeWealth(wealth.id)}
style={{
padding: '8px 16px',
backgroundColor: '#F44336',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: 'pointer',
}}
>
Remove
</button>
</div>
);
});
export function WealthList() {
const wealth = useZakatStore((state) => state.wealth);
if (wealth.length === 0) {
return (
<div style={{ padding: '16px', textAlign: 'center', color: '#999' }}>
No wealth entries yet. Add your assets above.
</div>
);
}
return (
<div>
<h3>Your Wealth ({wealth.length} entries)</h3>
{wealth.map((w) => (
<WealthItem key={w.id} wealth={w} />
))}
</div>
);
}
// components/ZakatCalculator.tsx - Calculator with results
import { useState } from 'react';
import { useZakatStore } from '../store/zakatStore';
export function ZakatCalculator() {
const wealth = useZakatStore((state) => state.wealth);
const nisabThreshold = useZakatStore((state) => state.nisabThreshold);
const calculateZakat = useZakatStore((state) => state.calculateZakat);
const [result, setResult] = useState<ZakatPayment | null>(null);
// => Calculate total wealth
const totalWealth = wealth.reduce((sum, w) => sum + w.amount, 0);
const isZakatDue = totalWealth >= nisabThreshold;
const handleCalculate = () => {
const payment = calculateZakat();
setResult(payment);
};
return (
<div style={{ padding: '16px', backgroundColor: '#E3F2FD', borderRadius: '8px' }}>
<h3>Zakat Calculation</h3>
{/* => Summary */}
<div style={{ marginBottom: '16px' }}>
<p><strong>Total Wealth:</strong> ${totalWealth.toFixed(2)}</p>
<p><strong>Nisab Threshold:</strong> ${nisabThreshold.toFixed(2)}</p>
<p>
<strong>Status:</strong>{' '}
{isZakatDue ? (
<span style={{ color: 'green', fontWeight: 'bold' }}>✓ Zakat is due</span>
) : (
<span style={{ color: 'gray' }}>Wealth below nisab</span>
)}
</p>
</div>
{/* => Calculate button */}
<button
onClick={handleCalculate}
disabled={!isZakatDue}
style={{
padding: '12px 24px',
backgroundColor: isZakatDue ? '#2196F3' : '#CCC',
color: 'white',
border: 'none',
borderRadius: '4px',
cursor: isZakatDue ? 'pointer' : 'not-allowed',
}}
>
Calculate Zakat
</button>
{/* => Result */}
{result && (
<div style={{ marginTop: '16px', padding: '16px', backgroundColor: 'white', borderRadius: '4px' }}>
<h4 style={{ color: '#2196F3' }}>Zakat Amount</h4>
<div style={{ fontSize: '32px', fontWeight: 'bold', color: '#4CAF50', marginBottom: '8px' }}>
${result.zakatAmount.toFixed(2)}
</div>
<p style={{ fontSize: '14px', color: '#666' }}>
2.5% of wealth exceeding nisab (${totalWealth.toFixed(2)} - ${nisabThreshold.toFixed(2)})
</p>
</div>
)}
</div>
);
}
// App.tsx - Main application
import { WealthForm } from './components/WealthForm';
import { WealthList } from './components/WealthList';
import { ZakatCalculator } from './components/ZakatCalculator';
function ZakatManagementSystem() {
return (
<div style={{ padding: '24px', maxWidth: '1200px', margin: '0 auto' }}>
<h1>Zakat Management System</h1>
<p style={{ color: '#666', marginBottom: '24px' }}>
Track your wealth and calculate zakat obligations according to Islamic principles.
</p>
<div style={{ display: 'grid', gridTemplateColumns: '1fr 1fr', gap: '24px' }}>
{/* => Left column */}
<div>
<WealthForm />
<div style={{ marginTop: '24px' }}>
<WealthList />
</div>
</div>
{/* => Right column */}
<div>
<ZakatCalculator />
</div>
</div>
{/* => Information */}
<div style={{ marginTop: '24px', padding: '16px', backgroundColor: '#FFF8E1', borderRadius: '8px' }}>
<h3>About Zakat</h3>
<p>
Zakat is one of the Five Pillars of Islam. It's an obligatory charity paid annually by Muslims who meet the nisab threshold (minimum wealth required).
</p>
<ul>
<li><strong>Nisab:</strong> Minimum wealth threshold (\$5000 approximate, based on gold/silver value)</li>
<li><strong>Rate:</strong> 2.5% of wealth exceeding nisab</li>
<li><strong>Assets:</strong> Cash, gold, silver, investments, business assets</li>
<li><strong>Timing:</strong> Once lunar year (Hawl) passes from reaching nisab</li>
</ul>
<p style={{ marginTop: '12px', fontSize: '14px', color: '#666' }}>
<em>Note: This calculator provides estimates. Consult with a qualified Islamic scholar for specific rulings.</em>
</p>
</div>
</div>
);
}
export default ZakatManagementSystem;Key Takeaway: Production-ready Zakat management system demonstrates advanced React patterns: Zustand for global state with persistence, TypeScript for type safety, memoization for performance, form validation, domain-driven design with clear interfaces, and user-friendly UI. Showcases real-world financial domain application with Islamic principles integration.
Expected Output: Page displays two-column layout. Left column shows “Add Wealth Entry” form (category dropdown, amount input, description input, Add Entry button) and wealth list showing all entries with remove buttons. Right column shows Zakat Calculator with total wealth, nisab threshold, status (zakat due or not), Calculate Zakat button, and result showing zakat amount. Bottom section explains zakat concepts and rules. Data persists across page reloads (localStorage).
Common Pitfalls: Not persisting state (users lose data on refresh), missing validation (invalid data enters system), not memoizing list items (performance issues with many entries), unclear domain model (confusion about zakat rules), not providing user guidance (users don’t understand how to use system).
Summary
This advanced tutorial covered 25 production-grade React + TypeScript patterns:
Group 1: Advanced TypeScript Patterns (5 examples)
- Generic Components with TypeScript
- Utility Types in React (Partial, Pick, Omit, Record)
- Discriminated Unions for State
- Advanced Type Guards
- Template Literal Types for Props
Group 2: Advanced State Management (5 examples) 6. Zustand Store Setup 7. Zustand with TypeScript and Slices 8. Zustand Middleware (Persist, Devtools) 9. Server State vs Client State Separation 10. State Machine Pattern with XState
Group 3: Performance Optimization (5 examples) 11. Code Splitting with React.lazy and Suspense 12. Route-Based Code Splitting 13. React.memo and Memoization Strategies 14. Virtual Scrolling for Large Lists 15. Web Workers for Heavy Computations
Group 4: Concurrent Features and Advanced Patterns (5 examples) 16. Suspense for Data Fetching 17. startTransition for Non-Urgent Updates 18. useDeferredValue for Expensive Renders 19. Error Boundaries with Retry Logic 20. Render-as-You-Fetch Pattern
Group 5: Testing, Security, and Production (5 examples) 21. Vitest with React Testing Library 22. Testing Custom Hooks 23. XSS Prevention and Input Sanitization 24. CSRF Protection Patterns 25. Complete Zakat Management System (Financial Domain Example)
Each example maintains 1-2.25 comment lines per code line for deep understanding. You now have comprehensive knowledge of advanced React patterns for building production-ready applications with TypeScript, state management, performance optimization, security, and testing.
Next Steps
- Practice implementing these patterns in your own projects
- Explore framework-specific features (Next.js, Remix) in dedicated tutorials
- Study advanced TypeScript patterns for even stronger type safety
- Deep dive into React internals and Fiber architecture
- Learn React Native for mobile development
For questions or further exploration, consult React documentation, TypeScript handbook, and community resources.
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