• Refactoring legacy JavaScript to modern syntax
• Implementing functional programming patterns
• Optimizing JavaScript performance
• Writing maintainable and readable code
• Working with asynchronous operations
• Building modern web applications
• Migrating from callbacks to Promises/async-await
• Implementing data transformation pipelines

1. Arrow Functions

Syntax and Use Cases:

```javascript

// Traditional function

function add(a, b) {

return a + b;

}

// Arrow function

const add = (a, b) => a + b;

// Single parameter (parentheses optional)

const double = x => x * 2;

// No parameters

const getRandom = () => Math.random();

// Multiple statements (need curly braces)

const processUser = user => {

const normalized = user.name.toLowerCase();

return { ...user, name: normalized };

};

// Returning objects (wrap in parentheses)

const createUser = (name, age) => ({ name, age });

```

Lexical 'this' Binding:

```javascript

class Counter {

constructor() {

this.count = 0;

}

// Arrow function preserves 'this' context

increment = () => {

this.count++;

};

// Traditional function loses 'this' in callbacks

incrementTraditional() {

setTimeout(function() {

this.count++; // 'this' is undefined

}, 1000);

}

// Arrow function maintains 'this'

incrementArrow() {

setTimeout(() => {

this.count++; // 'this' refers to Counter instance

}, 1000);

}

}

```

2. Destructuring

Object Destructuring:

```javascript

const user = {

id: 1,

name: 'John Doe',

email: 'john@example.com',

address: {

city: 'New York',

country: 'USA'

}

};

// Basic destructuring

const { name, email } = user;

// Rename variables

const { name: userName, email: userEmail } = user;

// Default values

const { age = 25 } = user;

// Nested destructuring

const { address: { city, country } } = user;

// Rest operator

const { id, ...userWithoutId } = user;

// Function parameters

function greet({ name, age = 18 }) {

console.log(Hello ${name}, you are ${age});

}

greet(user);

```

Array Destructuring:

```javascript

const numbers = [1, 2, 3, 4, 5];

// Basic destructuring

const [first, second] = numbers;

// Skip elements

const [, , third] = numbers;

// Rest operator

const [head, ...tail] = numbers;

// Swapping variables

let a = 1, b = 2;

[a, b] = [b, a];

// Function return values

function getCoordinates() {

return [10, 20];

}

const [x, y] = getCoordinates();

// Default values

const [one, two, three = 0] = [1, 2];

```

3. Spread and Rest Operators

Spread Operator:

```javascript

// Array spreading

const arr1 = [1, 2, 3];

const arr2 = [4, 5, 6];

const combined = [...arr1, ...arr2];

// Object spreading

const defaults = { theme: 'dark', lang: 'en' };

const userPrefs = { theme: 'light' };

const settings = { ...defaults, ...userPrefs };

// Function arguments

const numbers = [1, 2, 3];

Math.max(...numbers);

// Copying arrays/objects (shallow copy)

const copy = [...arr1];

const objCopy = { ...user };

// Adding items immutably

const newArr = [...arr1, 4, 5];

const newObj = { ...user, age: 30 };

```

Rest Parameters:

```javascript

// Collect function arguments

function sum(...numbers) {

return numbers.reduce((total, num) => total + num, 0);

}

sum(1, 2, 3, 4, 5);

// With regular parameters

function greet(greeting, ...names) {

return ${greeting} ${names.join(', ')};

}

greet('Hello', 'John', 'Jane', 'Bob');

// Object rest

const { id, ...userData } = user;

// Array rest

const [first, ...rest] = [1, 2, 3, 4, 5];

```

4. Template Literals

```javascript

// Basic usage

const name = 'John';

const greeting = Hello, ${name}!;

// Multi-line strings

const html = `

`;

// Expression evaluation

const price = 19.99;

const total = Total: $${(price * 1.2).toFixed(2)};

// Tagged template literals

function highlight(strings, ...values) {

return strings.reduce((result, str, i) => {

const value = values[i] || '';

return result + str + ${value};

}, '');

}

const name = 'John';

const age = 30;

const html = highlightName: ${name}, Age: ${age};

// Output: "Name: John, Age: 30"

```

5. Enhanced Object Literals

```javascript

const name = 'John';

const age = 30;

// Shorthand property names

const user = { name, age };

// Shorthand method names

const calculator = {

add(a, b) {

return a + b;

},

subtract(a, b) {

return a - b;

}

};

// Computed property names

const field = 'email';

const user = {

name: 'John',

[field]: 'john@example.com',

[get${field.charAt(0).toUpperCase()}${field.slice(1)}]() {

return this[field];

}

};

// Dynamic property creation

const createUser = (name, ...props) => {

return props.reduce((user, [key, value]) => ({

...user,

[key]: value

}), { name });

};

const user = createUser('John', ['age', 30], ['email', 'john@example.com']);

```

1. Promises

Creating and Using Promises:

```javascript

// Creating a promise

const fetchUser = (id) => {

return new Promise((resolve, reject) => {

setTimeout(() => {

if (id > 0) {

resolve({ id, name: 'John' });

} else {

reject(new Error('Invalid ID'));

}

}, 1000);

});

};

// Using promises

fetchUser(1)

.then(user => console.log(user))

.catch(error => console.error(error))

.finally(() => console.log('Done'));

// Chaining promises

fetchUser(1)

.then(user => fetchUserPosts(user.id))

.then(posts => processPosts(posts))

.then(result => console.log(result))

.catch(error => console.error(error));

```

Promise Combinators:

```javascript

// Promise.all - Wait for all promises

const promises = [

fetchUser(1),

fetchUser(2),

fetchUser(3)

];

Promise.all(promises)

.then(users => console.log(users))

.catch(error => console.error('At least one failed:', error));

// Promise.allSettled - Wait for all, regardless of outcome

Promise.allSettled(promises)

.then(results => {

results.forEach(result => {

if (result.status === 'fulfilled') {

console.log('Success:', result.value);

} else {

console.log('Error:', result.reason);

}

});

});

// Promise.race - First to complete

Promise.race(promises)

.then(winner => console.log('First:', winner))

.catch(error => console.error(error));

// Promise.any - First to succeed

Promise.any(promises)

.then(first => console.log('First success:', first))

.catch(error => console.error('All failed:', error));

```

2. Async/Await

Basic Usage:

```javascript

// Async function always returns a Promise

async function fetchUser(id) {

const response = await fetch(/api/users/${id});

const user = await response.json();

return user;

}

// Error handling with try/catch

async function getUserData(id) {

try {

const user = await fetchUser(id);

const posts = await fetchUserPosts(user.id);

return { user, posts };

} catch (error) {

console.error('Error fetching data:', error);

throw error;

}

}

// Sequential vs Parallel execution

async function sequential() {

const user1 = await fetchUser(1); // Wait

const user2 = await fetchUser(2); // Then wait

return [user1, user2];

}

async function parallel() {

const [user1, user2] = await Promise.all([

fetchUser(1),

fetchUser(2)

]);

return [user1, user2];

}

```

Advanced Patterns:

```javascript

// Async IIFE

(async () => {

const result = await someAsyncOperation();

console.log(result);

})();

// Async iteration

async function processUsers(userIds) {

for (const id of userIds) {

const user = await fetchUser(id);

await processUser(user);

}

}

// Top-level await (ES2022)

const config = await fetch('/config.json').then(r => r.json());

// Retry logic

async function fetchWithRetry(url, retries = 3) {

for (let i = 0; i < retries; i++) {

try {

return await fetch(url);

} catch (error) {

if (i === retries - 1) throw error;

await new Promise(resolve => setTimeout(resolve, 1000 * (i + 1)));

}

}

}

// Timeout wrapper

async function withTimeout(promise, ms) {

const timeout = new Promise((_, reject) =>

setTimeout(() => reject(new Error('Timeout')), ms)

);

return Promise.race([promise, timeout]);

}

```

1. Array Methods

Map, Filter, Reduce:

```javascript

const users = [

{ id: 1, name: 'John', age: 30, active: true },

{ id: 2, name: 'Jane', age: 25, active: false },

{ id: 3, name: 'Bob', age: 35, active: true }

];

// Map - Transform array

const names = users.map(user => user.name);

const upperNames = users.map(user => user.name.toUpperCase());

// Filter - Select elements

const activeUsers = users.filter(user => user.active);

const adults = users.filter(user => user.age >= 18);

// Reduce - Aggregate data

const totalAge = users.reduce((sum, user) => sum + user.age, 0);

const avgAge = totalAge / users.length;

// Group by property

const byActive = users.reduce((groups, user) => {

const key = user.active ? 'active' : 'inactive';

return {

...groups,

[key]: [...(groups[key] || []), user]

};

}, {});

// Chaining methods

const result = users

.filter(user => user.active)

.map(user => user.name)

.sort()

.join(', ');

```

Advanced Array Methods:

```javascript

// Find - First matching element

const user = users.find(u => u.id === 2);

// FindIndex - Index of first match

const index = users.findIndex(u => u.name === 'Jane');

// Some - At least one matches

const hasActive = users.some(u => u.active);

// Every - All match

const allAdults = users.every(u => u.age >= 18);

// FlatMap - Map and flatten

const userTags = [

{ name: 'John', tags: ['admin', 'user'] },

{ name: 'Jane', tags: ['user'] }

];

const allTags = userTags.flatMap(u => u.tags);

// From - Create array from iterable

const str = 'hello';

const chars = Array.from(str);

const numbers = Array.from({ length: 5 }, (_, i) => i + 1);

// Of - Create array from arguments

const arr = Array.of(1, 2, 3);

```

2. Higher-Order Functions

Functions as Arguments:

```javascript

// Custom forEach

function forEach(array, callback) {

for (let i = 0; i < array.length; i++) {

callback(array[i], i, array);

}

}

// Custom map

function map(array, transform) {

const result = [];

for (const item of array) {

result.push(transform(item));

}

return result;

}

// Custom filter

function filter(array, predicate) {

const result = [];

for (const item of array) {

if (predicate(item)) {

result.push(item);

}

}

return result;

}

```

Functions Returning Functions:

```javascript

// Currying

const multiply = a => b => a * b;

const double = multiply(2);

const triple = multiply(3);

console.log(double(5)); // 10

console.log(triple(5)); // 15

// Partial application

function partial(fn, ...args) {

return (...moreArgs) => fn(...args, ...moreArgs);

}

const add = (a, b, c) => a + b + c;

const add5 = partial(add, 5);

console.log(add5(3, 2)); // 10

// Memoization

function memoize(fn) {

const cache = new Map();

return (...args) => {

const key = JSON.stringify(args);

if (cache.has(key)) {

return cache.get(key);

}

const result = fn(...args);

cache.set(key, result);

return result;

};

}

const fibonacci = memoize((n) => {

if (n <= 1) return n;

return fibonacci(n - 1) + fibonacci(n - 2);

});

```

3. Composition and Piping

```javascript

// Function composition

const compose = (...fns) => x =>

fns.reduceRight((acc, fn) => fn(acc), x);

const pipe = (...fns) => x =>

fns.reduce((acc, fn) => fn(acc), x);

// Example usage

const addOne = x => x + 1;

const double = x => x * 2;

const square = x => x * x;

const composed = compose(square, double, addOne);

console.log(composed(3)); // ((3 + 1) * 2)^2 = 64

const piped = pipe(addOne, double, square);

console.log(piped(3)); // ((3 + 1) * 2)^2 = 64

// Practical example

const processUser = pipe(

user => ({ ...user, name: user.name.trim() }),

user => ({ ...user, email: user.email.toLowerCase() }),

user => ({ ...user, age: parseInt(user.age) })

);

const user = processUser({

name: ' John ',

email: 'JOHN@EXAMPLE.COM',

age: '30'

});

```

4. Pure Functions and Immutability

```javascript

// Impure function (modifies input)

function addItemImpure(cart, item) {

cart.items.push(item);

cart.total += item.price;

return cart;

}

// Pure function (no side effects)

function addItemPure(cart, item) {

return {

...cart,

items: [...cart.items, item],

total: cart.total + item.price

};

}

// Immutable array operations

const numbers = [1, 2, 3, 4, 5];

// Add to array

const withSix = [...numbers, 6];

// Remove from array

const withoutThree = numbers.filter(n => n !== 3);

// Update array element

const doubled = numbers.map(n => n === 3 ? n * 2 : n);

// Immutable object operations

const user = { name: 'John', age: 30 };

// Update property

const olderUser = { ...user, age: 31 };

// Add property

const withEmail = { ...user, email: 'john@example.com' };

// Remove property

const { age, ...withoutAge } = user;

// Deep cloning (simple approach)

const deepClone = obj => JSON.parse(JSON.stringify(obj));

// Better deep cloning

const structuredClone = obj => globalThis.structuredClone(obj);

```

```javascript

// Class syntax

class User {

// Private fields

#password;

// Public fields

id;

name;

// Static field

static count = 0;

constructor(id, name, password) {

this.id = id;

this.name = name;

this.#password = password;

User.count++;

}

// Public method

greet() {

return Hello, ${this.name};

}

// Private method

#hashPassword(password) {

return hashed_${password};

}

// Getter

get displayName() {

return this.name.toUpperCase();

}

// Setter

set password(newPassword) {

this.#password = this.#hashPassword(newPassword);

}

// Static method

static create(id, name, password) {

return new User(id, name, password);

}

}

// Inheritance

class Admin extends User {

constructor(id, name, password, role) {

super(id, name, password);

this.role = role;

}

greet() {

return ${super.greet()}, I'm an admin;

}

}

```

```javascript

// Exporting

// math.js

export const PI = 3.14159;

export function add(a, b) {

return a + b;

}

export class Calculator {

// ...

}

// Default export

export default function multiply(a, b) {

return a * b;

}

// Importing

// app.js

import multiply, { PI, add, Calculator } from './math.js';

// Rename imports

import { add as sum } from './math.js';

// Import all

import * as Math from './math.js';

// Dynamic imports

const module = await import('./math.js');

const { add } = await import('./math.js');

// Conditional loading

if (condition) {

const module = await import('./feature.js');

module.init();

}

```

```javascript

// Custom iterator

const range = {

from: 1,

to: 5,

[Symbol.iterator]() {

return {

current: this.from,

last: this.to,

next() {

if (this.current <= this.last) {

return { done: false, value: this.current++ };

} else {

return { done: true };

}

}

};

}

};

for (const num of range) {

console.log(num); // 1, 2, 3, 4, 5

}

// Generator function

function* rangeGenerator(from, to) {

for (let i = from; i <= to; i++) {

yield i;

}

}

for (const num of rangeGenerator(1, 5)) {

console.log(num);

}

// Infinite generator

function* fibonacci() {

let [prev, curr] = [0, 1];

while (true) {

yield curr;

[prev, curr] = [curr, prev + curr];

}

}

// Async generator

async function* fetchPages(url) {

let page = 1;

while (true) {

const response = await fetch(${url}?page=${page});

const data = await response.json();

if (data.length === 0) break;

yield data;

page++;

}

}

for await (const page of fetchPages('/api/users')) {

console.log(page);

}

```

```javascript

// Optional chaining

const user = { name: 'John', address: { city: 'NYC' } };

const city = user?.address?.city;

const zipCode = user?.address?.zipCode; // undefined

// Function call

const result = obj.method?.();

// Array access

const first = arr?.[0];

// Nullish coalescing

const value = null ?? 'default'; // 'default'

const value = undefined ?? 'default'; // 'default'

const value = 0 ?? 'default'; // 0 (not 'default')

const value = '' ?? 'default'; // '' (not 'default')

// Logical assignment

let a = null;

a ??= 'default'; // a = 'default'

let b = 5;

b ??= 10; // b = 5 (unchanged)

let obj = { count: 0 };

obj.count ||= 1; // obj.count = 1

obj.count &&= 2; // obj.count = 2

```

```javascript

// Debounce

function debounce(fn, delay) {

let timeoutId;

return (...args) => {

clearTimeout(timeoutId);

timeoutId = setTimeout(() => fn(...args), delay);

};

}

const searchDebounced = debounce(search, 300);

// Throttle

function throttle(fn, limit) {

let inThrottle;

return (...args) => {

if (!inThrottle) {

fn(...args);

inThrottle = true;

setTimeout(() => inThrottle = false, limit);

}

};

}

const scrollThrottled = throttle(handleScroll, 100);

// Lazy evaluation

function* lazyMap(iterable, transform) {

for (const item of iterable) {

yield transform(item);

}

}

// Use only what you need

const numbers = [1, 2, 3, 4, 5];

const doubled = lazyMap(numbers, x => x * 2);

const first = doubled.next().value; // Only computes first value

```

1. Use const by default: Only use let when reassignment is needed
2. Prefer arrow functions: Especially for callbacks
3. Use template literals: Instead of string concatenation
4. Destructure objects and arrays: For cleaner code
5. Use async/await: Instead of Promise chains
6. Avoid mutating data: Use spread operator and array methods
7. Use optional chaining: Prevent "Cannot read property of undefined"
8. Use nullish coalescing: For default values
9. Prefer array methods: Over traditional loops
10. Use modules: For better code organization
11. Write pure functions: Easier to test and reason about
12. Use meaningful variable names: Self-documenting code
13. Keep functions small: Single responsibility principle
14. Handle errors properly: Use try/catch with async/await
15. Use strict mode: 'use strict' for better error catching

1. this binding confusion: Use arrow functions or bind()
2. Async/await without error handling: Always use try/catch
3. Promise creation unnecessary: Don't wrap already async functions
4. Mutation of objects: Use spread operator or Object.assign()
5. Forgetting await: Async functions return promises
6. Blocking event loop: Avoid synchronous operations
7. Memory leaks: Clean up event listeners and timers
8. Not handling promise rejections: Use catch() or try/catch

• MDN Web Docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript
• JavaScript.info: https://javascript.info/
• You Don't Know JS: https://github.com/getify/You-Dont-Know-JS
• Eloquent JavaScript: https://eloquentjavascript.net/
• ES6 Features: http://es6-features.org/