Modern JavaScript Patterns

Comprehensive guide for mastering modern JavaScript (ES6+) features, functional programming patterns, and best practices for writing clean, maintainable, and performant code.

When to Use This Skill

• 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

ES6+ Core Features

1. Arrow Functions

Syntax and Use Cases:

// 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:

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:

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:

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:

// 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:

// 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

// Basic usage const name = "John"; const greeting = Hello, ${name}!;

// Multi-line strings const html = <div> <h1>${title}</h1> <p>${content}</p> </div>;

// 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 + <mark>${value}</mark>; }, ""); }

const name = "John"; const age = 30; const html = highlightName: ${name}, Age: ${age}; // Output: "Name: <mark>John</mark>, Age: <mark>30</mark>"

5. Enhanced Object Literals

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"]);

Asynchronous Patterns

1. Promises

Creating and Using Promises:

// 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:

// 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:

// 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:

// 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]); }

Functional Programming Patterns

1. Array Methods

Map, Filter, Reduce:

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:

// 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:

// 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:

// 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

// 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

// 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);

Modern Class Features

// 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; } }

Modules (ES6)

// 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(); }

Iterators and Generators

// 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); }

Modern Operators

// 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

Performance Optimization

// 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

Best Practices

• Use const by default: Only use let when reassignment is needed

• Prefer arrow functions: Especially for callbacks

• Use template literals: Instead of string concatenation

• Destructure objects and arrays: For cleaner code

• Use async/await: Instead of Promise chains

• Avoid mutating data: Use spread operator and array methods

• Use optional chaining: Prevent "Cannot read property of undefined"

• Use nullish coalescing: For default values

• Prefer array methods: Over traditional loops

• Use modules: For better code organization

• Write pure functions: Easier to test and reason about

• Use meaningful variable names: Self-documenting code

• Keep functions small: Single responsibility principle

• Handle errors properly: Use try/catch with async/await

• Use strict mode: 'use strict' for better error catching

Common Pitfalls

• this binding confusion: Use arrow functions or bind()

• Async/await without error handling: Always use try/catch

• Promise creation unnecessary: Don't wrap already async functions

• Mutation of objects: Use spread operator or Object.assign()

• Forgetting await: Async functions return promises

• Blocking event loop: Avoid synchronous operations

• Memory leaks: Clean up event listeners and timers

• Not handling promise rejections: Use catch() or try/catch