🤖system prompt•7 months ago
modern-javascript-patterns
Master ES6+ features including async/await, destructuring, spread
# 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:**
```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, ${name}!`;
// Multi-line strings
const html = `
<div>
<h1>${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 = highlight`Name: ${name}, Age: ${age}`;
// Output: "Name: <mark>John</mark>, Age: <mark>30</mark>"
```
### 5. Enhanced Object Literals
```javascript
const name = "John";
const age = 30;
// Shorthand property names
const user = { name, age };
// Shorthand method names
const calculator = {
add(a, b) {
return a + b;
},
subtract(a, b) {
return a - b;
},
};
// Computed property names
const field = "email";
const user = {
name: "John",
[field]: "john@example.com",
[`get${field.charAt(0).toUpperCase()}${field.slice(1)}`]() {
return this[field];
},
};
// Dynamic property creation
const createUser = (name, ...props) => {
return props.reduce(
(user, [key, value]) => ({
...user,
[key]: value,
}),
{ name },
);
};
const user = createUser("John", ["age", 30], ["email", "john@example.com"]);
```
## Asynchronous Patterns
### 1. Promises
**Creating and Using Promises:**
```javascript
// Creating a promise
const fetchUser = (id) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (id > 0) {
resolve({ id, name: "John" });
} else {
reject(new Error("Invalid ID"));
}
}, 1000);
});
};
// Using promises
fetchUser(1)
.then((user) => console.log(user))
.catch((error) => console.error(error))
.finally(() => console.log("Done"));
// Chaining promises
fetchUser(1)
.then((user) => fetchUserPosts(user.id))
.then((posts) => processPosts(posts))
.then((result) => console.log(result))
.catch((error) => console.error(error));
```
**Promise Combinators:**
```javascript
// Promise.all - Wait for all promises
const promises = [fetchUser(1), fetchUser(2), fetchUser(3)];
Promise.all(promises)
.then((users) => console.log(users))
.catch((error) => console.error("At least one failed:", error));
// Promise.allSettled - Wait for all, regardless of outcome
Promise.allSettled(promises).then((results) => {
results.forEach((result) => {
if (result.status === "fulfilled") {
console.log("Success:", result.value);
} else {
console.log("Error:", result.reason);
}
});
});
// Promise.race - First to complete
Promise.race(promises)
.then((winner) => console.log("First:", winner))
.catch((error) => console.error(error));
// Promise.any - First to succeed
Promise.any(promises)
.then((first) => console.log("First success:", first))
.catch((error) => console.error("All failed:", error));
```
### 2. Async/Await
**Basic Usage:**
```javascript
// Async function always returns a Promise
async function fetchUser(id) {
const response = await fetch(`/api/users/${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]);
}
```
## Functional Programming Patterns
### 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);
```
## Modern Class Features
```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 `${super.greet()}, I'm an admin`;
}
}
```
## Modules (ES6)
```javascript
// Exporting
// math.js
export const PI = 3.14159;
export function add(a, b) {
return a + b;
}
export class Calculator {
// ...
}
// Default export
export default function multiply(a, b) {
return a * b;
}
// Importing
// app.js
import multiply, { PI, add, Calculator } from "./math.js";
// Rename imports
import { add as sum } from "./math.js";
// Import all
import * as Math from "./math.js";
// Dynamic imports
const module = await import("./math.js");
const { add } = await import("./math.js");
// Conditional loading
if (condition) {
const module = await import("./feature.js");
module.init();
}
```
## Iterators and Generators
```javascript
// Custom iterator
const range = {
from: 1,
to: 5,
[Symbol.iterator]() {
return {
current: this.from,
last: this.to,
next() {
if (this.current <= this.last) {
return { done: false, value: this.current++ };
} else {
return { done: true };
}
},
};
},
};
for (const num of range) {
console.log(num); // 1, 2, 3, 4, 5
}
// Generator function
function* rangeGenerator(from, to) {
for (let i = from; i <= to; i++) {
yield i;
}
}
for (const num of rangeGenerator(1, 5)) {
console.log(num);
}
// Infinite generator
function* fibonacci() {
let [prev, curr] = [0, 1];
while (true) {
yield curr;
[prev, curr] = [curr, prev + curr];
}
}
// Async generator
async function* fetchPages(url) {
let page = 1;
while (true) {
const response = await fetch(`${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);
}
```
## Modern Operators
```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
```
## Performance Optimization
```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
```
## Best Practices
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
## Common Pitfalls
1. **this binding confusion**: Use arrow functions or bind()
2. **Async/await without error handling**: Always use try/catch
3. **Promise creation