JavaScript Cheatsheet

JavaScript is the programming language of the web, running in browsers and on servers via Node.js. This cheatsheet covers modern (ES6+) syntax and common patterns.

Variables

Prefer const and let over var.

const name = "Ada";      // block-scoped, constant
let count = 0;            // block-scoped, reassignable
var old = "avoid";        // function-scoped (legacy)

Functions

Arrow functions and defaults.

function add(a, b) { return a + b; }
const mul = (a, b) => a * b;
const greet = (name = "friend") => `Hi ${name}`;

Arrays

Powerful built-in methods.

const nums = [1, 2, 3];
nums.map(n => n * 2);        // [2, 4, 6]
nums.filter(n => n > 1);     // [2, 3]
nums.reduce((a, b) => a + b) // 6
nums.find(n => n === 2);     // 2
nums.includes(3);            // true
[...nums, 4];                // spread -> [1,2,3,4]

Objects

Destructuring and spread.

const user = { id: 1, name: "Ada" };
const { name } = user;           // destructure
const copy = { ...user };        // shallow copy
const merged = { ...user, age: 36 };
Object.keys(user);               // ['id', 'name']

Template Literals

String interpolation and multiline.

const msg = `Hello ${name}, you have ${count} items`;
const html = `
  <div>${name}</div>
`;

Conditionals

Ternaries and short-circuiting.

const label = count > 0 ? "has" : "empty";
const value = input ?? "default";   // nullish coalescing
user?.address?.city;                 // optional chaining

Async / Await

Handle promises cleanly.

async function getUser(id) {
  try {
    const res = await fetch(`/api/users/${id}`);
    return await res.json();
  } catch (err) {
    console.error(err);
  }
}

Promises

Compose asynchronous work.

fetch("/api")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

Promise.all([p1, p2]).then(([a, b]) => {});

Modules

Import and export.

export const pi = 3.14;
export default function () {}

import defaultFn, { pi } from "./math.js";

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JavaScript powers interactivity on the web and full-stack apps on Node.js. Learn these fundamentals, then explore frameworks like React and runtimes like Node and Deno.

For full documentation, see https://developer.mozilla.org/en-US/docs/Web/JavaScript