The language of the web browser, and, via Node, of a huge share of backend and tooling code too.

Setup

Runs natively in every browser. For local scripting/tooling, install Node.js (node -v to confirm), which also gives you npm.

Core syntax

// variables, prefer const, use let when reassigned, avoid var
const name = "Ada";
let count = 0;

// functions
function add(a, b) { return a + b; }
const multiply = (a, b) => a * b;

// template literals
console.log(`${name} counted to ${count}`);

// arrays & objects
const nums = [1, 2, 3];
const doubled = nums.map(n => n * 2);
const user = { name: "Ada", age: 30 };
const { name: n2, age } = user; // destructuring

// async/await
async function fetchUser(id) {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  return res.json();
}

// classes
class Animal {
  constructor(name) { this.name = name; }
  speak() { return `${this.name} makes a sound.`; }
}

Equality: always ===

== coerces types in surprising ways ("" == 0 is true). Use ===/!== unless you have a specific, commented reason not to.

Modules

// math.js
export function add(a, b) { return a + b; }
export default class Calculator {}

// main.js
import Calculator, { add } from "./math.js";

The event loop, briefly

JS is single-threaded with a task queue: synchronous code runs first, then microtasks (resolved Promises), then macrotasks (setTimeout, I/O callbacks). This is why a resolved Promise's .then() runs before a setTimeout(fn, 0).

Popular libraries/frameworks

Common gotchas

• this inside a regular function depends on how it's called, not where it's defined, arrow functions capture the surrounding this instead, which is why they're preferred for callbacks inside class methods.
• Array/object equality is by reference: [1,2] === [1,2] is false.
• Floating point: 0.1 + 0.2 !== 0.3, use a small epsilon or a decimal library for money.

See also: TypeScript for the typed superset most new projects now use by default.