JavaScript is a high-level programming language created in 1995 by Brendan Eich at Netscape. It was originally designed to add interactivity to web pages and is now used across the web (in browsers and servers), in mobile apps, in desktop applications, in databases, and in many other environments.

JavaScript is one of the three core technologies of the web alongside HTML (structure) and CSS (presentation). The language is standardized as ECMAScript (ES); the names are used interchangeably in most contexts.

Where JavaScript runs

Environment	Runtime
Web browsers	Built into Chrome (V8), Firefox (SpiderMonkey), Safari (JavaScriptCore)
Servers	Node.js (V8), Deno (V8), Bun (JavaScriptCore)
Mobile apps	React Native, NativeScript, Capacitor
Desktop apps	Electron, Tauri
Edge networks	Cloudflare Workers, Vercel Edge, Deno Deploy
Databases	MongoDB scripts, PostgreSQL functions (with extensions)
Embedded systems	Espruino, Johnny-Five for hardware

The same language with different runtimes and APIs.

Basic syntax

// Variables
const name = "Jane";
let count = 0;

// Functions
function greet(person) {
  return `Hello, ${person}`;
}

// Arrow functions
const double = (n) => n * 2;

// Conditionals
if (count > 0) {
  console.log("Positive");
} else {
  console.log("Zero or negative");
}

// Arrays
const fruits = ["apple", "banana", "cherry"];
fruits.forEach((fruit) => console.log(fruit));

// Objects
const user = {
  name: "Jane",
  email: "jane@example.com",
  greet() {
    return `Hello, ${this.name}`;
  }
};

// Async / await
async function fetchData(url) {
  const response = await fetch(url);
  return await response.json();
}

What JavaScript does in the browser

JavaScript in the browser interacts with the page through the Document Object Model (DOM):

Manipulate page content. Add, remove, or modify HTML elements
Respond to user events. Click, hover, scroll, keyboard input, form submission
Make network requests. Fetch data from APIs without reloading the page
Animate elements. Smooth transitions and complex animations
Handle forms. Validation, submission, autofill
Store data. Cookies, localStorage, sessionStorage, IndexedDB
Use device APIs. Geolocation, camera, microphone, push notifications, vibration
Render graphics. Canvas, WebGL, WebGPU
Build single-page applications. Frameworks like React, Vue, Svelte, Angular

JavaScript on the server

Node.js (introduced in 2009) brought JavaScript to server environments. Server-side JavaScript powers:

Web server applications (Express, Fastify, Hono, Koa)
API backends
Build tools (webpack, Vite, esbuild)
Static site generators (Astro, Eleventy, Next.js)
Real-time applications (chat, collaboration, dashboards)
Command-line tools

Deno (2020) and Bun (2023) are newer alternatives to Node.js with different design choices.

ECMAScript versions

JavaScript is standardized as ECMAScript. Major versions:

Version	Year	Notable additions
ES5	2009	`strict mode`, JSON, array methods
ES6 / ES2015	2015	`let`/`const`, arrow functions, classes, modules, promises, template literals
ES2016 onwards	Annual	Incremental additions: `async`/`await`, optional chaining, nullish coalescing, etc.

Most modern JavaScript uses ES2015+ features. Browsers receive new features regularly.

Common frameworks and libraries

For browser-based UI:

Framework / library	Approach
React	Component-based UI; massive ecosystem
Vue	Component-based UI; gentler learning curve
Svelte	Compiles components to vanilla JS at build time
Angular	Full-featured framework from Google
Solid	Fine-grained reactivity, similar API to React
Lit	Lightweight web components
htmx	HTML-driven interactivity, minimal JavaScript
Alpine.js	Lightweight reactivity for sprinkles of interactivity

For state management, routing, animations, data fetching, etc., there are many specialized libraries.

TypeScript

TypeScript is a superset of JavaScript that adds static type checking. It compiles to standard JavaScript and is widely used in larger projects:

function greet(name: string): string {
  return `Hello, ${name}`;
}

TypeScript catches many common errors at compile time and provides better tooling (autocomplete, refactoring) than vanilla JavaScript. It is increasingly the default for new projects.

JavaScript and performance

JavaScript performance affects:

Page load time. Large JS bundles take time to download and execute
Interactivity. Heavy JS execution blocks the main thread
Battery and CPU. Particularly on mobile devices
Bundle size. Even unused code adds to download time without tree-shaking

Common performance practices:

Use code splitting to load only what’s needed
Defer non-critical scripts
Tree-shake unused exports at build time
Avoid large dependencies for small features
Measure with browser DevTools Performance tab

How much JavaScript a site needs

The right amount of JavaScript varies by site type:

Site type	Typical JavaScript footprint
Marketing site	Minimal, perhaps 0–50 KB for forms and interactivity
Documentation	Moderate, search, navigation, possibly framework
Blog	Small to moderate, depends on features
Web application	Large, 200 KB to several MB
Single-page application	Largest, entire app loads upfront

Modern frameworks like Astro and Eleventy emphasize shipping minimal JavaScript by default; React-based frameworks (Next.js, Remix) ship more by default.

Common misconceptions

“JavaScript is the same as Java.” They share part of a name but are unrelated languages with different design and use cases.
“You need JavaScript to build a website.” Many sites work without JavaScript; static HTML is sufficient for content-focused pages.
“More JavaScript means a better user experience.” Often the opposite; lighter pages tend to feel faster and more responsive.
“JavaScript only runs in browsers.” It runs in browsers, servers, mobile apps, desktop apps, edge networks, and many other environments.