What is a JS file?

JavaScript (JS) is a programming language that enables interactive web pages. Originally created by Brendan Eich at Netscape in 1995 (in just 10 days), it’s now the most widely used programming language in the world. JS files contain executable code that browsers interpret directly — no compilation step is needed. Beyond browsers, JavaScript runs on servers via Node.js, in mobile apps via React Native, and in desktop apps via Electron.

The ECMAScript (ES) standard governs the language. Modern JS (ES2015+) introduced classes, arrow functions, modules, async/await, and destructuring, significantly improving developer ergonomics over the original language.

How to open JS files

• VS Code (Windows, macOS, Linux) — IntelliSense, debugging, integrated terminal
• Any web browser — Console (F12) and DevTools for interactive execution
• Node.js — Server-side runtime to execute .js files outside the browser
• Sublime Text (Windows, macOS, Linux) — Fast editing
• WebStorm (Windows, macOS, Linux) — Full JavaScript IDE with refactoring tools

Technical specifications

Common use cases

• Web interactivity: DOM manipulation, form validation, event handling
• Server-side: Node.js, Deno, Bun for backend APIs and services
• Mobile apps: React Native (iOS + Android from one codebase)
• Desktop apps: Electron (VS Code, Slack, Discord are all Electron apps)
• Build tools: Webpack, Vite, esbuild, Rollup for bundling assets

Modern JavaScript features

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

// Destructuring
const { name, age } = user;

// Async/await
const data = await fetch('/api/users').then(r => r.json());

// Optional chaining
const city = user?.address?.city;

// Nullish coalescing
const timeout = config.timeout ?? 5000;

JavaScript vs TypeScript

TypeScript (.ts) is a superset of JavaScript that adds static type checking. It compiles to plain .js for deployment. TypeScript is the preferred choice for large codebases and teams — it catches type errors at compile time rather than at runtime. All valid .js code is also valid .ts code, making migration incremental.

Security considerations

JavaScript that handles user input must be sanitized to prevent Cross-Site Scripting (XSS) attacks — never insert raw user content into the DOM via innerHTML. Use textContent instead, or a sanitization library like DOMPurify. When handling sensitive data, avoid storing secrets in client-side JS code, as all JS sent to the browser is publicly visible in DevTools.

Performance

Large JS bundles slow down page load. Best practices: code-split (load JS only when needed), tree-shake (remove unused exports), minify for production, and use async/defer attributes on script tags to avoid blocking HTML parsing. Core Web Vitals — especially Total Blocking Time (TBT) — are directly affected by JS execution on the main thread.