Everything you need to know about JS
JavaScript (.js) is the language of the web. Created in 10 days by Brendan Eich in 1995 to add interactivity to Netscape Navigator, JavaScript is now the most-used programming language on Earth - powering the front-end of every website, the back-end of millions (Node.js), mobile apps (React Native), and even desktop apps (Electron).
How it works under the hood
- Single-threaded with event loop. JavaScript runs on one thread but uses an event loop to handle async work without blocking - this is why `setTimeout`, `fetch`, and Promises don't freeze the UI.
- Prototypal inheritance. Objects inherit from other objects, not from classes. The `class` keyword (ES2015) is syntactic sugar over prototypes.
- ES Modules. Modern JavaScript uses `import`/`export`. CommonJS (`require`) is the older Node.js convention; ESM is the standard going forward.
- Type coercion gotchas. `'5' + 3 === '53'` (string concat); `'5' - 3 === 2` (numeric subtraction). Use `===` (strict equality) instead of `==` to avoid surprises.
Where you'll actually use it
- Browser scripting (universal)
- Server-side runtime (Node.js, Deno, Bun)
- Cross-platform mobile (React Native, Capacitor)
- Desktop apps (Electron, Tauri)
How it compares to alternatives
JS vs TypeScript: TypeScript adds static types - catches bugs at compile time. Most modern projects use TS. JS vs Python: JS is single-threaded async, Python is sync-first. JS vs WebAssembly: WASM is for performance-critical code; JS is for orchestration.
Things that will trip you up
- `this` binding is context-dependent - arrow functions inherit; regular functions create new context
- `==` does type coercion (`null == undefined` is true!) - always use `===`
- Hoisting: `var` declarations are hoisted but assignments aren't - `let` and `const` have temporal dead zones
Test it yourself: Jest/Vitest for unit tests, Playwright/Cypress for browser tests, ESLint for linting, Prettier for formatting. `node --inspect` for debugging.