JavaScript
Brendan Eich wrote the first version in 10 days (1995) under the name Mocha → LiveScript → JavaScript. Today the web's only native language.
Dynamically typed, prototype-based, single-threaded with an event loop. ECMAScript is the language standard; new versions every year (ES2015 was the big modernisation — let/const, arrow, modules, promises, classes). TC39 steers development.
Runs in V8 (Chrome, Node, Cloudflare Workers), JavaScriptCore (Safari, Bun), SpiderMonkey (Firefox), Hermes (React Native). Outside the browser: Node.js, Deno, Bun. TypeScript is its popular strict-typed superset.