What is JavaScript, and where can JavaScript code run?

Direct answer

JavaScript is a high-level, dynamically typed programming language standardized as ECMAScript; it runs in browsers and in non-browser runtimes such as Node.js.

JavaScript is the programming language most closely associated with interactive web applications, but the language itself is not limited to browsers. Its standardized language specification is called ECMAScript.

Browsers run JavaScript through engines such as V8, SpiderMonkey, or JavaScriptCore and expose Web APIs such as the DOM, fetch, timers, and storage.
Server runtimes such as Node.js execute JavaScript outside the browser and provide APIs for files, networking, processes, and servers.
Other environments include edge runtimes, desktop shells, mobile frameworks, build tools, and embedded systems.