TC39

Ecma International's Technical Committee 39 — the committee that specifies JavaScript, as ECMAScript (ECMA-262), through a staged proposal process where a feature advances from an idea (stage 0) to shipping in engines (stage 4) and a yearly edition of the language

The stage is the fact to know about a feature. Stage 3 means the design is done and engines are implementing; stage 4 means it is in the spec and safe to write. Build tools and TypeScript let a codebase use a stage-3 feature early, which is convenient and a small bet. The committee owns the language only: the DOM, fetch and the rest of the browser's API are WHATWG and W3C, and Node's are Node's.