You may want to read https://developer.mozilla.org/en-US/docs/Web/JavaScript/Even...

Note that it does not mention promises and async/await but it does include it. Promises are essentially syntactic sugar for a setTimeout calling success and failure methods. This also means that promise code will be executed in parallel but on one thread and one heap, so each promise will fight with all other for priority. This is generally fine for async stuff like fetching resources but it would completely destroy performance if it would do anything heavy, like, say unzipping zips with jszip.

Historically the event-driven model of JS enabled Node.js's API to be callback-driven. This lead to what is called "callback hell" and eventually to a proper introduction of promises and async/await to the language.