The tools the browser gives JavaScript beyond the language itself.
Fetch, localStorage, the DOM, geolocation, setTimeout — none of these are part of the JavaScript language specification. They're Web APIs, provided by the browser (the host environment) and exposed to your JS code through global objects like window. This is why the same JS engine (V8) behaves differently in a browser versus in Node.js — the language is identical, but the surrounding APIs are completely different runtime-provided surfaces.
Many of these APIs are asynchronous by design and integrate directly with the event loop — fetch returns a promise, IntersectionObserver and MutationObserver fire callbacks when something changes, and requestAnimationFrame schedules work to run right before the next repaint rather than on a fixed timer. Knowing which APIs are synchronous (localStorage, most DOM reads) versus asynchronous (fetch, IndexedDB) matters directly for avoiding janky UI or accidentally blocking the main thread.
What you'll walk away knowing
No questions match "".