21 — The browser

One line: Everything a page does beyond computing — its document, events, painting, network rules and storage — comes from the browser, not from JavaScript, and each part has rules that Node never enforces.

This chapter is about the host, not the language. It starts with the DOM as an API the browser provides, then how events travel through the document tree. Next comes the rendering loop, which paints only between tasks, then fetch under the browser's cross-origin rules, then the places a page can keep data, and last how <script> tags load code. Node cannot run these pages' browser examples; where Node offers a contrast, a second example runs it.

The pages

Page	Level	In one line	Status
The DOM	101	document, window, setTimeout and console are not JavaScript: ECMA-262 defines none of them. A browser supplies them from the DOM and HTML standards, and Node supplies its own set, where typeof document is "undefined" but setTimeout exists.	stub
DOM events	201	A click is dispatched along a path through the tree — down from window in the capture phase, at the target, then back up in the bubble phase — so one listener on a <ul> hears clicks on every <li>, even items added later.	stub
The rendering loop	201	The page repaints only between tasks: text changed 1,000 times inside one loop is painted once, and while a 3-second loop runs no click is handled and nothing the script changes appears; requestAnimationFrame runs a callback just before the next paint.	stub
fetch and CORS	201	CORS is enforced by the browser, not the server: a simple cross-origin GET still reaches the server and runs, but without Access-Control-Allow-Origin in the reply the page's fetch rejects with a bare TypeError, while the same request from Node succeeds.	stub
Storage	201	localStorage keeps only strings — setItem("n", 1) reads back "1" and an object becomes "[object Object]" — and it is synchronous; cookies ride along with every request to their site, and an HttpOnly one is invisible to document.cookie.	stub
Loading scripts	201	A plain <script src> stops the HTML parser until it has downloaded and run; defer runs scripts after parsing in document order, async runs each the moment it arrives in any order, and type="module" behaves like defer without being asked.	stub

Boundaries

The event loop itself is in Async and the event loop; fetch in Node, where no CORS applies, is in The Node.js runtime; ES modules as a language feature are in Modules.