What is the difference between Node.js and browser JavaScript?

Direct answer

Same language (ECMAScript), different runtimes — browsers add DOM, window, and layout; Node adds fs, process, http, and libuv-backed I/O with no document or rendering APIs.

JavaScript syntax you learn for React applies on the server, but globals and APIs differ. Code using document or localStorage fails in Node; code using fs or process fails in the browser unless bundled with polyfills.

	Browser	Node.js
Global object	window	globalThis (global in CommonJS)
UI / DOM	document, alert, canvas	Not available
I/O	fetch, IndexedDB (sandboxed)	fs, net, child_process, pg client
Module systems	ES modules + bundlers	CommonJS + native ESM
Event loop	Rendering + tasks	libuv phases, no paint

Shared core — Promises, async/await, classes, Map/Set, optional chaining work in both.
Isomorphic code — Zod schemas in packages/shared validate on web and API without DOM imports.
Security model — browser sandboxes tabs; Node server has full filesystem and env access — validate inputs.

runtime-check.js

// Works in both
const total = [1, 2, 3].reduce((a, b) => a + b, 0);

// Browser only
if (typeof document !== "undefined") {
  document.title = "AceDevHub";
}

// Node only
if (typeof process !== "undefined" && process.versions?.node) {
  console.log("Node", process.version);
}