Node.js and the browser

Both environments execute JavaScript, but the host decides which APIs are available. V8 is the JavaScript engine used by Google Chrome/Chromium and Node.js: it parses and executes JavaScript and optimizes frequently used code. Chrome provides DOM and browser APIs on top of V8; Node provides fs, process, streams, and event-loop/I/O integration on top of V8 and libuv. V8 itself provides neither document nor filesystem access.

Question	Browser	Node.js
What does the code interact with?	Pages, DOM, user input, Web APIs	Files, processes, servers, CLI, Node APIs
Can it use document/window?	Yes, in a page	No, not by default
Can it use node:fs?	Not as a normal browser API	Yes, subject to OS permissions
What module styles are typical?	ES modules (import)	CommonJS (require) and ES modules (import)
Who chooses the runtime version?	The user's browser	The application/deployment owner

// Browser: document.querySelector('button')
// Node.js: process.argv[2], require('node:fs')

The browser isolates websites with origin-based protections and does not give arbitrary scripts full filesystem access. Node runs with the permissions of its process: never execute untrusted code or install packages blindly. Some APIs overlap (for example modern Node also includes fetch); the distinction is the runtime's capabilities, not two different JavaScript languages.

Firefox uses SpiderMonkey and Safari uses JavaScriptCore; sharing the ECMAScript language does not require sharing V8. Browser and Node scheduling also differ: Node's libuv phases and process.nextTick() are not browser APIs; see the event loop.

Node vs browser (opens new window) · The V8 JavaScript engine (opens new window)