1.13.b — Where JavaScript Runs

In one sentence: JavaScript is one language with multiple runtimes — most often the browser (with DOM and Web APIs) or Node.js (with files, networking, and no window/document by default).

Navigation: ← 1.13.a — What is JavaScript? · 1.13.c — Linking JS with HTML →

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1. Browser JavaScript

When JS runs in a web page, the host is the browser. You typically have:

Capability	Examples
DOM	document.querySelector, creating elements, reading form values
Web APIs	fetch, localStorage, setTimeout, addEventListener
Globals	window, document, console

Security model: Code from one origin is isolated from others (same-origin policy for many APIs). User-triggered actions and permissions (camera, location) are gated for safety.

Use cases: Interactive UIs, client-side validation, SPAs, progressive enhancement on top of HTML/CSS.

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2. Node.js

Node.js is a JavaScript runtime built on Chrome’s V8 engine, designed for servers, CLIs, and tooling. It does not include the browser DOM.

Capability	Examples
File system	Reading/writing files (fs module)
HTTP	Building APIs, proxy servers
Process	Environment variables, process.argv for CLI args
Tooling	Running tests, bundlers (Vite, webpack), formatters

Globals differ: No window or document. You have global / globalThis, process, Buffer (in many setups), and CommonJS require or ES modules import depending on project config.

Use cases: REST/GraphQL backends, static site generation, build scripts, desktop tooling (e.g. with Electron — which does embed a browser for UI).

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3. Same language, different environments

  Your .js file
        │
        ├──► Browser:  V8 / SpiderMonkey / JSC  +  DOM + Web APIs
        │
        └──► Node.js:  V8  +  libuv (I/O)  +  Node core modules

Core syntax (if, for, function, const) is the same. What changes is which objects and modules exist at runtime.

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4. ECMAScript (ES)

The language specification is called ECMAScript (e.g. ES2015, ES2020). Engines and Node implement that spec plus host-specific APIs. When people say "JavaScript," they usually mean ECMAScript + host APIs.

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5. Key takeaways

1. Browser JS = DOM + Web APIs + window / document.
2. Node.js = server/tooling runtime; no DOM unless you embed a browser (e.g. Puppeteer).
3. Learn core JS once; learn environment APIs where you deploy.

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Explain-It Challenge

Explain without notes:

1. Name three things you can do in a browser with JS that you typically cannot do in plain Node without extra libraries.
2. Why does document.body throw or fail in a normal Node script?
3. What does it mean that Node uses the V8 engine?

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Navigation: ← 1.13.a — What is JavaScript? · 1.13.c — Linking JS with HTML →