Node.js vs browser JavaScript

JavaScript is one language with two major runtimes. Syntax overlaps (functions, classes, async/await), but globals, APIs, and security boundaries differ sharply.

Browser-only

• document, window, DOM APIs, fetch (also in modern Node)
• Sandboxed—no raw file system or arbitrary shell (without extensions)
• User-controlled environment—never trust client code for secrets

Node-only

• process, __dirname (CommonJS), import.meta.url (ESM)
• node:fs, node:http, node:path, node:crypto
• Full server privileges—validate input, guard file paths, manage secrets via env vars

Shared

Modern ECMAScript features—const/let, arrow functions, classes, modules, promises, Map/Set—work in both when supported by the runtime version. Always check Node LTS release notes for feature support.

Important interview questions and answers

1. Q: Can you use document in Node?
   A: No—there is no DOM. Use templates on the server (EJS, React SSR) or return JSON for SPAs.
2. Q: Is fetch available in Node?
   A: Yes in modern Node (18+ globally)—earlier versions needed node-fetch or http.
3. Q: Why share JS on client and server?
   A: One language for full-stack teams, shared validation schemas (e.g. Zod), and less context switching—trade-off is coupling if not modularized.

Self-check

1. Name two globals available in Node but not in the browser.
2. Why must secrets live on the server only?