Runtime Vs Browser

🌍 1. JavaScript in the Browser

When you run JavaScript inside a browser (Chrome, Firefox, Safari, etc.):

• Purpose:
  Mostly for client-side (front-end) tasks like manipulating the DOM, handling events, animations, and making network requests.

• Available APIs:

  • document, window, navigator, localStorage, fetch, etc.

  • These are provided by the browser environment, not JavaScript itself.

• Execution:
  JavaScript code is executed by the browser’s JavaScript engine (e.g., V8 in Chrome).
  But it runs inside a sandboxed environment with strict security (can’t directly touch the file system or OS).

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⚙️ 2. Node.js Runtime Environment

When you run JavaScript with Node.js:

• Purpose:
  Built for server-side tasks like building APIs, reading/writing files, networking, process management.

• Available APIs:

  • fs (file system), http, os, path, process, etc.

  • These come from Node.js, not from the language or browser.

• Execution:
  Still uses the V8 engine (like Chrome), but Node.js adds bindings to C++ libraries for low-level operations.

• No Browser APIs:
  You don’t get document, window, or fetch (until recently — Node 18+ added a WHATWG fetch implementation).

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📊 Key Differences (Browser vs Node.js)

Aspect	Browser JS	Node.js
Environment	Runs inside a browser	Runs on the OS
APIs Available	DOM, BOM, Web APIs (fetch, localStorage)	Core modules (fs, http, os, process)
Security	Sandboxed (no direct FS/OS access)	Full OS & network access
Global Object	window	global
Use Case	UI interactivity, client-side logic	Backend servers, tooling, scripts

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✅ In short:

• Browser JS = limited sandbox, DOM-focused, client-side.

• Node.js = full runtime environment with extra APIs for server-side & system-level tasks.