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The JavaScript Browser Environment

When JavaScript runs in a browser, it gets access to a special environment with three main parts:

• ECMAScript — the core language (variables, functions, loops, etc.)
• DOM (Document Object Model) — APIs to interact with HTML/XML pages
• BOM (Browser Object Model) — APIs to interact with the browser itself

┌─────────────────────────────────────┐
│           JavaScript                │
│  ┌──────────┐ ┌──────┐ ┌────────┐   │
│  │    JS    │ │ DOM  │ │  BOM   │   │
│  │ Language │ │ APIs │ │ APIs   │   │
│  └──────────┘ └──────┘ └────────┘   │
└─────────────────────────────────────┘

The Global Object: window

In a browser, window is the global object. Every global variable, DOM API, and BOM method lives on it:

<p id='output'></p>

const out = document.getElementById('output');
out.innerHTML = 'Window inner width: ' + window.innerWidth + 'px<br>';
out.innerHTML += 'Page location: ' + window.location.href;

DOM vs BOM

DOM Specs

The DOM is maintained by the W3C and WHATWG. Modern browsers implement the DOM Living Standard which includes:

• Core DOM — the tree structure model
• HTML DOM — HTML-specific elements and attributes
• CSSOM — CSS object model for styling

BOM APIs

The BOM provides browser-level objects:

• navigator — browser info (user agent, geolocation, media devices)
• location — URL information and navigation
• history — session history (back, forward, pushState)
• screen — screen dimensions
• fetch / XMLHttpRequest — network requests
• localStorage / sessionStorage — client-side storage

<p id='bom-output'></p>

const el = document.getElementById('bom-output');
el.innerHTML = 'Browser: ' + navigator.userAgent + '<br>';
el.innerHTML += 'Platform: ' + navigator.platform + '<br>';
el.innerHTML += 'Screen size: ' + screen.width + 'x' + screen.height;

Key Takeaways

• JavaScript in the browser has three layers: language, DOM, BOM
• window is the global object — all browser APIs hang off it
• DOM lets you read and modify the page
• BOM lets you interact with the browser itself
• Both DOM and BOM are evolving standards — always check compatibility