What is window?

JavaScriptJunior~2 min read

The window object is the global browser object, which represents the tab's window in which JavaScript runs.

It is the root object for the entire browser environment, and through it you can access all global variables, functions, and document itself.

1. What `window` is

window is an object that describes the current browser window and contains:

the web page itself (document),
window control methods (opening, closing, scrolling, and so on),
timers (setTimeout, setInterval),
browser APIs (localStorage, history, location, and others),
all global variables (for example, those declared via var).

2. Example:

javascript

console.log(window.document); // access to the DOM
console.log(window.location.href); // page URL
console.log(window.innerWidth); // browser window width

In practice, in the browser almost everything "lives" inside window.

3. `window` is a global object

When you write:

javascript

alert("Hello");

this is the same as:

javascript

window.alert("Hello");

Almost all global functions and properties actually belong to window. The same applies to variables declared via var:

javascript

var x = 10;
console.log(window.x); // 10

But - variables declared via let and const are not added to window:

javascript

let y = 5;
console.log(window.y); // undefined

4. Main properties and objects inside `window`

Property / Object	What it does
`window.document`	The HTML document (DOM tree)
`window.location`	URL information and navigation methods
`window.history`	Navigation history
`window.navigator`	Information about the browser and system
`window.localStorage`	Persistent data storage
`window.sessionStorage`	Temporary storage for the session
`window.console`	Access to the console (`console.log`)
`window.alert()`, `confirm()`, `prompt()`	Dialog windows
`window.setTimeout()` / `setInterval()`	Timers
`window.innerWidth`, `window.innerHeight`	Viewport dimensions
`window.scrollTo()`, `window.scrollBy()`	Page scrolling

5. Examples:

javascript

console.log(window.location.href); // https://example.com
window.location.href = "https://example.org"; // navigate to another site

Timers:

javascript

window.setTimeout(() => {
  console.log("2 seconds have passed");
}, 2000);

Page scrolling:

javascript

window.scrollTo({ top: 0, behavior: "smooth" });

Storage:

javascript

window.localStorage.setItem("theme", "dark");
console.log(window.localStorage.getItem("theme")); // "dark"

6. Relationship:

javascript

window
 ├── document   -> HTML document (DOM)
 ├── location   -> URL and navigation
 ├── history    -> navigation history
 ├── navigator  -> browser information
 ├── localStorage / sessionStorage -> storage
 ├── console
 ├── timers (setTimeout, setInterval)
 └── ... and much more

7. Key difference from `document`:

Property	`window`	`document`
What it represents	The browser tab (window)	The page content (DOM)
Global object	Yes	No
Contains	The whole environment (DOM, timers, APIs)	Only the tree of HTML elements
Example	`window.innerWidth`	`document.body`

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