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The window property of a window object points to the window object itself. Thus the following expressions all return the same window object:

window.window
window.window.window
window.window.window.window
  ...

In web pages, the window object is also a global object. This means:

  1. global variables of your script are in fact properties of window:
    var global = {data: 0};
    alert(global === window.global); // displays "true"
    
  2. you can access built-in properties of the window object without having to type window. prefix:
    setTimeout("alert('Hi!')", 50); // equivalent to using window.setTimeout.
    alert(window === window.window); // displays "true"
    

The point of having the window property refer to the object itself, was likely to make it easy to refer to the global object. Otherwise, you'd have to do a manual var window = this; assignment at the top of your script.

Another reason, is that without this property you wouldn't be able to write, for example, "window.open('http://google.com/')". You'd have to use "open('http://google.com/')" instead.

Yet another reason to use this property, is for libraries which wish to offer OOP-versions, and non-OOP versions (especially JavaScript modules). For example, if we refer to "this.window.location.href", a JavaScript module could define a property called "window" inside of a class it defined (since no global "window" variable exists for it by default) which could be created after passing in a window object to the module class' constructor.  Thus, "this.window" inside of its functions would refer to that window object. In the non-namespaced version, "this.window" would refer back to "window", and also be able to readily get the document location. Another advantage, is that the objects of such a class (even if the class were defined outside of a module) could change their reference to the window at will, they would not be able to do this if they had hard-coded a reference to "window". The default in the class could still be set as the current window object.

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'Window.window' in that specification.
Living Standard No difference from the latest snapshot HTML 5.1
HTML 5.1
The definition of 'Window.window' in that specification.
Recommendation No difference from the HTML5
HTML5
The definition of 'Window.window' in that specification.
Recommendation First snapshot containing the definition of Window.window.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
Basic supportChrome Full support YesEdge ? Firefox Full support YesIE ? Opera ? Safari ? WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support YesOpera Android ? Safari iOS ? Samsung Internet Android ?

Legend

Full support  
Full support
Compatibility unknown  
Compatibility unknown

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