Are global variables just properties on the `windo

问题:

Whenever I see a website on the browser an instance of javascript is running. And I can declare a global variable in the console (DevTools);

var a = 1234567890; 

This variable has been declared in global scope such that I can get the value of the variable like so;

> a 
1234567890

However, I can also do this;

> window.a 
1234567890 

Am I understanding it correctly that the window object is the object that contains all the global variables within the website instance on the browser? If so to what scope does the window object belong? This is confusing me a little bit;

> window 
Window {top: Window, window: Window, location: Location, external:, ...}  
> window.window 
Window {top: Window, window: Window, location: Location, external:, ...}  
> window.window.window 
Window {top: Window, window: Window, location: Location, external:, ...}  

Is the window object the ultimate global object and does that have an object called window that refers back to itself?

回答1:

Is the window object the ultimate global object and does that have an object called window that refers back to itself?

Yes, and yes. This, for instance, returns true:

window.window.window.window.window === window.window;

You can, if you are interested, get a list of all the properties of the window object (and hence all global variables) with Object.keys:

console.log(Object.keys(window));

Note, however, that if you are spending too much time thinking about global variables, there is probably a problem with the architecture of your code.

回答2:

Yes, the window object is

The Global Object_{(§15.1 ES5 Specification)}

The unique global object is created before control enters any execution context.

Unless otherwise specified, the standard built-in properties of the global object have attributes {[[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}.

The global object does not have a [[Construct]] internal property; it is not possible to use the global object as a constructor with the new operator.

The global object does not have a [[Call]] internal property; it is not possible to invoke the global object as a function.

The values of the [[Prototype]] and [[Class]] internal properties of the global object are implementation-dependent.

In addition to the properties defined in this specification the global object may have additional host defined properties. This may include a property whose value is the global object itself; for example, in the HTML document object model the window property of the global object is the global object itself.

回答3:

All global variables become properties of the window object.

>>> window.somevar = 1;
1
>>> somevar
1

And all of the core JavaScript functions are methods of the window object.

回答4:

Yes, Window is the root element of the DOM (Document Object Model) object hierarchy.