First look at our source code.
Mozilla Firefox :
Why is this happening?
We view the structure of the Window object in the browser console;
It turns out that the Window object contains some APIs implemented by browser manufacturers, such as the standard sessionStorage in html5;
It also has a window attribute, and the value of this attribute points to the Window object;
My understanding: The Window object serves browser manufacturers. We cannot directly operate the properties of the Window object. New APIs added by Window will be reflected in the window object;
The properties we operate on window will be reflected in the Window object.
For example, define a global variable window.a = 'aaa';
All JavaScript objects exist in a running environment. This running environment itself is also an object, called a "top-level object". This means that all objects in JavaScript are subordinate to the "top-level object". Different running environments have different "top-level objects". In the browser environment, this top-level object is the window object.
All global variables of the browser environment are properties of the window object.
Window can be understood as JavaScriptContext context environment.
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
