window.onload函数如何工作？

Question

I don't know javascript well. So may be stupid question here:

Suppose, we can add a prototype or say property to the window object like

window.something = function(){..}

or,

window.something = "somestring"

But how window.onload = function(){....} is not creating onload function but assigning it to run when loaded....

And if the onload is built-in function then window.onload = function(){...} should override the onload function....?!?!

Answer 1

That's called by the browser !

window.onload gets fired after the main HTML, all CSS, all images and all other resources have been loaded and rendered.

Answer 2

The window.onload property is created by the browser and exists by default. By default is has a value of null . But, if you assign a function to it (so it contains a valid function instead of null ), then the browser will call that function when the page resources have finished loading.

Here's a little demo that illustrates this: http://jsfiddle.net/jfriend00/7z48j/

Answer 3

Many properties of the window and document object are recognized specially by the browser. In this case, when the window.onload property is set, the browser automatically runs the function after loading the document into the browser window.