Why does jQuery has a “window=this” at the very begining and say it would speed up references to window?

问题

When I open jQuery's source code I find this line.

var 
// Will speed up references to window, and allows munging its name.
window = this

Why and how this line will speed up?

回答1:

1. javascript functions have lexical scope
2. jQuery wraps its entire implementation in an anonymous function
3. when said function begins execution it is executing in the "global" scope (ie this == window).

"window = this;" simply creates a local identifier in that scope so that references to it do not have to "bubble up" outside of the local scope to resolve.

回答2:

this would be faster for javascript to reference to, as compared to window which would have to be resolved to the window object.

来源：https://stackoverflow.com/questions/563875/why-does-jquery-has-a-window-this-at-the-very-begining-and-say-it-would-speed