Question

This value is set before jQuery has been exposed to the global scope:

_jQuery = window.jQuery,

Near the end of the IIFE and after the above statement there is

window.jQuery = window.$ = jQuery;

This is from the jQuery development source.

What compiler mechanism allows jQuery to do an assignment from an unset variable.

Also, what is _jQuery for in general?

Answer 1

As the comment says, _jQuery backs anything that already exists in the global scope up (like older versions of jQuery). This is probably for the jQuery.noConflict feature.