Why does jquery need _$ in case of overwrite?

Question

var 
// Will speed up references to window, and allows munging its name.
window = this,
// Will speed up references to undefined, and allows munging its name.
undefined,
// Map over jQuery in case of overwrite
_jQuery = window.jQuery,
// Map over the $ in case of overwrite
_$ = window.$,

jQuery = window.jQuery = window.$ = function( selector, context ) {
    // The jQuery object is actually just the init constructor 'enhanced'
    return new jQuery.fn.init( selector, context );
};

Why does jQuery need "_$ = window.$" or "_jQuery = window.jQuery"? It doesn't make sense to me, but without this two lines, the framework doesn't work.

Thanks for any help..

Answer 1

It's saving a reference to the values of $ and jQuery before it overwrites them so that you can call jQuery.noConflict() and restore the values.