js 中var that=this

js中经常出现var that=this,为什么这么做？

http://stackoverflow.com/questions/4886632/what-does-var-that-this-mean-in-javascript 

问题，回答：

一个回答：
$('#element').click(function(){
    // this is a reference to the element clicked on

    var that = this;

    $('.elements').each(function(){
        // this is a reference to the current element in the loop
        // that is still a reference to the element clicked on
    });
});

Because this frequently changes when you change the scope by calling a new function, you can't access the original value by using it. Aliasing it to that allows you still to access the original value of this.

另一个回答：From Crockford

By convention, we make a private that variable. This is used to make the object available to the private methods. This is a workaround for an error in the ECMAScript Language Specification which causes this to be set incorrectly for inner functions.