Question

In a JavaScript file I saw:

function Somefunction(){
   var that = this; 
   ... 
}

What is the purpose of declaring that and assigning this this to it?

Answer 1

I’m going to begin this answer with an illustration:

var colours = ['red', 'green', 'blue'];
document.getElementById('element').addEventListener('click', function() {
    // this is a reference to the element clicked on

    var that = this;

    colours.forEach(function() {
        // this is undefined
        // that is a reference to the element clicked on
    });
});

My answer originally demonstrated this with jQuery, which is only very slightly different:

$('#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.

Personally, I dislike the use of that as the alias. It is rarely obvious what it is referring to, especially if the functions are longer than a couple of lines. I always use a more descriptive alias. In my examples above, I’d probably use clickedEl.