Question

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What does var that = this; mean in javascript?

I often find this assignment in Javascript code:

var that = this;

This is an example:

function Shape(x, y) {
    var that= this;

    this.x = x;
    this.y = y;

    this.toString= function() {
        return 'Shape at ' + that.x + ', ' + that.y;
    };
}

Can you please explain why that is needed?

Please bear in mind I am very familiar with PHP or Java but not with the Javascript object model.

Answer 1

The value is this is set when a function is called.

Setting that to this preserves that value for a function defined inside that function (since it would otherwise get a value for this that would depend on how it (the inner function) was called.