[javascript] What does 'var that = this;' mean in JavaScript?

In a JavaScript file I saw:

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

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

Sometimes this can refer to another scope and refer to something else, for example suppose you want to call a constructor method inside a DOM event, in this case this will refer to the DOM element not the created object.

HTML

<button id="button">Alert Name</button>

JS

var Person = function(name) {
  this.name = name;
  var that = this;
  this.sayHi = function() {
    alert(that.name);
  };
};

var ahmad = new Person('Ahmad');
var element = document.getElementById('button');
element.addEventListener('click', ahmad.sayHi); // => Ahmad

Demo

The solution above will assing this to that then we can and access the name property inside the sayHi method from that, so this can be called without issues inside the DOM call.

Another solution is to assign an empty that object and add properties and methods to it and then return it. But with this solution you lost the prototype of the constructor.

var Person = function(name) {
  var that = {};
  that.name = name;
  that.sayHi = function() {
    alert(that.name);
  };
  return that;
};

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.

JS Fiddle

function usesThis(name) {
    this.myName = name;

    function returnMe() {
        return this;        //scope is lost because of the inner function
    }

    return {
        returnMe : returnMe
    }
}

function usesThat(name) {
    var that = this;
    this.myName = name;

    function returnMe() {
        return that;            //scope is baked in with 'that' to the "class"
    }

    return {
        returnMe : returnMe
    }
}

var usesthat = new usesThat('Dave');
var usesthis = new usesThis('John');
alert("UsesThat thinks it's called " + usesthat.returnMe().myName + '\r\n' +
      "UsesThis thinks it's called " + usesthis.returnMe().myName);

This alerts...

UsesThat thinks it's called Dave

UsesThis thinks it's called undefined

Here is an example `

$(document).ready(function() {
        var lastItem = null;
        $(".our-work-group > p > a").click(function(e) {
            e.preventDefault();

            var item = $(this).html(); //Here value of "this" is ".our-work-group > p > a"
            if (item == lastItem) {
                lastItem = null;
                $('.our-work-single-page').show();
            } else {
                lastItem = item;
                $('.our-work-single-page').each(function() {
                    var imgAlt = $(this).find('img').attr('alt'); //Here value of "this" is '.our-work-single-page'. 
                    if (imgAlt != item) {
                        $(this).hide();
                    } else {
                        $(this).show();
                    }
                });
            }

        });
    });`

So you can see that value of this is two different values depending on the DOM element you target but when you add "that" to the code above you change the value of "this" you are targeting.

`$(document).ready(function() {
        var lastItem = null;
        $(".our-work-group > p > a").click(function(e) {
            e.preventDefault();
            var item = $(this).html(); //Here value of "this" is ".our-work-group > p > a"
            if (item == lastItem) {
                lastItem = null;
                var that = this;
                $('.our-work-single-page').show();
            } else {
                lastItem = item;
                $('.our-work-single-page').each(function() {
                   ***$(that).css("background-color", "#ffe700");*** //Here value of "that" is ".our-work-group > p > a"....
                    var imgAlt = $(this).find('img').attr('alt'); 
                    if (imgAlt != item) {
                        $(this).hide();
                    } else {
                        $(this).show();
                    }
                });
            }

        });
    });`

.....$(that).css("background-color", "#ffe700"); //Here value of "that" is ".our-work-group > p > a" because the value of var that = this; so even though we are at "this"= '.our-work-single-page', still we can use "that" to manipulate previous DOM element.

This is a hack to make inner functions (functions defined inside other functions) work more like they should. In javascript when you define one function inside another this automatically gets set to the global scope. This can be confusing because you expect this to have the same value as in the outer function.

var car = {};
car.starter = {};

car.start = function(){
    var that = this;

    // you can access car.starter inside this method with 'this'
    this.starter.active = false;

    var activateStarter = function(){
        // 'this' now points to the global scope
        // 'this.starter' is undefined, so we use 'that' instead.
        that.starter.active = true;

        // you could also use car.starter, but using 'that' gives
        // us more consistency and flexibility
    };

    activateStarter();

};

This is specifically a problem when you create a function as a method of an object (like car.start in the example) then create a function inside that method (like activateStarter). In the top level method this points to the object it is a method of (in this case, car) but in the inner function this now points to the global scope. This is a pain.

Creating a variable to use by convention in both scopes is a solution for this very general problem with javascript (though it's useful in jquery functions, too). This is why the very general sounding name that is used. It's an easily recognizable convention for overcoming a shortcoming in the language.

Like El Ronnoco hints at Douglas Crockford thinks this is a good idea.

The use of that is not really necessary if you make a workaround with the use of call() or apply():

var car = {};
car.starter = {};

car.start = function(){
    this.starter.active = false;

    var activateStarter = function(){
        // 'this' now points to our main object
        this.starter.active = true;
    };

    activateStarter.apply(this);
};