Accessing `this` in Ajax callback, all within an Object

Question

I'm dealing with a problem about an Ajax callback inside of an Object. Please consider this code :

Search.prototype =
{
    ask : function( query )
    {
        // Display loader
        $('.loader').show();

        $.ajax({
            dataType : 'jsonp',
            type : 'GET',
            url : 'http://api.deezer.com/search/track/',
            data : {
                output : 'jsonp',
                q : query
            }
        }).done(function(res) {

            this.loadResults( res );
            // [Error] Object success has no method 'loadResult'

        });
    },

    loadResults : function (res)
    {
        // Hide loader
        $('.loader').hide();

        console.log( res );

        // doing some stuff
        // ...
    }
}

var search = new Search();
search.ask( 'eminem' );

I get an error Object success has no method loadResult , which makes sense as the callback is part of an anonymous jQuery function.

But how to get my initial object instance ?

I've been trying with a var that = this; before the Ajax call, but I't won't works for the same reasons.

I don't know if it's possible to do this or if the problem comes from my code global organization. Feel free to advise me about the best practices :)

Thanks by advance.

[Update (solved)]

I obfuscated some things in my code which I though it was unnecessary to post here, but I finally found out the problem a little bit earlier in my code. Sorry about that.

Here's my full code, which is now working :

define(['jquery'], function($) {

    var Search = function()
    {
        this._keyDownTimer = 0;
        this._searchDelay = 1000; // ms
    };

    Search.prototype =
    {
        // The init function that I though it was unnecessary to post here. Sorry :/
        init : function()
        {
            $('#q').on('keydown', (function(evt) {

                clearTimeout( this._keyDownTimer );

                this._keyDownTimer = setTimeout( (function() {

                    this.ask( $('#q').val() );

                }).bind( this ), this._searchDelay); /* <-- Here's the solution.
                                                        I forgot to bind `this`
                                                        to the timeout callback function,
                                                        so the `this` under all of my code
                                                        was referring to the anonymous function
                                                        of this callback. */

            }).bind( this ));
        },

        ask : function( query )
        {
            // Display loader
            $('.loader').show();

            console.log(this); // Now `this` refers to my object :)

            var req = $.ajax({
                dataType : 'jsonp',
                type : 'GET',
                url : 'http://api.deezer.com/search/track/',
                context : this,
                data : {
                    output : 'jsonp',
                    q : query
                }
            });

            req.done(function(res) {
                this.loadResults(res);
            });
        },

        loadResults : function (res)
        {
            // Hide loader
            $('.loader').hide();

            // doing some stuff
            // ...
        }
    };

    return new Search;

});

Thanks for your replies, it really helped.

Pb solved.

Answer 1

There are several ways to do this.

You can set the context setting for the ajax options:

jQuery.ajax context setting

$.ajax({
    context: this

Function.prototype.bind

    .done(function (res) {

    }.bind(this));

However, this is not as widely supported as...

jQuery.proxy

Created for this purpose.

    .done($.proxy(function (res) {

    }, this);

Assigning this to another value

var self = this;
$.ajax({
/* snip */
.done(function (res) {
    self.loadResults(res);

This is commonly done in JavaScript to give access to this in lower scopes.

Arrow functions with lexical binding

$.ajax({
/* snip */
.then(res => this.loadResults(res));

Answer 2

You can use the ES5 .bind function to set this correctly:

$.ajax(...).done(this.loadResults.bind(this));

(use the shim at the above link, or the jQuery $.proxy equivalent on older browsers).

Alternatively, add context: this to the $.ajax options:

$.ajax({
    ...,
    context: this
}).done(this.loadResults);

Note that in either case you will override jQuery's default behaviour of passing the ajax option object in this.

p.s. it's also good practise to return the result of the $.ajax() chain so that the user of your object can chain additional callbacks (e.g. a .fail handler) to it.

Answer 3

You can use the $.ajax() context object here like:

$.ajax({
    url : 'http://api.deezer.com/search/track/',
    context: this,
    ...
}).done(function (res) {
    this.loadResults( res );
});