A callback is a programming term that refers to a function that is passed as a parameter to another function. Callbacks are frequently used in asynchronous programming. For example in JavaScript when you make an AJAX request to the server, you pass the AJAX request callback functions that it will execute when the request completes or fails.

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Overcoming Scope Limitations in jQuery AJAX Callbacks

Update: Not long after writing this post, I realized that $.ajax() has a configuration option named context, which allows you to assign scope to a callback method. This knowledge renders the specifics of what I say below pretty much irrelevent, but closures remain a very slick solution to scoping problems in other situations.

If learning jQuery is teaching me anything, it’s the power of closures.  Recently I’ve been working on a jQuery-powered gallery for a client.  My code dynamically retrieves JSON data from the server. The problem is that such a construct as the one below doesn’t work:

Gallery = function(id)
{
    this.id = id;
 
    this.loadGallery = function()
    {
        $.ajax({
            'url' : '/gallery.php',
            'data' : { 'gallery_id' : this.id },
            'dataType' : 'json',
            'success' : this.populate
        });
    };
 
    this.populate = function(data)
    {
        // populate() gets called, but not in the scope of the Gallery object.
        // So the following call will output "undefined".
        alert(this.id);
    };
};

Unfortunately jQuery doesn’t provide us with a mechanism to set the scope of our callbacks like some frameworks but fortunately for us JavaScript provides a way around that: the closure.

Gallery = function(id)
{
    // ...
 
    this.loadGallery = function()
    {
        $.ajax({
            'url' : '/gallery.php',
            'data' : { 'gallery_id' : this.id },
            'dataType' : 'json',
            'success' : function(gallery) {
                return function(data) { gallery.populate(data) };
            }(this)
        });
    };
 
    // ...
};

What the heck does this all mean? In JavaScript it’s completely legal to create an anonymous function (also known as a closure) and call it, all in a single statement. So:

function(gallery){ ... }(this)

creates an anonymous function and immediately calls it, passing this, which is a reference to the Gallery instance on which the loadGallery is being called, as the first parameter to the function. What are we doing inside that function?

return function(data) { gallery.populate(data) };

Remember: in JavaScript everything is an Object including functions, so it’s completely legal for one function to return a reference to another function or, in this case, an anonymous function .

The closure being returned has a single argument named data, and because it is defined within the scope of the outer closure, we can gall methods on gallery, such as populate(). It’s a long way to go to resolve the scoping issue, but it does work.