Understanding $.proxy() in jQuery

I have written this function:

function my_proxy (func,obj)
{
    if (typeof(func)!="function")
        return;

    // If obj is empty or another set another object 
    if (!obj) obj=this;

    return function () { return func.apply(obj,arguments); }
}

What it ultimately does is it ensures that the value of this in a function will be the value you desire.

A common example is in a setTimeout that takes place inside a click handler.

Take this:

$('#myElement').click(function() {
        // In this function, "this" is our DOM element.
    $(this).addClass('aNewClass');
});

The intention is simple enough. When myElement is clicked, it should get the class aNewClass. Inside the handler this represents the element that was clicked.

But what if we wanted a short delay before adding the class? We might use a setTimeout to accomplish it, but the trouble is that whatever function we give to setTimeout, the value of this inside that function will be window instead of our element.

$('#myElement').click(function() {
    setTimeout(function() {
          // Problem! In this function "this" is not our element!
        $(this).addClass('aNewClass');
    }, 1000);
});

So what we can do instead, is to call $.proxy(), sending it the function and the value we want to assign to this, and it will return a function that will retain that value.

$('#myElement').click(function() {
   // ------------------v--------give $.proxy our function,
    setTimeout($.proxy(function() {
        $(this).addClass('aNewClass');  // Now "this" is again our element
    }, this), 1000);
   // ---^--------------and tell it that we want our DOM element to be the
   //                      value of "this" in the function
});

So after we gave $.proxy() the function, and the value we want for this, it returned a function that will ensure that this is properly set.

How does it do it? It just returns an anonymous function that calls our function using the .apply() method, which lets it explicitly set the value of this.

A simplified look at the function that is returned may look like:

function() {
    // v--------func is the function we gave to $.proxy
    func.apply( ctx );
    // ----------^------ ctx is the value we wanted for "this" (our DOM element)
}

So this anonymous function is given to setTimeout, and all it does is execute our original function with the proper this context.


Without going into greater detail (which would be necessary because this is about Context in ECMAScript, the this context variable etc.)

There are three different types of "Contexts" in ECMA-/Javascript:

  • The global context
  • Function context
  • eval context

Every code is executed in its execution context. There is one global context and there can be many instances of function (and eval) contexts. Now the interesting part:

Every call of a function enters the function execution context. An execution context of a function looks like:

The Activation Object
Scope Chain
this value

So the this value is a special object which is related with the execution context. There are two functions in ECMA-/Javascript which may change the this value in a function execution context:

.call()
.apply()

If we have a function foobar() we can change the this value by calling:

foobar.call({test: 5});

Now we could access in foobar the object we passed in:

function foobar() { 
    this.test // === 5
}

This is exactly what jQuery.proxy() does. It takes a function and context (which is nothing else than an object) and links the function by invoking .call() or .apply() and returns that new function.

Tags:

Javascript

Jquery

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