“this” keyword in event methods when using JavaScr

Question

I'm trying to access the member variables of a prototype class in JavaScript in an event handler -- something I'd typically use the "this" keyword for (or "that" [copy of this] in the case of event handlers). Needless to say, I'm running into some trouble.

Take, for example, this HTML snippet:

<a id="myLink" href="#">My Link</a>

And this JavaScript code:

function MyClass()
{
  this.field = "value"
  this.link = document.getElementById("myLink");
  this.link.onclick = this.EventMethod;
}

MyClass.prototype.NormalMethod = function()
{
  alert(this.field);
}

MyClass.prototype.EventMethod = function(e)
{
  alert(this.field);
}

Instantiating a MyClass object and calling NormalMethod works exactly like I expect it to (alert saying "value"), but clicking the link results in an undefined value because the "this" keyword now references the event target (the anchor () HTML element).

I'm new to the prototype JavaScript style, but in the past, with closures, I've simply made a copy of "this" in the constructor:

var that = this;

And then I could access members variables in event methods via the "that" object. That doesn't seem to work with prototype code. Is there another way to achieve this?

Thanks.

Answer 1

You need:

this.link.onclick = this.EventMethod.bind(this);

...'bind' is part of Prototype, and returns a function which calls your method with 'this' set correctly.

Answer 2

You should try

this.link.onclick = this.EventMethod.bind(this);

Answer 3

Your "that=this" closure idiom is still applicable:

function MyClass()
{
    ...

    var that = this;
    this.link.onclick = function() {
        return that.EventMethod.apply(that, arguments);

        // that.EventMethod() works too here, however
        // the above ensures that the function closure
        // operates exactly as EventMethod itself does.

    };
}

Answer 4

As stated above, using bind which is a part of the Prototype library is a clean way to solve this problem. This question is a duplicate of another SO question which is answered here with implementation of the bind method without including the whole prototype library :

https://stackoverflow.com/a/2025839/1180286