"this" keyword in event methods when using JavaScript prototype object

Question

Hi everyone,

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()
{
  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 is being overridden by the event method (in this case, it instead references the anchor DOM 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

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 2

You should try

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

Answer 3

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.