I'm not sure exactly how to phrase my question, so let me present an example:

function foo() {
  window.addEventListener("keydown", function(event) {
        bar(event.keycode);
}

foo.prototype.bar = function (keycode) {
//code
}

I've tried using this.bar(), but that results in using the window as this. Is there a way to do this, or will I have to call another initialize method manually?

I'm not sure exactly how to phrase my question, so let me present an example:

function foo() {
  window.addEventListener("keydown", function(event) {
        bar(event.keycode);
}

foo.prototype.bar = function (keycode) {
//code
}

I've tried using this.bar(), but that results in using the window as this. Is there a way to do this, or will I have to call another initialize method manually?

• javascript
• html
• oop
• prototype

3 Answers 3

Bind this.bar to this before you pass it.

function foo() {
    window.addEventListener("keydown", this.bar.bind(this), false);
}


foo.prototype.bar = function (event) {
    console.log(event.keyCode);
}

demo http://jsfiddle.net/2tee4/

An alternative solution if you don't have Function.prototype.bind available*, and you're unwilling to add extra functions to Function.prototype would be to close over the call to this.bar:

function foo() {
    var self;
    self = this;
    window.addEventListener('keydown', function (e) {
        self.bar(e);
    }, false);
}
foo.prototype.bar = function (e) {
    console.log(e.keyCode);
}

_{* although your use of addEventListener without attachEvent leads me to believe that Function.prototype.bind would be an acceptable choice}

Additionally, libraries such as jQuery may include their own form of bind, jQuery's is jQuery.proxy.

If your intention is to add a new listener for every foo created, another option is to make foo implement the EventListener interface, and simply pass this in place of the handler.

function Foo() {
    window.addEventListener("keydown", this, false);
}

Foo.prototype.bar = "foobar";

Foo.prototype.handleEvent = function(e) {
    console.log(e.type);   // "mousedown" (for example)
    console.log(this.bar); // "foobar"
};

new Foo();

Note that this only works with addEventListener().

DEMO: http://jsfiddle.net/k93Pr/