Javascript scope addEventListener and this

Question

I am a C# developer experimenting with JavaScript and I'm trying to get my head around the scope :)

I have the following code which contains an addEventListener in which I want to use a field from my object:

(function(window) {

    function Keyboard() {
        this.keys = {};
    }

    Keyboard.prototype.handle_keydown = function(args) {
        this.keys[args.keyCode] = true;
    }

    Keyboard.prototype.listen = function() {
        window.addEventListener('keydown', this.handle_keydown);
    }

    app.util.keyboard = new Keyboard();

})(window);

I would like to use the keys array in my hander, but understand that I cannot access is by using this, because this is the window in that context (correct?). If I change it to

app.util.keyboard.keys[args.keyCode] = true;

it works, but I'm not sure that's a good way to fix it.

I found this question, which seems rather similar, but Im not sure how I can fit it into my example.

Thanks for your help!

Answer 1

A few things:

• Most people will suggest something like var self = this because it's fast and easy.

• But var self = this does not separate the view object entirely from the view logic, which coming from a more formal C# background and looking at your code, sounds like something you want to do.

• In order to have the callback execute only when the event fires, wrap the handler in a function, so that it's evaluated right away, but only executed when and if a keydown event fires (see the code below).

• Understanding scope in JS: Whatever the execution context is, is also the current scope. Your listener was added in a method (called listen) on Keyboard.prototype, but the keydown event is actually fired on window -- the handler is executing in a different context than where it was defined; it's executing within the context of what is invoking it, in this case, window, so it's scoped to window unless you bind it to another object via bind or apply when it's defined.

In your code, window is the view a user's interacting with, and Keyboard is that view's controller. In MVC patterns like what you're probably used to in C#/.NET, views don't tell themselves what to do when things happen, controllers tell views what to do. So, if you were to assign a reference to the controller by using var self = this like so many do, the view would be managing itself -- but only for that specific handler for keydown events. This is inconsistent and would become hard to manage in a large project.

A solution:

Keyboard.prototype.listen = function() {
    window.addEventListener('keydown', function(e) {
        this.handle_keydown(e);
    }.bind(this), false);
}

A better solution:

Keyboard.prototype.view = window;

Keyboard.prototype.listen = function() {
    this.view.addEventListener('keydown', function(e) {
        this.handle_keydown(e);
    }.bind(this), false);
}

The best solution (until ES6 class is ready):

// define
function addViewController(view) {

    function ViewController() {

        this.handle_keydown = function(args) {
            // handle keydown events
        };

        this.listen = function() {
            this.view.addEventListener('keydown', function(e) {
                this.handle_keydown(e);
            }.bind(this), false);
        };

        this.view = view;
        return this;

    }

    return new ViewController(view);

}

// implement
var keyboard = addViewController(window);
keyboard.listen();

• Note: .bind() is compatible with ECMAScript 5+; if you need a solution for older browsers, Mozilla has posted a great alternative to .bind() using functions and .call():

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Function/bind

Edit: Here's what your instantiated keyboard object will look like using this new, modular solution: