Dealing with Scope in Object methods containing 'this' keyword called by Event Listeners

I've generalized my lack of understanding of the situation to this small problem. Here's what I think I know so far:

I have an object myDog (a global variable). Dog has a member variable el that is an html element; because it's an element, I can add event listeners to it. So, when you click on myDog.el, it logs to the console the values of this.name and myDog.name. As expected because of scope, this.name is undefined and myDog.name is 'tye'. this inside of Dog.speak when invoked by the click event listener refers to the element that was clicked, the member variable el, not the object Dog. Since myDog is a global variable, it's able to pick back up regardless of the function's scope and get to myDog.name just fine.

See code below:

function Dog(name,id) {
    this.name = name ? name : "spot";
    this.id = id ? id : "dog";
    this.el = document.getElementById(this.id); // given there is a div with a matching    
    this.el.addEventListener("click",this.speak); // ignore IE for simplicity (attachEvent has its own 'this' scope issues)
}

Dog.prototype = {
    speak: function() {
        console.log("this.name: "+this.name+"\nmyDog.name: "+myDog.name);
    }
};

var myDog = new Dog("tye","dog1");

So... my questions are

1) What are some strategies for attaching objects to html elements, so that I can go from this.el back to myDog (this.el's owner) without myDog being a global variable?

2) Are global variables in this case a necessary evil? And if so, what are so good strategies in this case to gracefully use them? For example, what if I wanted 100 dogs instantiated? How would I handle all those global variables in Dog.speak?

Here's a jsfiddle version if you want to play with it: http://jsfiddle.net/chadhutchins/Ewgw5/

Answers:

Answer

"1) What are some strategies for attaching objects to html elements..."

Since you're using .addEventListener(), I'd suggest taking advantage of a feature of it that few people seem to know about... making your Dog object implement the EventListener interface.

This establishes a very clean relationship between your Dog data and its associated element.

Only minor changes are required. Code first... explanation below.


DEMO: http://jsfiddle.net/Ewgw5/1/

function Dog(name,id) {
    this.name = name ? name : "spot";
    this.id = id ? id : "dog";
    this.el = document.getElementById(this.id);

    // ---------------------------------v----no function!
    this.el.addEventListener("click", this);
}

Dog.prototype = {
    // Implement the `EventListener` interface   
    handleEvent: function(event) {
        switch (event.type) {
            case "click": return this.speak();
        }
    },
    speak: function() {
        console.log("this.name: "+this.name+"\nmyDog.name: "+myDog.name);
    }
};

var myDog = new Dog("tye","dog1");

So all I did was pass this in the constructor to addEventListener() instead of passing a function, and then I added a handleEvent() method to Dog.prototype.

Now when a "click" event occurs, it will invoke the handleEvent() method. The value of this in that method will be your Dog instance. So from there you can call whatever method(s) you need.

Because you made the element a property of this, you can access the element via this.el. But that's technically not even necessary, since the element is also available via the event object as event.currentTarget.


"2) Are global variables in this case a necessary evil..."

Thankfully no!


This behavior should be part of your shim for .addEventListener().

Answer

You can try something like this:

function Dog(name,id) {
   var self = this;   // <---- NEW BIT saving a reference to this
   this.name = name || "spot"; // <-- unrelated tidy-up to use || instead of ?:
   this.id = id || "dog";
   this.el = document.getElementById(this.id); // given there is a div with a matching    
   this.el.addEventListener("click",function(){ self.speak(); });
        // ignore IE for simplicity (attachEvent has its own 'this' scope issues)
}

Through the magic of closures the anonymous function I've added within addEventListener() has access to the scope of the containing function even after the containing function returns, so it is able to use self which holds a reference to the original object saved from this when Dog() was called as a constructor.

EDIT: Sorry, I didn't directly address the questions you numbered (1) and (2), but as you can see you don't need global variables to fix this this issue. With the technique I described you could instantiate 100 dogs and they'd all work. (Well, they'd all speak anyway: they'd all work if you added Dog.prototype.work = function() { }.)

Answer

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

 this.el.addEventListener("click", this.speak.bind(this));

This way is preferable because it doesn't require allocating a scope in order to pass the binding. Scope allocation is one of the more costly things in JS.

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