Type overloading in Javascript

One of the core aspects to the Simpli5 Javascript library is that the components you build don’t wrap DOM elements. They actually are DOM elements. This is accomplished using what I call type overloading. How it works is you take an HTML element in the DOM and make it a different type. It is important–or maybe just ethical ;) –that this new type is a subclass of the original element type. You

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don’t want to find out that a div is no longer a div, and Firefox has issues with this anyway. So how do you type overload? First create your new type.

 function MyDiv() { this.addClass('my-div'); } 

Then it needs to extend the HTMLDivElement type. There is a trick to this using a dummy type.

 function dummy(){} dummy.prototype = HTMLDivElement.prototype; MyDiv.prototype = new dummy(); 

Then we can add methods to the MyDiv.prototype which we may later call.

 MyDiv.prototype.addClass = function(className) { var classes = this.className ? this.className.split(/\s+/) : []; if (classes.indexOf(className) == -1) classes.push(className); this.className = classes.join(' '); }; 

Now we have our new type. We can’t just create it because to get an HTML element you have to go through document.createElement(). We can either do that or grab an element already in the DOM. Let’s do the latter.

 var div = document.getElementsByTagName('div')[0]; 

Now we have our div. The next part is type overloading. On every Javascript object is a special property “__proto__” (two underscores on both sides) which points to the object’s prototype. It also has “constructor” which points to its type function. We just have to replace these two with our own, changing the type of the object at runtime.

 div.__proto__ = MyDiv.prototype; div.constructor = MyDiv; div.constructor(); // call the constructor for any initialization that needs to happen. 

We now have a div which is of type MyDiv. The following code all returns true and works.

 alert(div instanceof HTMLDivElement); // true since we subclassed it alert(div instanceof MyDiv); // also true since this is our new type div.addClass('foo'); // works, no errors, the method exists on div alert(div.className); // "my-div foo" because our constructor added my-div and we just added foo 

It’s a really neat thing that you can make objects become new types at runtime. And since every HTML5 browser supported it at the time I wrote Simpli5 it seemed like a great solution. Unfortunately, not all browsers support the __proto__ property. Now that IE9 is publicly available in beta I’ve given it a look. It does not support __proto__. After all, __proto__ isn’t a standardized property. So for Simpli5 I’ll have to for-i-in copy all the methods of my new type over to the instance of an element. It will be slower, and “div instanceof MyDiv” won’t return true, but it should still work for our needs. Hey Microsoft, I know it isn’t a standard property, but do you think you could make the __proto__ object a publicly accessible read/write property? I would really appreciate it!