Tuesday, October 2, 2012

JavaScript's Pseudo Classical Inheritance diagram

The following is a chart of JavaScript Pseudo Classical Inheritance.  The constructor
Foo is just a class name for an imaginary class.  The foo object is an instance of Foo.

Note that the prototype in Foo.prototype is not to form a prototype chain.  Foo.prototype points to some where in a prototype chain, but this prototype property of Foo is not to form the prototype chain.  What constitute a prototype chain are the __proto__ pointing up the chain, and the objects pointed to by __proto__, such as going from foo.__proto__, going up to foo.__proto__.__proto__, and so forth, until null is reached.

JavaScript's Pseudo Classical Inheritance works like this way: I am a constructor, and I am just a function, and I hold a prototype reference, and whenever foo = new Foo() is called, I will let foo.__proto__ point to my prototype object.  So Foo.prototype and obj.__proto__ are two different concepts.  Foo.prototype indicates that, when an object of Foo is created, this is the point where the prototype chain of the new object should point to -- that is, foo.__proto__ should point to where Foo.prototype is pointing at.

In the ECMA-262 Edition 5.1 spec, the term [[Prototype]] is used.  And that's the same as __proto__.  It is often mentioned as the "[[Prototype]] internal property".  And don't confuse this with a function's prototype property.  One of the key points regarding [[Prototype]] is: "All objects have an internal property called [[Prototype]]. The value of this property is either null or a reference to an object and is used for implementing inheritance."

And now we can see from the diagram why when we inherit Dog from Animal, we would do:

    function Dog() {}    // the usual constructor function
    Dog.prototype = new Animal();
    Dog.prototype.constructor = Dog;

It is to say: first of all, define a function Dog.  This function is an object, as every function in JavaScript is an object.  Now, add a property prototype to the object Dog.  And then, new Animal() will create a new object (and it will become part of the prototype chain), and since this new object is created using the Animal constructor, this new object's __proto__ will point to where Animal.prototype is pointing to.  Now, make Dog.prototype point to this new object.  And that's how the object pointed to by Dog.prototype is created as shown in the diagram.

Next, since that new object was created by the Animal constructor, the new object's constructor property will point to Animal (an object created by constructor Foo will have a constructor property pointing to Foo).  This constructor property is not Dog.prototype's own property -- it is the own property of Dog.prototype.__proto__, so it is an inherited property.  Note that in this case, we actually want the Dog.prototype object's constructor property to point to Dog, and that why we have the second line of code above: Dog.prototype.constructor = Dog;

Note that we can use an empty function F() to set up the above relationship as well:

    function Dog() {}    // the usual constructor function
    function F() {}
    F.prototype = Animal.prototype;
    Dog.prototype = new F();
    Dog.prototype.constructor = Dog;

Because Animal() may take a longer time or more complex logic to run, and it can also create properties in that new object that we don't need.  A way to set up the Dog.__proto__ to point at Animal.prototype is what we need, and F() can already accomplish that.

If there are 3 Dog instances, they would point to the middle of that long prototype chain.  It is still a complete prototype chain, but a shorter one:

Now we can understand why when we add a method to the Animal class, we would use

    Animal.prototype.move = function() { ... };

That's because when we say


If woofie the object doesn't have the move method, it will go up the prototype chain, just like any prototypal inheritance scenario, first to the object pointed to by woofie.__proto__, which is the same as the object that Dog.prototype refers to.  If the method move is not a property of that object (meaning that the Dog class doesn't have a method move), go up one level in the prototype chain, which is woofie.__proto__.__proto__, or the same as Animal.prototype.  Remember we already did

    Animal.prototype.move = function() { ... };

earlier, and so now move is found, and the method can be invoked.

Note again that this is how prototypal inheritance works, and see how "classical inheritance" is simulated: by the help of prototypal inheritance.

Using the diagram, we can also see the working of instanceof.  For foo instanceof Animal, it is true, because we take foo and look at the whole prototype chain, and the Animal.prototype object is part of that chain.  Therefore, it returns true.  woofie instanceof Animal is true for the similar reason: take woofie's whole prototype chain, and the Animal.prototype object is part of that chain.  woofie instanceof Bichon is false because the Bichon.prototype is not part of that chain.  Note that woofie.__proto__ instanceof Animal is true, the same as Dog.prototype instanceof Animal, because instanceof checks for whether the right operand's prototype object is part of the left operand's prototype chain. (Note that Dog.prototype instanceof Dog used to be true, but it has changed in later implementation of JavaScript: so it will go up to see if Dog.prototype is part of the chain, but it won't include the object itself (the left operand) to check against Dog.prototype).

Note that in reality, each constructor function has a __proto__ property as well, and if the Function constructor is also shown here,  a more complete picture is:

Even though foo.constructor === Foo, the constructor property is not foo's own property.  It is actually obtained by going up the prototype chain, to where foo.__proto__ is pointing at.  The same is for Function.constructor.  The diagram can be complicated, and sometimes confusing when we see Constructor.prototype, foo.__proto__, Foo.prototype.constructor.  Note that Firefox, Chrome, Safari, and node.js support __proto__, but IE doesn't support it, and it can be obtained by Object.getPrototypeOf(foo).  (IE 9 or above is needed.  Before IE 9, it can be defined as in John Resig's post, and it requires that the constructor property is set properly.)  To verify the diagram, note that even though foo.constructor will show a value, the property constructor is not foo's own property, but is obtained by following up the prototype chain, as foo.hasOwnProperty("constructor") can tell.