Javascript inheritance: call super-constructor or use prototype chain?

Quite recently I read about JavaScript call usage in MDC

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

one linke of the example shown below, I still don't understand.

Why are they using inheritance here like this

Prod_dept.prototype = new Product();

is this necessary? Because there is a call to the super-constructor in

Prod_dept()

anyway, like this

Product.call

is this just out of common behaviour? When is it better to use call for the super-constructor or use the prototype chain?

function Product(name, value){
  this.name = name;
  if(value >= 1000)
    this.value = 999;
  else
    this.value = value;
}

function Prod_dept(name, value, dept){
  this.dept = dept;
  Product.call(this, name, value);
}

Prod_dept.prototype = new Product();

// since 5 is less than 1000, value is set
cheese = new Prod_dept("feta", 5, "food");

// since 5000 is above 1000, value will be 999
car = new Prod_dept("honda", 5000, "auto");

Thanks for making things clearer

Answers:

Answer

The answer to the real question is that you need to do both:

  • Setting the prototype to an instance of the parent initializes the prototype chain (inheritance), this is done only once (since the prototype object is shared).
  • Calling the parent's constructor initializes the object itself, this is done with every instantiation (you can pass different parameters each time you construct it).

Therefore, you should not call the parent's constructor when setting up inheritance. Only when instantiating an object that inherits from another.

Chris Morgan's answer is almost complete, missing a small detail (constructor property). Let me suggest a method to setup inheritance.

function extend(base, sub) {
  // Avoid instantiating the base class just to setup inheritance
  // Also, do a recursive merge of two prototypes, so we don't overwrite 
  // the existing prototype, but still maintain the inheritance chain
  // Thanks to @ccnokes
  var origProto = sub.prototype;
  sub.prototype = Object.create(base.prototype);
  for (var key in origProto)  {
     sub.prototype[key] = origProto[key];
  }
  // The constructor property was set wrong, let's fix it
  Object.defineProperty(sub.prototype, 'constructor', { 
    enumerable: false, 
    value: sub 
  });
}

// Let's try this
function Animal(name) {
  this.name = name;
}

Animal.prototype = {
  sayMyName: function() {
    console.log(this.getWordsToSay() + " " + this.name);
  },
  getWordsToSay: function() {
    // Abstract
  }
}

function Dog(name) {
  // Call the parent's constructor
  Animal.call(this, name);
}

Dog.prototype = {
    getWordsToSay: function(){
      return "Ruff Ruff";
    }
}    

// Setup the prototype chain the right way
extend(Animal, Dog);

// Here is where the Dog (and Animal) constructors are called
var dog = new Dog("Lassie");
dog.sayMyName(); // Outputs Ruff Ruff Lassie
console.log(dog instanceof Animal); // true
console.log(dog.constructor); // Dog

See my blog post for even further syntactic sugar when creating classes. http://js-bits.blogspot.com/2010/08/javascript-inheritance-done-right.html

Technique copied from Ext-JS and http://www.uselesspickles.com/class_library/ and a comment from https://stackoverflow.com/users/1397311/ccnokes

Answer

The ideal way to do it is to not do Prod_dept.prototype = new Product();, because this calls the Product constructor. So the ideal way is to clone it except for the constructor, something like this:

function Product(...) {
    ...
}
var tmp = function(){};
tmp.prototype = Product.prototype;

function Prod_dept(...) {
    Product.call(this, ...);
}
Prod_dept.prototype = new tmp();
Prod_dept.prototype.constructor = Prod_dept;

Then the super constructor is called at construction time, which is what you want, because then you can pass the parameters, too.

If you look at things like the Google Closure Library you'll see that's how they do it.

Answer

If you have done Object Oriented Programming in JavaScript, you will know that you can create a class as follows:

Person = function(id, name, age){
    this.id = id;
    this.name = name;
    this.age = age;
    alert('A new person has been accepted');
}

So far our class person only has two properties and we are going to give it some methods. A clean way of doing this is to use its 'prototype' object. Starting from JavaScript 1.1, the prototype object was introduced in JavaScript. This is a built in object that simplifies the process of adding custom properties and methods to all instances of an object. Let's add 2 methods to our class using its 'prototype' object as follows:

Person.prototype = {
    /** wake person up */
    wake_up: function() {
        alert('I am awake');
    },

    /** retrieve person's age */
    get_age: function() {
        return this.age;
    }
}

Now we have defined our class Person. What if we wanted to define another class called Manager which inherits some properties from Person. There is no point redefining all this properties again when we define our Manager class, we can just set it to inherit from the class Person. JavaScript doesn't have built in inheritance but we can use a technique to implement inheritance as follows:

Inheritance_Manager = {};//We create an inheritance manager class (the name is arbitrary)

Now let's give our inheritance class a method called extend which takes the baseClass and subClassas arguments. Within the extend method, we will create an inner class called inheritance function inheritance() { }. The reason why we are using this inner class is to avoid confusion between the baseClass and subClass prototypes. Next we make the prototype of our inheritance class point to the baseClass prototype as with the following code: inheritance.prototype = baseClass. prototype; Then we copy the inheritance prototype into the subClass prototype as follows: subClass.prototype = new inheritance(); The next thing is to specify the constructor for our subClass as follows: subClass.prototype.constructor = subClass; Once finished with our subClass prototyping, we can specify the next two lines of code to set some base class pointers.

subClass.baseConstructor = baseClass;
subClass.superClass = baseClass.prototype;

Here is the full code for our extend function:

Inheritance_Manager.extend = function(subClass, baseClass) {
    function inheritance() { }
    inheritance.prototype = baseClass.prototype;
    subClass.prototype = new inheritance();
    subClass.prototype.constructor = subClass;
    subClass.baseConstructor = baseClass;
    subClass.superClass = baseClass.prototype;
}

Now that we have implemented our inheritance, we can start using it to extend our classes. In this case we are going to extend our Person class into a Manager class as follows:

We define the Manager class

Manager = function(id, name, age, salary) {
    Person.baseConstructor.call(this, id, name, age);
    this.salary = salary;
    alert('A manager has been registered.');
}

we make it inherit form Person

Inheritance_Manager.extend(Manager, Person);

If you noticed, we have just called the extend method of our Inheritance_Manager class and passed the subClass Manager in our case and then the baseClass Person. Note that the order is very important here. If you swap them, the inheritance will not work as you intended if at all. Also note that you will need to specify this inheritance before you can actually define our subClass. Now let us define our subClass:

We can add more methods as the one below. Our Manager class will always have the methods and properties defined in the Person class because it inherits from it.

Manager.prototype.lead = function(){
   alert('I am a good leader');
}

Now to test it let us create two objects, one from the class Person and one from the inherited class Manager:

var p = new Person(1, 'Joe Tester', 26);
var pm = new Manager(1, 'Joe Tester', 26, '20.000');

Feel free to get full code and more comments at: http://www.cyberminds.co.uk/blog/articles/how-to-implement-javascript-inheritance.aspx

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