Question

I am reading this one JS book that talks about inheritance and I am really confused at what each line is doing. Can someone let me know if my understanding is correct or not.

function classA(sColor){
   this.color = sColor;
   this.sayColor = function (){
      alert(this.color);
   };
}

function ClassB(sColor, sName){
   this.newMethod = classA; //assigning a reference to a parent class
   this.newMethod(sColor);  //calling the constructor of the parent class

   //question: so what is 'this' then? from my understanding when we call 'new ClassB('a','b')'
   //we instantiate a new object and that new object becomes the 'this' in this case. Does this
   //line mean we are converting the this to classA?

   delete this.newMethod;   //deleting reference to the CONSTRUCTOR of the parent class

   this.name = sName;
   this.sayName = function(){
      alert(this.name);
   }

}

Answer 1

How this works

function ClassB(sColor, sName){
   this.newMethod = classA; 
   this.newMethod(sColor);  

   delete this.newMethod;

   this.name = sName;
   this.sayName = function(){
      alert(this.name);
   }
}

Is a mediocre way of doing this

function ClassB(sColor, sName){
   classA.call(this, sColor); 

   this.name = sName;
   this.sayName = function(){
      alert(this.name);
   }
}

var f = new ClassB("green", "boy");
f.sayName(); // boy
f.sayColor(); // green

Your basically calling the classA constructor with your this object.

JavaScript does not have classes, it just has objects and functions that manipulate objects. Your ClassA function manipulates this and so does ClassB.

ClassA is just a function that manipulates an object. In this case it manipulates the context object which is this. All ClassB is doing is saying

• let ClassA manipulate this
• add a property called name to this
• add a method called sayName to this

Bonus:

There’s a better way to do OO in JavaScript

// A is a blueprint for an object with a method sayColor
var A = (function() {
  var self = Object.create({});
  self.sayColor = function() { 
    alert(this.color);
  };
  return self;
}());

// B is a blueprint for an object with methods sayColor and sayName
var B = (function() {
  // clone the methods from A
  var self = Object.create(A);
  self.sayName = function() {
    alert(this.name);
  };
  return self;
}());

// clone B and set the properties for name and color
var b = Object.create(B, {
  name: { value: "boy" },
  color: { value: "green" }
});

/* or use a factory

var bFactory = function(name, color) {
  return Object.create(B, {
    name: { value: name },
    color: { value: boy }
  });
}

b = bFactory("boy", "green");

*/

b.sayName();
b.sayColor();

Use Object.create which is ES5 so use the ES5-shim to support legacy browsers.