Question

In the book “JavaScript the definitive guide 5 edition“, section 9.2 Prototypes and Inheritance, I find the following words:

In the previous section, I showed that
the new operator creates a new, empty
object and then invokes a constructor
function as a method of that object.
This is not the complete story,
however. After creating the empty
object, new sets the prototype of that
object. The prototype of an object is
the value of the prototype property of
its constructor function. All
functions have a prototype property
that is automatically created and
initialized when the function is
defined. The initial value of the
prototype property is an object with a
single property. This property is
named constructor and refers back to
the constructor function with which
the prototype is associated. (You
may recall the constructor property
from Chapter 7 ; this is why every
object has a constructor property.)
Any properties you add to this
prototype object will appear to be
properties of objects initialized by
the constructor.

Now, if that is true, how could prototypal inheritance exists? I mean, let’s say the prototype object of a constructor function has a constructor property initially. Because the prototype object itself is an object, to determine its constructor we often use prototype_object.constructor. But now the prototype_object already has a constructor property itself, and it points to the constructor function with which the prototype is associated. In this situation how can inheritance exists?

Answer 1

Let say, Dog is a Mammal.

function Mammal() {
  this.milk = true;
};

function Dog() {
this.bark = true;
}
Dog.prototype = new Mammal;

So prototype of Dog points to an object of Mammal. This Mammal object has a reference to its constructor so when Dog is new, JavaScript see that Dog prototype is a Mammal so Mammal's constructor is called to produce a valid Mammal object (another one) then make it a Dog object using Dog constructor.

From this, the constructor of Dog.prototype is a Mammal (a Mammal Object that has extra fields and functions added) BUT constructor of Dog is Dog. The inheritance exist because the an instance of Dog has a Mammal as a prototype; hence, Dog is a Mammal. When a method is called and JS cannot find it from Dog.prototype, JS look in Mammal.prototype (which is an Object that has extra fields and functions added).

Hope this helps.