JavaScript原型继承

Question

I've some code like

function Vehicle(){
  this.isMovable = true;
}
Vehicle.prototype = {
  hasTyres:function(){ return true;},
  needsFuel:true
};

var Car  = function(){
  Vehicle.call(this);
  this.type = "Car";
};

Now

It works even if I create prototype like this

Car.prototype = Object.create(Vehicle.prototype);

or

Car.prototype = Vehicle.prototype;

What is the difference ?

I was under the impression that

Car.prototype = Object.create(Vehicle); 

will cause Car to inherit from vehicle ,but it's not.

Can anyone explain what's happening inside Object.create method

Thanks, SRK

Answer 1

Car.prototype = Object.create(Vehicle.prototype);

This one creates an object whose prototype is Vehicle.prototype . In this object, you put your shared methods for Car instances while "inheriting" from Vehicle . This is the right way to go.

Car instance -> Car prototype -> Vehicle prototype

---

Car.prototype = Vehicle.prototype;

This one uses the same prototype for Vehicle to Car . This means that you'll be clobbering the same object for both classes. Adding to Car.prototype would mean also adding it to Vehicle.prototype , which you don't want.

Car instance -> Car prototype (which is also Vehicle prototype)

---

Car.prototype = Object.create(Vehicle); , Car.prototype is an object whose prototype is Vehicle , a function. You don't want this either.

Car instance -> Car prototype -> Vehicle function

Answer 2

Vehicle is a function. Calling Object.create(Vehicle); will create an object whose prototype is that function.
That's not what you want.

Writing Car.prototype = Vehicle.prototype; will use the same prototype object for both classes, making it impossible to add a function to the derived class only.

For more details, see my blog .