How can I call prototype child method from parent

Question

I want to call child object method form prototype object:

function Hamster() {
    this.sleep = function(){
      console.log('sleep');
    }
}

Hamster.prototype = new function(){

    var _self = this;

    this.test = function(){
      this.sleep();
      //_self.sleep(); //not working
    }
}

lazy = new Hamster();
lazy.test();

Why when I use this.sleep() everything works fine, but when I use _self.sleep() ( _self variable has this value) I get error? Why _self and this inside the test() function are not same?

Answer 1

The this reference stored in self is different from the this reference you call inside the anonymous function you assigned to test() . The point is that the this scope of the function you're creating and assigning to Hamster.prototype is an empty object. It's so because you used the new keyword.

Hamster.prototype = new function(){

    //Stores the reference to the empty object that's the scope of the anonymous 
    //function because it was called using the new keyword.
    var _self = this; 

    this.test = function(){
      //Works because the this keyword is a reference to Hamster.
      this.sleep(); 

      //Does not work because the empty object doesn't have a sleep method.
      //_self.sleep(); 
    }
}

Answer 2

You're confused about how prototypes work.

You probably want nothing more than

Hamster.prototype = {
    test: function() {
        this.sleep();
    }
};

What you are doing is creating a new object for the prototype. The _self variable you are setting is not pointing to the this of individual Hamster objects, but rather to the this of the new prototype object you are creating, which is lost to the world.

Answer 3

Let's write it with a function which returns the two things, letting us compare them more easily

function Constructor() {}

Constructor.prototype = new function () {
    var foo = this;
    this.bar = function () {
        return {"this": this, "foo": foo};
    };
};

var instance = new Constructor(),
    o = instance.bar();

And now test what we've got

o.this === instance; // true
o.foo === instance; // fase
// so what is o.foo ?
o.foo === Constructor.prototype; // true

What does this tell us?

• When we invoke a method from instance , this refers to that instance
• When we invoke a function with new , then this refers to a new object instance created by the new ; in this case it is the object we've then set as the prototype for Constructor

When are these references set?

• The this in instance.bar is only set when instance.bar is actually invoked
• The this from the new function () { ... } is set immediately as it is created and does not change after that (you've basically used an IIFE ), it is also what is returned by the new so what the Constructor.prototype gets set to in this example

Answer 4

_self是您定义的匿名函数之外的局部变量，您没有通过它，因此未在内部定义