Javascript nested prototypes

Question

The other day I was fiddling with Javascript, and I noticed that I can't write a prototype to an object within a prototype function. Like this:

var obj = function() { }

obj.prototype.First = function() {
    this.prototype.Second = function() {
        alert("Second Prototype");
    }
}

obj.First();
obj.Second();

For some reason, the second prototype won't work and the code doesn't run. Is my syntax wrong or are there some prototype limitations that I don't know about? Thanks!

Edit:
I'm not trying to add a prototype to a prototype... that wouldn't make much sense. This is what I'm trying to do: Add two separate prototypes to obj. Prototype 2 is defined when prototype 1 is called. I thought that this would contain a reference to object, so this.prototype would be the same as obj.prototype, but it doesn't work as expected.

`obj` is a function. `obj.prototype` is a property of the function and only used when you call the function with `new`. E.g. `var foo = new obj();`. `foo.First` would now be available because `obj.prototype` is the prototype of `foo`. However, `obj.First` can't work because you never created the `First` property on `obj`. — Felix Kling, Sep 30 '14 at 20:53

score 2 · Answer 1 · answered Jan 31 '17 at 17:11

This is an old question, but I thought I'd add my two cents. This code is trying to add functions on 'prototype'. However, this can only be done on the class name. What you have is a variable pointing to an anonymous class. To access the variable of an anonymous variable use 'proto'. The below is the same as your example, except, using proto it is 'successful'. Although, I don't see the benefit of using prototypes like this as the prototype added methods only apply to the anonymous instance 'obj'. http://jsbin.com/zapocamipi/edit?js,console

var obj = function() { }

obj.__proto__.First = function() { 
  console.log("First Prototype");
    this.__proto__.Second = function() {
        console.log(this);
    }
}

obj.First();
obj.Second();

score 1 · Answer 2 · answered Sep 30 '14 at 00:08

1

this.prototype doesn't exist.

If you want to add a property to the instance, use this.

If you want to add a property to the prototype, use Constructor.prototype.

Also, obj is a function (class), not an instance,
You want to create an instance using the new keyword, and you should name the constructor function as UpperCamelCase.

answered Sep 30 '14 at 00:08

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@Blue0500 I changed my answer. When reviewing your question it looks like you would like the instance to change type when calling a certain function. – HMR Sep 30 '14 at 03:17

score 1 · Accepted Answer · edited May 23 '17 at 12:09

Maybe this could help you out understanding the role of a constructor function and the prototype.

Depending on what you're trying to do (obj, First and Second doesn't really show your intention) you could do:

A Person has Eyes. This can be done through composition.

An Employer is a Person but a Person is not necessarily an Employer (can be Client or Consultant too). This could be done through inheritance.

A Cat can move. In a Class based language Cat has to implement Movable but in JavaScript you can use mix ins and leave the implementation to the default implementation that Movable provides or override it. JavaScript does not compile time check if you do implement certain things.

If you would like to change the type of the object instance after calling a certain function then it's dangerous to meddle with the prototype because that will affect all instances of that type.

Maybe you should return an instance of another type.

var Customer = function(name) { 
  this.name=name || 'unknown';
};
Customer.createVipCustomer = function() {
    return new VipCustomer(this);
}
var VipCustomer=function(customer){
  //re use Customer constructor
  Customer.call(this,customer.name);
  this.isVip=true;
}
//inherit the protype defined members
VipCustomer.prototype=Object.create(Customer.prototype);
VipCustomer.prototype.constructor=VipCustomer;
VipCustomer.prototype.second=function(){
  console.log('this is second');
}
var aCustomer = new Customer('Ben');
//update to VipCustomer
aCustomer = Customer.createVipCustomer(aCustomer);
aCustomer.second();

Javascript nested prototypes

3 Answers3