javascript reference to prototype function is undefined in constructor

Question

Given the following object:

function MyObject() {
  window.myFunc1 = this.myFunc1;
  window.myFunc2 = this.__proto__.myFunc2;
}

MyObject.prototype.anotherFunc = function () {
  window.myFunc3 = this.myFunc3;
  window.myFunc4 = this.__proto__.myFunc4;
}

MyObject.prototype.myFunc1 = function () { console.log(1); }

MyObject.prototype.myFunc2 = function () { console.log(2); }

MyObject.prototype.myFunc3 = function () { console.log(3); }

MyObject.prototype.myFunc4 = function () { console.log(4); }

var o = new MyObject();
o.anotherFunc();
window.myFunc1();
window.myFunc2();
window.myFunc3();
window.myFunc4();

In fiddle I'm getting what is expected, but when testing with an ios app under ionic framework the results (tested with safari debugger) are:

  window.myFunc1 -> undefined
  window.myFunc2 -> it works!
  window.myFunc3 -> it works!
  window.myFunc4 -> it works!

Isn't the prototype attached to the object before it is created?

EDIT

attached fiddle and working example, fine tuned the question

Uh, are you actually calling MyObject and anotherFunc anywhere? — Bergi
– Bergi, Commented Oct 22, 2014 at 20:13

shock_one · Accepted Answer · 2015-04-02 22:28:42Z

1

It may happen if you create your object in an unusual way, and this is bound to something different from what you expect. In my case I was exporting a constructor from the node module, and creating an object with new require('module')();, which had the same effect of prototype functions being undefined.

answered Apr 2, 2015 at 22:28

shock_one

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Comments

Bergi · Accepted Answer · 2014-10-22 20:19:19Z

0

I'm getting the following results […]

I cannot reproduce that. Try this:

function MyObject() {
  console.log(this.myFunc1);
  console.log(Object.getPrototypeOf(this).myFunc2);
}
MyObject.prototype.anotherFunc = function () {
  console.log(this.myFunc3);
  console.log(Object.getPrototypeOf(this).myFunc4);
}

MyObject.prototype.myFunc1 = function () { }
MyObject.prototype.myFunc2 = function () { }
MyObject.prototype.myFunc3 = function () { }
MyObject.prototype.myFunc4 = function () { }

var o = new MyObject();
o.anotherFunc()

Isn't the prototype attached to the object before it is created?

Uh, no. The prototype cannot really be attached to a non-existing object :-) Best think of it as happening simultaneously, that an object is created with a prototype, it doesn't exist without one.

What you might have meant that the object (with its prototype) is created before the constructor is executed - yes.

answered Oct 22, 2014 at 20:19

Bergi

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7 Comments

Miquel Over a year ago

Then I'm not understanding why this.myFunc1 in object creator is undefined

Bergi Over a year ago

It was not. Your example is unreproducable. Maybe you can create a fiddle or so that demonstrates the issue?

Miquel Over a year ago

Well I was doing so when I realized that in fiddle it works right. I was testing it in an ios ionic app (angular) in the safari debugger I have always seen window.myFunc1 is undefined...

Bergi Over a year ago

Maybe you had a typo in one of the three places where the identifier myFunc1 was used?

Miquel Over a year ago

Let us continue this discussion in chat.

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