599719 - Objects created from functions with reassigned prototype will have Object as constructor, not the function

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[A. Sel.]

User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 6.1; de; rv:1.9.2.10) Gecko/20100914 Firefox/3.6.10
Build Identifier: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b6) Gecko/20100101 Firefox/4.0b6

Assigning a new prototype-object to a function instead of adding members to the existing prototype will result in newly created objects having the wrong constructor.

Given the following code:

----- BEGIN -----

function TestClass()
{
	this.foo = "bar";
}

TestClass.prototype = {
	testfunc: function testfunc()
	{
		 
	}
}

var test = new TestClass();

----- END -----

test.constructor will be: 

function Object() {
    [native code]
}

Having this code:

----- BEGIN -----

function TestClass()
{
	this.foo = "bar";
}

TestClass.prototype.testfunc =  function testfunc()
{
	
};

var test = new TestClass();

----- END -----

test.constructor will be: 

function TestClass() {
    this.foo = "bar";
}

... which is the correct constructor function.

Checked this in Firefox 3.6 and 4.0b6

Reproducible: Always

Comment 1

[Tom S. (please needinfo tschuster)]

Actually this is correct behaviour. I will try to explain.

TestClass.prototype = {/* ... */}
Will asign a new object to prototype. And the constructor of objects is the Object Function.

So TestClass.prototype.constructor points to ({}).constructor, wich is 
function Object () { }.
If you know cal new TestClass, it will just return the prototype property. And 'constructor' will be the constructor of the object you assigned to 'prototype'.

In the other case, you dont change the TestClass.prototype.constructor property, wich is automaticly assigned when creating a new function.

So to fix this, either use:
TestClass.prototype.a =  function () {};
Or:
TestClass.prototype = {
  constructor: TestClass,
  a: function () {}
}

Comment 2

[A. Sel.]

Thanks for the explanation. Makes perfect sense. I always thought the constructor is set as a part of the "new"-operation. Didn't know it was part of the prototype-magic ...

Marking it as RESOLVED INVALID now.

Thanks! :)