Question

Okay, I’m hating Javascript right now, and I hope someone can help me.

I have code which is set up like the following:

function Obj1() {
    var me = this;

    this.something = "yay";

    this.getThis = function(){
        return me;
    }
}

Obj1.prototype.method = function() {
    return this.something;
};

function Obj2() {
    this.something = "nay";

}
Obj2.prototype.method = function() {
    return this.something;
}; 



var o1 = new Obj1();
var o2 = new Obj2();

document.write(o1.method()); // Returns yay
document.write(o1.method.call(o2));   // Returns nay, but I need "yay" here

(JSFiddle @ http://jsfiddle.net/A9u9K/)

My Problem is, that I need to call Obj1.method in the second case, but I am absolutely unable to get a reference to the object 🙁

How can I work around this?

Edit: Sorry, I got my example code pretty wrong 🙁 Updated it. I took most of the code from a previous answer, because it is much nicer and still illustrates my problem.

Answer 1

Updated Answer:

document.write(o1.method.call(o2)); // Returns nay, but I need "yay" here

You’ve said you’ve got it sorted now, but as the answer to that isn’t actually shown here on SO, I figured I may as well update to show it.

If it’s method you want to have access me, even if it’s been called with a different this value, you have to define it like getThis, as a closure over me:

function Obj1() {
    var me = this;

    this.something = "yay";

    this.method = function() {
        return me.something;
    };

    this.getThis = function(){
        return me;
    };
}

function Obj2() {
    this.something = "nay";

}
Obj2.prototype.method = function() {
    return this.something;
};

…or of course, if you don’t need the “something” to be a property on the object, just make it a var within the constructor (a private variable, like me):

function Obj1() {
    var me = this;
    var something = "yay";

    this.method = function() {
        return something;
    };

    this.getThis = function(){
        return me;
    };
}

function Obj2() {
    this.something = "nay";

}
Obj2.prototype.method = function() {
    return this.something;
};

---

Original Answer: (To Revision 1 of the question, which didn’t have me.)

but I thought that, when creating a closure (as I do in 4) Javascript should preserve “this”.

this is set entirely by how a function is called, not where it’s defined; more about that here and here. But the way you’ve defined your getThis function, you can use the fact it closes over the constructor call to solve this (no pun) without using this:

function Obj1() {
    var me = this;               // <== Use a variable to remember `this`

    this.something = "yay";

    this.method = function() {
        return this.something;
    };

    this.getThis = function(){
        return me;               // <== Return it
    };
}

Live example

More about closures and the plumbing that makes the me thing work here.

There is a cost involved in this, and just generally in your pattern of defining functions within the constructor function: Each individual object created by Obj1 and Obj2 gets its own copy of each function. This can have memory implications if there are lots of these objects running around (but unless you have lots, you needn’t worry and you get benefits like the me thing and other private variables). In constrast, if you use a function assigned to the prototype, all instances will share a single, common copy of the function.

In your sample code, only the getThis function really needs to be duplicated for every instance (because you’re relying on the closure), so you can do this to avoid unnecessary function proliferation:

function Obj1() {
    var me = this;

    this.something = "yay";

    this.getThis = function(){
        return me;
    };
}
Obj1.prototype.method = function() {
    return this.something;
};

function Obj2() {
    this.something = "nay";

}
Obj2.prototype.method = function() {
    return this.something;
};