Reference Instance Variables in Javascript Constructor

Question

I'm trying to maintain state on an object by doing something like this:

obj = function() { 
    this.foo = undefined; 
    this.changeState = function () { 
        (function () { this.foo = "bar" })(); // This is contrived, but same idea.
    }; 
};

I want to set the instance variable foo to "bar" when I call the changeState method.

For instance:

o = new obj();
o.changeState();
alert(o.foo); // This should say "bar"

As far as I can tell, what is happening is that "this" in the inner anonymous function is pointing to window. I'm not sure what's going on.

Am I on the right track? Is there a better approach?

Answer 1

Unless you specify a this context when calling a function the default will be the global (which in browsers is window).

Alternatives are:-

obj = function() { 
  this.foo = undefined; 
  this.changeState = function () { 
    (function () { this.foo = "bar" }).call(this); // This is contrived, but same idea.
  }; 

};

or:-

obj = function() {
  var self = this;
  this.foo = undefined; 
  this.changeState = function () { 
    (function () { self.foo = "bar" })(); // This is contrived, but same idea.
  }; 

};

Answer 2

this topic comes up a lot, but it's hard to serach for since "this" is removed from SO searches.

Basically, in JavaScript, this always refers to the calling object, not the context object. Since here we call o.changeState() from the global scope, this refers to window.

You actually don't need the inner function for the closure to work in this case - the changeState function itself is enough to close a lexical scope.

obj = function()
{
  var self = this; 
  this.foo = undefined; 
  this.changeState = function()
  {
    self.foo = "bar";
  }
} 

Answer 3

function obj() { 
    this.foo = undefined; 
    this.changeState = function () { this.foo = "bar" };
};

var o = new obj();
o.changeState();
alert(o.foo);

works for me. I'm not sure why you would want to use a self-invoking function just to assign a function reference, nor why you use a function expression for your constructor rather than a function declaration.

Answer 4

I figured it out. Just needed to save a reference to the current context and use that in the inner anonymous function:

obj = function() { 
    this.foo = undefined; 
    var self = this; 
    this.changeState = function () { 
        (function () { self.foo = "bar" })();
    }; 
};