Question

I have a question concerning how the “this” pointer is treated in a nested function scenario.

Say I insert this following sample code into a web page. I get an error when I call the nested function “doSomeEffects()”. I checked in Firebug and it indicates that when I am in that nested function, the “this” pointer is actually pointing to the global “window” object – which I did not expect. I must not be understanding something correctly because I thought since I declared the nested function within a function of the object, it should have “local” scope in relation to the function (i.e. the “this” pointer would be referring to the object itself like how it is in my first “if” statement).

Any pointers (no pun intended) would be appreciated.

var std_obj = {
  options : { rows: 0, cols: 0 },
  activeEffect : "none",
  displayMe : function() {

    // the 'this' pointer is referring to the std_obj
    if (this.activeEffect=="fade") { }

    var doSomeEffects = function() {

      // the 'this' pointer is referring to the window obj, why?
      if (this.activeEffect=="fade") { }

    }

    doSomeEffects();   
  }
};

std_obj.displayMe();

Answer 1

In JavaScript the this object is really based on how you make your function calls.

In general there are three ways to setup the this object:

1. someThing.someFunction(arg1, arg2, argN)
2. someFunction.call(someThing, arg1, arg2, argN)
3. someFunction.apply(someThing, [arg1, arg2, argN])

In all of the above examples the this object will be someThing.
Calling a function without a leading parent object will generally get you the global object which in most browsers means the window object.