Ability To Refer To Function By Variable

I have a function that sets a value of this.name. If I call a function of that function (is there a better term for that?), I can get the value of that name with this.name. However, if I save that function's function as a variable, and THEN call it, this.name is undefined.

I don't think my vocabulary is clear enough, so here's my example:
function Ninja(name){
this._name = name;
this.getName = function(){
return "ninja "+this._name
}}

sam = new Ninja("sam");
assert( sam.getName() == "ninja sam", "Ninja's name is ninja sam" );
sam2 = new Ninja("sam2");
sam2getName = sam2.getName;
assert( sam2getName() == "ninja sam2", "Ninja 2's name is ninja sam2" ); //fails
log("Ninja 2's name is actually "+sam2getName()) // actually "ninja undefined"

(Note: Use of "ninja" and assertions are because I was playing with the code in John Resig's java runner thingie: [URL]). The reason I want to be able to refer to a function by a variable is because I want to pass the function to something else that requires a function to be passed in, such as an ajax callback or something. Anyway... why is this.name undefined in the second example?