Help With JavaScript

Okay, I think you're confusing two different ways of doing this. You can write

function assembleGreeting(NAME, MOOD) {
  return "Hello my name is " + (NAME || "you") + " and I am in a " + (MOOD || "good") + " mood!";
}

var greeting = assembleGreeting("jim","awesome"); //Don't forget quotes!

That will get you the output you expected.

However, you can also do it this way, which it looks like you were getting mixed up at:

var GenericGreet = function(name, mood) {

    this.name = name || "you";

    this.mood = mood || "good";

    this.speak = function() {
    return "Hello my name is " + this.name + " and I am in a " + this.mood + " mood!";
    };
};

var greeter = new GenericGreet("jim", "awesome"); 

Let's break down what's going on here. This is called a Constructor function. Instead of using this function, our greeter variable could have been constructed like this:

var literalConstructed = {
  name: "jim",
  mood: "awesome",
  speak: function() {
    return "Hello my name is " + this.name + " and I am in a " + this.mood + " mood!";
  };
};

With a constructor though, you can do way more to your input data, reuse the structure, have local variables, etc. Anyways now you can say alert(greeter.speak()) or you can say alert(greeter.mood), and you can edit them directly like greeter.mood = "happy";

Anyways, that's what it seems like you were being mixed up with, and why people were telling you to use apply. You can use .apply, with this method too. For example, assuming the Constructor is defined as above

var changeMe = {
  existingProperty: "Golden!"
};

GenericGreet.apply(changeMe, ["jim","awesome"]);

//or you can use

GenericGreet.call(changeMe, "jim", "awesome");

What's going on here, is that you are basically running the Constructor over an existing object, and the first argument you give to .apply or .call is the variable to be used as "this" in that process.