We're building a web game where everybody wins and we are all friends forever.

It's simple—you click on one of three boxes to see what nice thing you've won. You always win something nice. Because we love you.

Here's what we have so far. Something's going wrong though. Can you tell what it is?

The syntax is just fine—the problem is some unexpected behavior.

The user's prize is always undefined!

The Problem

The anonymous method we're assigning to the buttons' onclicks has access to variables in the scope outside of it (this is called a closure). In this case, it has access to btnNum.

When a method accesses a variable outside its scope, it accesses that variable, not a frozen copy. So when the value held by the variable changes, the method gets that new value. By the time the user starts pressing buttons, our loop will have already completed and btnNum will be 3, so this is what each of our anonymous methods will get for btnNum!

Why 3? The for loop will increment btnNum until the conditional in the middle is no longer met—that is, until it's not true that btnNum < prizes.length. So the code in the for loop won't run with btnNum = 3, but btnNum will be 3 when the loop is done.

Why undefined? prizes has 3 elements, but they are at indices 0,1,2. Array indices start at 0, remember? (Write this down—forgetting this is an easy way to create an off-by-one error in a whiteboard interview.) In JavaScript, accessing a nonexistent index in an array returns undefined (Python throws an IndexError, but Ruby returns nil).

The Solution

We can solve this by wrapping our anonymous method in another anonymous method that takes btnNum as an argument. Like so:

This "freezes" the value of btnNum. Why? Well...

Primitives vs. Objects

btnNum is a number, which is a primitive type in JavaScript.

Primitives are "simple" data types (string, number, boolean, null, and undefined in JavaScript). Everything else is an object in JavaScript (methods, arrays, Date() values, etc).

Arguments Passed by Value vs. Arguments Passed by Reference

One important property of primitives in JS is that when they are passed as arguments to a method, they are copied ("passed by value"). So for example:

Heads up: This is not well-formed JavaScript. We're using it to prove a point.

The threatLevel inside inspireFear is a new number, initialized to the same value as the threatLevel outside of inspireFear. Giving these different variables the same name might cause confusion here. If we change the two variables to have different names we get the exact same behavior:

In contrast, when a method takes an object, it actually takes a reference to that very object. So changes you make to the object in the method persist after the method is done running. This is sometimes called a side effect.

Bringing it home

Back to our solution:

So when we pass btnNum to the outer anonymous method as its one argument, we create a new number inside the outer anonymous method called frozenBtnNum that has the value that btnNum had at that moment (0, 1, or 2).

Our inner anonymous method is still a closure because it still reaches outside its scope, but now it closes over this new number called frozenBtnNum, whose value will not change as we iterate through our for loop.

Like several common Javascript interview questions, this question hinges on a solid understanding of closures and pass by reference vs pass by value. If you're shaky on either of those, look back at the examples in the solution.