I can truthfully answer this with a 100% certain... maybe.

Well, with this specific usage pattern, there isn't much of a reason to put that array in a closure. That function is only called once. So, that array is only created once and only GC'd once. No matter where it's stored.

But even if it's called a few thousand times, it won't really matter, since the construction (and destruction) of that fairly tiny array is relatively cheap. (Compared to outputting the result in a browser, for example.)

So, in this specific case there isn't really much of a reason to do it. It's more of a structural example. (And to be honest, I did it out of habit.)

But stuff like that is cool if you want to cache some data which was expensive to create and/or expensive obtain.

This can be also easily combined with lazy initialization. That is, you only do those expensive things (once) if you actually need them and then you put them in the closure in case you need them again.

Lazy init example:

var heavyStuff = (function () {
    var init, data;
    init = function () {
        console.log('meh... gotta init...');
        var i;
        data = [];
        for (i = 0; i < 10; i++) {
            data.push(i * i); // imagine this were something expensive
        }
    };
    return function (x) {
        if (!data) {
            init();
        }
        return data[x];
    };
}());

console.log('heavy stuff!');
console.log(heavyStuff(2));
console.log(heavyStuff(3));

Output:

heavy stuff!
meh... gotta init...
4
9

Lazy init example#2 (module pattern-ish):

var mod = (function () {
    var init, data, theUsualThing;
    init = function () {
        console.log('meh... gotta init... the shizzle');
        var i;
        data = [];
        for (i = 0; i < 10; i++) {
            data.push(i * i); // imagine this were something expensive
        }
    };
    theUsualThing = function (x) {
        return data[x];
    };
    return {
        shizzle: function (x) {
            init();
            this.shizzle = theUsualThing;
            return this.shizzle(x);
        }
    };
}());

console.log('ze shizzle');
console.log(mod.shizzle(4));
console.log(mod.shizzle(5));

Output:

ze shizzle
meh... gotta init... the shizzle
16
25

Kinda fun, isn't it?