JavaScript Closures

Slow: digit_name creates the array each time it's invoked

var digit_name = function (n) {
    var names = ['zero', 'one', 'two', 'three'];

    return names[n];
};

console.log(digit_name(2));  // two

An example of a closure

Fast: digit_name immediately invokes a function and holds the result, a pointer to the array. This is a pointer to a variable that lives on outside of the scope of the function it was created in.

names is closed over the outer function.

var digit_name = (function () {
    var names = ['zero', 'one', 'two', 'three'];

    return function (n) {
        return names[n];
    };
}());

console.log(digit_name(2));  // two

JS Fiddle:  http://jsfiddle.net/stevenhollidge/22Ab7/