JavaScript functions are variables, and this is a big difference between JavaScript and many other programming languages. It can be a bit of a paradigm shift for people new to JavaScript, but it allows for some really cool things you can't do in many other languages.

When I say functions are variables, I mean that they're treated much the same as arrays, numbers, strings and objects. That means you can do some neat things.

You can define and redefine functions as local variables:

var myFunc;

if (Math.random() < 0.5) {
    myFunc = function() {
        alert('heads');
    };
} else {
    myFunc = function() {
        alert('tails');
    };
}

myFunc(); // alerts "heads" or "tails" depending on random value

You can also pass functions as parameters to other functions, which is very useful for callback functions:

function do_something(callback_function) {
    alert('hello');

    if (callback_function) {
        callback_function();
    }
}

var my_callback = function() {
    alert('goodbye');
};

do_something(my_callback); // alerts "hello" and then "goodbye"

You can also return a function from a function:

function get_multiplier(factor) {
    return function(num) {
        return num * factor;
    };
}

var times_5 = get_multiplier(5);
var result = times_5(10);

alert(result); // alerts "50"

var six_times_two = get_multiplier(6)(2);

alert(six_times_two); // alerts "12"

You can also define "anonymous" functions without a name and even execute them:

(function(first_name, get_last_name) {
    var last_name = get_last_name();

    alert(first_name + ' ' + last_name); // alerts "Jesse Skinner";
})('Jesse', function(){ return 'Skinner'; });

Note that in order to execute an anonymous function you have to wrap it in () parentheses first.

So that's just some of the unusual stuff you can do with JavaScript. Once you get familiar with the concept, you can really start to reuse code in great new ways.