The callback pattern found in many node.js functions.

callback_pattern.js

var invokee1 = function(err, callback) {
    // cant change this function
    callback();
};

var invokee2 = function(err, optional1, optional2, callback) {
    // work with optional 1 or optional 2
    console.log(optional1);
    console.log(optional2);
    // cant change this function
    callback();
};

var invoker = function(invokee) {
    var cb = function() {
        console.log('this is the callback');
    };

    // call the invokee with the right amount of arguments
    var args = [null, 'a', 'b', 'c', 'd']; // null is the error arg
    args[invokee.length - 1] = cb;
    invokee.apply(null, args);
};

invoker(invokee1);
invoker(invokee2);