I'd say that while, most of the time, there is little value in distinguishing between the two, the cases where there is value tend to be quite interesting.

Take, for example, a function which can be given a callback. undefined might indicate that some default callback should be used (as if the parameter weren't specified), but null could indicate that no callback should be made at all:

function asyncWorker(data, callback, timeout) {

    if (typeof callback === undefined) {

        callback = function() { $(#log).append(<p>Done!</p>); };

    }



    // ...



    if (callback) callback();

}



asyncWorker([1, 2, 3]); // default callback, no timeout

asyncWorker([4, 5, 6], null); // no callback, no timeout

asyncWorker([7, 8, 9], undefined, 10000); // default callback, 10s timeout

Of course, false or 0 could be used instead of null here, but that might not be the case in a more complex example. And whether your code benefits from the additional parameter complexity is entirely up to you. :-)