jQuery function missing parameter

This is the Ajax GET function:
jQuery.get( url, [ data ], [ callback(data, textStatus, XMLHttpRequest) ], [ dataType ] )

However, I see examples of code like this where the first and third parameters are provided but the second is missing:
$.get(‘ajax/test.html’, function(data) {
alert(‘Load was performed.’);

I’m familiar with providing a ‘’ or NULL value when a parameter is optional, so how does jQuery/javascript handle missing parameters?

From the jQuery source

    get: function( url, data, callback, type ) {
        // shift arguments if data argument was omited
        if ( jQuery.isFunction( data ) ) {
            type = type || callback;
            callback = data;
            data = null;

        return jQuery.ajax({
            type: "GET",
            url: url,
            data: data,
            success: callback,
            dataType: type

it checks to see if the data parameter is a function, in which case it sets type to type or callback (first defined value.), callback to data, and data to null then calls jQuery.ajax() with everything where it should be. That keeps developers from being forced to add empty strings and nulls, needlessly complicating their own code.