argnames.js

// return an array of argument names
function getArgNames(fn) {
	var matches = fn.toString().match(/\(([a-z_, ]+)\)/i);
	if (matches.length > 1) {
		return matches[1].replace(/\s+/g, '').split(',');
	}
	return [];
}

// some sample implementation
function somethingCool(callback, context) {
	var names = getArgNames(callback),
		args = [];

	if (!names.length) {
		// default invokation
		return callback();
	}
	// deal with named args appropriately
	for (i=0; i<names.length; i++) {
		switch(name) {
			case 'name':
				args.push('Randy');
				break;
			case 'math':
				args.push(0.1 + 0.2);
				break;
			default:
				args.push(void 0);
				break;
		}
	}
	callback.apply(context || this, args);
}

// invoking the sample with various callbacks
somethingCool(function() {
	console.log('hello world');
});
somethingCool(function(name) {
	console.log('hello ' + name);
});
somethingCool(function(math, name) {
	console.log('math is hard, ' + name);
	console.log(math);
});

// notice how the order of the argument names doesn't even matter!