masala.js

var not = function(fn){ return function(){ return !fn.apply(this, arguments); }; };

var nullOrUndefined = function(e){ return this[e] == null; };
var notNullOrUndefined = not(nullOrUndefined);

var existsIn = function(e){ return this.indexOf(e) != -1; };
var doesntExistIn = not(existsIn);

var merge = function(dest, source){
	var validKeys = Object.keys(source).filter(notNullOrUndefined, source);

	validKeys.forEach(function(e){
		dest[e] = source[e];
	});

	return dest;
};

var genSauce = function(fn, existingArgs, argsRemaining){
	return function(args){
		var argsGiven = Object.keys(args).filter(notNullOrUndefined, args),
		    nextArgsRemaining = argsRemaining.filter(doesntExistIn, argsGiven);

		existingArgs = merge(existingArgs, args);

		if ( nextArgsRemaining.length === 0 ) return fn.call(this, existingArgs);

		return genSauce(fn, existingArgs, nextArgsRemaining);
	};
};

var masala = function(fn, args){
	var argsRemaining = Object.keys(args).filter(nullOrUndefined, args),
	    defaultArgs = merge({}, args);

	return genSauce(fn, defaultArgs, argsRemaining);
};

module.exports = masala;

/* Use:

function add(options) {
	options.done(options.a + options.b);
}

var addSauce = masala(add, { done:null, a:null, b:null });

var addAndLog = addSauce({ done: console.log.bind(console, "answer:") });

var add2AndLog = addAndLog({ a: 2 });

add2AndLog({b:3}); //=> "answer: 5"

*/