Decorates a given JavaScript function with checking/assignment of default values to undefined parameters

decorateWithDefaultsChecking.js

//decorates a given function with checking/assignment of default values to undefined parameters
function decorateWithDefaultsChecking(targetFunc /* ,default1, default2... */){
  var defaults = [].slice.call(arguments, 1)
	if (!defaults)
		return targetFunc;
	var opStart = targetFunc.length - defaults.length;
	return function(){
		for(i=0; i<defaults.length; i++){
			if (arguments[opStart + i] === undefined){
				arguments[opStart + i] = defaults[i];
			}
		}
		//the next line below here is essential, remember that arguments is not a real array, but an "array like" object, 
		//so in order for this to work we need to set the length propety on our own, otherwise, the new assigned values are ignored when invoking apply
		arguments.length = arguments.length + defaults.length;
		return targetFunc.apply(this, arguments);
	};
}

test.js

function formatWords(word1, word2, op1, op2, op3){
  return op1 + word1 + op2 + word2 + op3;
}

console.log("before decoration");
console.log(formatWords("Asturies", "Xixon"));

var defaultsAware = decorateWithDefaultsChecking(formatWords, "->", ":", ".");

console.log("after decoration");
console.log(defaultsAware("Asturies", "Xixon"));
console.log(defaultsAware("Asturies", "Xixon", "+"));
console.log(defaultsAware("Asturies", "Xixon", "+", ";"));
console.log(defaultsAware("Asturies", "Xixon", "[", ",", "]"));