JavaScript: Utility function to default function in parameters

Recently I came across a interesting discussion on reddit. And that was about a utility function which will give you flexibility to define default function in parameters.

I found it very interesting, so wrote one for me.

/*
in param: defaultArg1, defaultArg2, .... function
return : function 
*/
var defaultParams = function(){
  var args = arguments,
   len = args.length,
   fn = args[--len],
   defaultArg = [];
  
  if(typeof fn !== 'function'){
   throw new Error('invalid arguments');
  }
  while(len--){
   defaultArg[len] = args[len];
  }
  return function(){  
   var args = arguments,
    fnlen = fn.length,
    params = [];
   while(fnlen--){
     params[fnlen] = args[fnlen] == null ?  defaultArg[fnlen] : args[fnlen];
   }
   return fn.apply(this, params);
  }
}
 
 
 
//how to use 
var alert_my_age = defaultParams(21, function(age) {
  console.log(age);
})
 
alert_my_age()   //#=> Alerts 21
alert_my_age(45) //#=> Alerts 45