a map function for arrays *or* objects

map.js

function map(obj, fn, thisArg){
  // suggest you polyfill if you need Javascript < 1.6
  var result = {}; 
  if(obj instanceof Array){ 
    result = obj.map(fn, thisArg || null);
  } else {
    Object.keys(obj).forEach(function(key){
      result[key] = fn.call(thisArg || null, obj[key], key, obj);
    });
  }
  return result;
}