How to find keys of a hash?

var h = {a:'b',c:'d'};

var k = h.keys() ; // k = ['a','c'];

Object.prototype.keys = function ()
{
  var keys = [];
  for(var i in this) if (this.hasOwnProperty(i))
  {
    keys.push(i);
  }
  return keys;
}

var obj = { "a" : 1, "b" : 2, "c" : 3};
alert(Object.keys(obj)); // will output ["a", "b", "c"]

if(!Object.keys) Object.keys = function(o){
   if (o !== Object(o))
      throw new TypeError('Object.keys called on non-object');
   var ret=[],p;
   for(p in o) if(Object.prototype.hasOwnProperty.call(o,p)) ret.push(p);
   return ret;
}

var keys = [];
for (var k in h)keys.push(k);

function getKeys(h) {
  Array keys = new Array();
  for (var key in h)
    keys.push(key);
  return keys;
}

_.keys({one : 1, two : 2, three : 3});
// => ["one", "two", "three"]

Object.prototype.keys = function () ...

for (var key in h) ...