How to find keys of a hash?

There is function in modern JavaScript (ECMAScript 5) called Object.keys performing this operation:

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

Compatibility details can be found here.

On the Mozilla site there is also a snippet for backward compatibility:

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;
}

For production code requiring a large compatibility with client browsers I still suggest Ivan Nevostruev's answer above with shim to ensure Object.keys in older browsers. However, it's possible to get the exact functionality requested using ECMA's new defineProperty feature.

As of ECMAScript 5 - Object.defineProperty

As of ECMA5 you can use Object.defineProperty() to define non-enumerable properties. The current compatibility still has much to be desired, but this should eventually become usable in all browsers. (Specifically note the current incompatibility with IE8!)

Object.defineProperty(Object.prototype, 'keys', {
  value: function keys() {
    var keys = [];
    for(var i in this) if (this.hasOwnProperty(i)) {
      keys.push(i);
    }
    return keys;
  },
  enumerable: false
});

var o = {
    'a': 1,
    'b': 2
}

for (var k in o) {
    console.log(k, o[k])
}

console.log(o.keys())

# OUTPUT
# > a 1
# > b 2
# > ["a", "b"]

However, since ECMA5 already added Object.keys you might as well use:

Object.defineProperty(Object.prototype, 'keys', {
  value: function keys() {
    return Object.keys(this);
  },
  enumerable: false
});

Original answer

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

---

Edit: Since this answer has been around for a while I'll leave the above untouched. Anyone reading this should also read Ivan Nevostruev's answer below.

There's no way of making prototype functions non-enumerable which leads to them always turning up in for-in loops that don't use hasOwnProperty. I still think this answer would be ideal if extending the prototype of Object wasn't so messy.

you can use Object.keys

Object.keys(h)