Number of elements in a javascript object

AFAIK, there is no way to do this reliably, unless you switch to an array. Which honestly, doesn't seem strange - it's seems pretty straight forward to me that arrays are countable, and objects aren't.

Probably the closest you'll get is something like this

// Monkey patching on purpose to make a point
Object.prototype.length = function()
{
  var i = 0;
  for ( var p in this ) i++;
  return i;
}

alert( {foo:"bar", bar: "baz"}.length() ); // alerts 3

But this creates problems, or at least questions. All user-created properties are counted, including the _length function itself! And while in this simple example you could avoid it by just using a normal function, that doesn't mean you can stop other scripts from doing this. so what do you do? Ignore function properties?

Object.prototype.length = function()
{
  var i = 0;
  for ( var p in this )
  {
      if ( 'function' == typeof this[p] ) continue;
      i++;
  }
  return i;
}

alert( {foo:"bar", bar: "baz"}.length() ); // alerts 2

In the end, I think you should probably ditch the idea of making your objects countable and figure out another way to do whatever it is you're doing.