function empty (mixed_var) {
  // Checks if the argument variable is empty
  // undefined, null, false, number 0, empty string,
  // string "0", objects without properties and empty arrays
  // are considered empty
  //
  // http://kevin.vanzonneveld.net
  // +   original by: Philippe Baumann
  // +      input by: Onno Marsman
  // +   bugfixed by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +      input by: LH
  // +   improved by: Onno Marsman
  // +   improved by: Francesco
  // +   improved by: Marc Jansen
  // +      input by: Stoyan Kyosev (http://www.svest.org/)
  // +   improved by: Rafal Kukawski
  // *     example 1: empty(null);
  // *     returns 1: true
  // *     example 2: empty(undefined);
  // *     returns 2: true
  // *     example 3: empty([]);
  // *     returns 3: true
  // *     example 4: empty({});
  // *     returns 4: true
  // *     example 5: empty({'aFunc' : function () { alert('humpty'); } });
  // *     returns 5: false
  var undef, key, i, len;
  var emptyValues = [undef, null, false, 0, "", "0"];

  for (i = 0, len = emptyValues.length; i < len; i++) {
    if (mixed_var === emptyValues[i]) {
      return true;
    }
  }

  if (typeof mixed_var === "object") {
    for (key in mixed_var) {
      // TODO: should we check for own properties only?
      //if (mixed_var.hasOwnProperty(key)) {
      return false;
      //}
    }
    return true;
  }

  return false;
}

empty(null);

true

empty(undefined);

true

empty([]);

true