JavaScript String.IsNullOrEmpty

I’ve been doing a lot of JavaScript lately and in my JS I’ve been testing if a string is empty, null or undefined.   C# has string.IsNullOrEmtpy and I believe JS should have the same.  I was about to write my on function, but decided to first see if someone else has done this.  I found an example, but didn’t completely like it. So I made some changes and here’s String.IsNullOrEmpty
 String.IsNullOrEmpty = function(value) {
  var isNullOrEmpty = true;
  if (value) {
   if (typeof (value) == ‘string’) {
    if (value.length > 0)
     isNullOrEmpty = false;
  return isNullOrEmpty;
I don’t know why it’s "String.IsNullOrEmpty" instead of "String.prototype.IsNullOrEmpty", but it works.  Maybe someone can answer this for me.