Testing for Null and Undefined

HTML

<ul id="l"></ul>

JavaScript

function isNullOrUndefined(a)
{
   var rc = false;

   if(a===null || typeof(a) === "undefined")
   {
      rc = true;
   }

   return rc;
}

var b=null;
$('<li></li>').html('null -> ' + isNullOrUndefined(b)).appendTo('#l');
var c;
$('<li></li>').html('undefined -> ' + isNullOrUndefined(c)).appendTo('#l');
var d=1;
$('<li></li>').html('1 -> ' + isNullOrUndefined(d)).appendTo('#l');
var e="apple";
$('<li></li>').html('"apple" -> ' + isNullOrUndefined(e)).appendTo('#l');