object.Exists(key) 

function keyExists(k)
{
   var fso, s = "";
   d = new ActiveXObject("Scripting.Dictionary");
   d.Add("a", "Athens");
   d.Add("b", "Belgrade");
   d.Add("c", "Cairo");
   if (d.Exists(k))
      s += "Specified key exists.";
   else 
      s += "Specified key doesn't exist.";
   return(s);
}

Function KeyExistsDemo
   Dim d, msg   ' Create some variables.
   Set d = CreateObject("Scripting.Dictionary")
   d.Add "a", "Athens"   ' Add some   keys and items.
   d.Add "b", "Belgrade"
   d.Add "c", "Cairo"
   If d.Exists("c") Then
      msg = "Specified key exists."
   Else
      msg = "Specified key doesn't exist."
   End If
   KeyExistsDemo = msg
End Function