how to compare those operations .js file with two attributes

obj = document.getElementById('txtstart');
   if (obj.value == "") {
       alert("Enter Start date");
       return false;
   }
   obj = document.getElementById('txtenddate');
   var on = document.getElementById('txtstart');
   var oob = document.getElementById('txtenddate').value;
   if (obj.value == "" && on.value <= oob.value) {
       alert("Enter correct date");
       return false;