Updating Submit Disable to Include isNaN

The datanumber should be numeric, so I wanted to check on the frontend, before they even submit it. It was easy to add an else to the previous javascript function SearchVal().

function searchVal(){
 if(document.getElementById('fname').value == 0
  && document.getElementById('lname').value == 0
  && document.getElementById('datanum').value == 0){
  msg = 'At least one search field must be filled';

 }else if(isNaN(document.getElementById('datanum').value)){
  msg = 'Data number must be numeric';

 }else{
  dis = false;
  msg = '';
 }

 if(dis == true){
  document.getElementById('sub').disabled = true;
 }else{
  document.getElementById('sub').disabled = false;
 }
 document.getElementById('validmsg').innerHTML = msg;
}