function onKeyDown(event) {

   // Only hijack INPUT keydown.
   if(event.target.localName != 'INPUT' ) return true;

   if ( 0x0d == event.KeyCode) {
      // What you want to do here?
      event.KeyCode = 0x09;
      // Pass through key
      return true;
   }

   // Other case.
   return true;
}

document.addEventListener("keydown", function(e) { return onKeyDown(e); }, true);