function ajax_lookup( event ) {
  // Perform an AJAX lookup on $(this).val();
};

// Console logging happens on keyup, for every single key
// pressed, which is WAAAY more often than you want it to.
$('input').keyup( ajax_lookup );

// Console logging happens on window keyup, but only after
// the user has stopped typing for 250ms.
$('input').keyup( $.debounce( 250, ajax_lookup ) );