JQuery library was developed specifically to simplify and to unify certain JavaScript functionality. However if you need to check a variable against undefined value, there is no need to invent any special method, since JavaScript has a typeof operator, which is simple, fast and cross-platform.
<script type="text/javascript">
$('#testing_sales').click(function(){
var testing_fuel_check = $('.testing_fuel_check').val();
if(typeof testing_fuel_check === "undefined")
{
alert('Please complete Fuel Sales after than complete testing!!');
}
else
{
}
});
</script>