Ajax POST request with jQuery and PHP - Clue Mediator

<script>
 $(document).ready(function() {
 
 $("#submit").click(function() {
 
 var firstName = $("#firstName").val();
 var lastName = $("#lastName").val();
 var email = $("#email").val();
 var message = $("#message").val();
 
 if(firstName==''||lastName==''||email==''||message=='') {
 alert("Please fill all fields.");
 return false;
 }
 
 $.ajax({
 type: "POST",
 url: "submission.php",
 data: {
 firstName: firstName,
 lastName: lastName,
 email: email,
 message: message
 },
 cache: false,
 success: function(data) {
 alert(data);
 },
 error: function(xhr, status, error) {
 console.error(xhr);
 }
 });
 
 });
 
 });
</script>