The oninvalid event is triggered if the field value added in <input> is invalid. Add a message if the user forgets to fill the form before submission.
Example
You can try to run the following code to learn how to implement oninvalid event in JavaScript.
<!DOCTYPE html>
<html>
<body>
<script>
function resetFunct() {
alert("The form was reset");
}
</script>
<form>
Enter Name: <input type="text"
required oninvalid = "alert('Fill the form before submitting!');" > <br>
Enter birth month: <input type= "text"> <br>
<input type = "submit">
<input type = "reset">
</form>
</body>
</html>