jquery form validation

JavaScript
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.19.2/jquery.validate.min.js"></script>reference : https://www.sitepoint.com/basic-jquery-form-validation-tutorial/

// Wait for the DOM to be ready
$(function() {
  // Initialize form validation on the registration form.
  // It has the name attribute "registration"
  $("form[name='registration']").validate({
    // Specify validation rules
    rules: {
      // The key name on the left side is the name attribute
      // of an input field. Validation rules are defined
      // on the right side
      firstname: "required",
      lastname: "required",
      email: {
        required: true,
        // Specify that email should be validated
        // by the built-in "email" rule
        email: true
      },
      password: {
        required: true,
        minlength: 5
      }
    },
    // Specify validation error messages
    messages: {
      firstname: "Please enter your firstname",
      lastname: "Please enter your lastname",
      password: {
        required: "Please provide a password",
        minlength: "Your password must be at least 5 characters long"
      },
      email: "Please enter a valid email address"
    },
    // Make sure the form is submitted to the destination defined
    // in the "action" attribute of the form when valid
    submitHandler: function(form) {
      form.submit();
    }
  });
});<script src="https://cdn.jsdelivr.net/jquery.validation/1.16.0/jquery.validate.min.js"></script>
then add javascript code:
$(document).ready(function() {
   $("#form").validate();
});
</script>function submitFunction(event){
	event.preventDefault();
}
$("#form_id").submit(submitFunction);$("#myform").validate({
  debug: true
});<html>
<body>
<h2>Validation of a form</h2>
<form id="form" method="post" action="">
First name:<br>
<input type="text" name="firstname" value="john">
<br>
Last name:<br>
<input type="text" name="lastname" value="Doe">
<br>
Email:<br>
<input type="email" name="u_email" value="[email protected]">
<br>
<br><br>
<input type="submit" value="Submit">
</form>
</body>
</html>
Source

Also in JavaScript: