Custom validation allows you to define your own validation rules for form fields. This can be useful when you have specific requirements that are not covered by built-in validation methods. For example, you may want to validate that a username is unique or that a phone number is in a specific format.

To implement custom validation, you can use JavaScript to define a function that checks the validity of a form field. This function should return true if the field is valid and false if it is not. Here's an example of how to implement custom validation for a password input field:

1function validatePassword(password) {
2  // Custom validation logic
3  if (password.includes('123456')) {
4    return false;
5  }
6  return true;
7}
8
9const passwordInput = document.getElementById('password');
10
11passwordInput.addEventListener('input', function() {
12  const password = passwordInput.value;
13  if (validatePassword(password)) {
14    passwordInput.setCustomValidity('');
15  } else {
16    passwordInput.setCustomValidity('Password is not valid');
17  }
18});