When HTML built-in verification is not enough, a custom validator can implement more complex form validation through JavaScript. First, listen to the form's submit event and prevent the default submission behavior from being executed to execute custom logic; second, error message elements can be inserted and styled when verification fails to improve the user experience; finally, custom verification can be incorporated into the browser's native verification process using Constraint Validation APIs such as setCustomValidity() and checkValidity() to achieve more granular control.
When you need more control over form validation than what HTML's built-in rules offer, custom validators are the way to go. Whether it's checking a complex password pattern or verifying that two fields match, writing your own validation logic gives you the flexibility to handle just about any case.
Understanding When You Need Custom Validation
HTML has some great built-in validation features — things like required , minlength , and pattern work well for most basic forms. But when you start dealing with more complex requirements, those tools might not be enough.
For example:
- Making sure a password contains both uppercase and lowercase letters
- Checking that a confirmation field matches another (like confirming an email)
- Validating a phone number format that varies by country
These are cases where a simple regex or required check won't cut it. That's when rolling your own validator becomes useful.
How to Create a Basic Custom Validator with JavaScript
To implement custom validation, you can use JavaScript to hook into the form submission process and run your own checks. The key is to listen for the submit event on the form and prevent the default action if validation fails.
Here's a basic approach:
document.querySelector('form').addEventListener('submit', function(event) {
const password = document.getElementById('password').value;
const confirmPassword = document.getElementById('confirm-password').value;
if (password !== confirmPassword) {
alert('Passwords do not match.');
event.preventDefault(); // Stops form from submitting
}
});This kind of setup lets you add as many checks as you want. Just make sure to give users clear feedback — whether through alerts, error messages next to the field, or highlighting the input in red.
Showing Friendly Error Messages
Built-in validation usually shows browser-generated popups, which can look out of place or feel too technical. With custom validation, you have full control over how errors are presented.
A good way to show errors:
- Insert a
<div>or<span>near the input field to display the message - Style it so it stands out but doesn't overwhelm
- Clear the message once the input is corrected
Example structure:
<div class="input-group"> <label for="email">Email:</label> <input type="email" id="email" required /> <div class="error-message"></div> </div>
Then in JavaScript:
const emailInput = document.getElementById('email');
const errorMessage = document.querySelector('.error-message');
if (!isValidEmail(emailInput.value)) {
errorMessage.textContent = 'Please enter a valid email address.';
event.preventDefault();
} else {
errorMessage.textContent = '';
}This keeps the user experience smooth and consistent with your site's design.
Using Constraint Validation API for More Control
If you want to integrate with the native validation system rather than bypass it entirely, you can use the Constraint Validation API . This allows you to add custom validity messages without completely overriding the browser's validation flow.
Key methods:
-
setCustomValidity()– sets a custom error message -
checkValidity()– manually checks if the element is valid
Useful when you want to tie custom logic into the same flow as standard validation.
For instance:
const passwordInput = document.getElementById('password');
passwordInput.addEventListener('input', function() {
if (passwordInput.value.length > 0 && passwordInput.value.length < 8) {
passwordInput.setCustomValidity('Password must be at least 8 characters long.');
} else {
passwordInput.setCustomValidity('');
}
});This way, the browser will treat your condition just like any other validation rule.
Basically that's it. Custom validators aren't hard to set up once you get the hang of them, but they do require attention to detail—especially around user feedback and integration with existing validation systems.
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