Chris Padilla/Blog / Tech

HTML Form Validation is Pretty Good!

After spending a fair chunk of time working in forms the React way, I've gotta say — we already get a lot of goodies with the basic HTML inputs.

There's a project I worked on recently that had me working in Vanilla JavaScript. No React, no libraries, just raw HTML, JS, and CSS.

I went in mentally preparing to have to write my own library. I was prepped to recreate Formik for this project. But I didn't really have to!

Here are some of the niceties that saved me a ton of time:

Input Pattern Attribute

Without needing to write any JS, you can check to ensure a text input matches a regex pattern. Here's an example from w3schools:

<form action="/action_page.php">
  <label for="country_code">Country code:</label>
  <input type="text" id="country_code" name="country_code"
  pattern="[A-Za-z]{3}" title="Three letter country code"><br><br>
  <input type="submit">

The title attribute is the message that shows when there is a discrepancy between the input value and your pattern regex.

Constrain Validation API

That may not cover all use cases, but modern browsers come with an API for further validation customization.

The Constrain Validation API is available on most form inputs. There are a couple of methods that are useful here: setCustomValidity() and reportValidity()

setCustomValidity allows us to set a custom error message. reportValidity will then show the message when we call it on an element.

When we get to handling form submission, these give us a way of still working with the browser's built in UI and form API.

const handleSubmit = (e) => {
    // We use our custom name validation here
    const nameElm = document.querySelector('#contact-name');
    const isNameValid = validateName(nameElm.value);

    // And integrate with the API here
    if (isNameValid) {
        // If valid, we set the message to an empty string, meaning it passes.
    } else {
        // If invalid, setting an error message will mark the input as invalid.
        // Report Validity then shows the message.
        nameElm.setCustomValidity('Please enter a valid phone number');

Bonus: The CSS psuedo-classes are available when working with the form in this way. We can still make of use CSS such as this:

input:invalid {
  box-shadow: 0 0 5px 1px red;

input:focus:invalid {
  box-shadow: none;

More details and examples are available on the MDN article for HTML form validation.