form-validation

A sign-up form that checks each field and explains exactly what's wrong, right under the field that's wrong. It teaches form events, a readable regex, and how to give feedback that screen readers announce too.

Run

Get it: git clone https://git.devai.io/templates/form-validation.git

Double-click index.html — it opens in your browser and works. Nothing to install, no build step.

Or serve it like production:

docker compose up --build

Then open http://localhost:8080.

How it works

Each field is described once, in the fields object: its input, its error <p>, and a validate(value) rule that returns an error message, or "" when the value is fine. One function, checkField, runs a rule and shows the result.

• On blur (leaving a field) just that field is checked — early feedback without nagging while you type.
• On submit every field is checked so all errors appear at once, then the cursor jumps to the first one to fix. preventDefault() stops the form from being sent; this demo only validates.
• Accessibility: aria-invalid="true" marks a wrong field, and each input's aria-describedby points at its error <p>, so screen readers read the message aloud.
• novalidate on the <form> turns off the browser's own popups so ours show instead. The email rule is deliberately simple: something@something.something.

Try it: require a digit in the password (&& /\d/.test(value)), add a "terms" checkbox that must be ticked, or disable the button until every field passes.

Layout

index.html   the form: each input followed by its (empty) error <p>
app.js       the fields object, checkField() and the two event listeners
styles.css   the look — invalid fields get a red border

Deploy

Push to your own GitHub repo and the shipped workflow (.github/workflows/ci.yml) tests the compose stack, publishes the image to GHCR, and — once you set the DEPLOY_HOST / DEPLOY_USER variables and DEPLOY_KEY secret — deploys it to your server over ssh.

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Part of devai.io — the Web Basics track: HTML, CSS & JavaScript, one concept at a time. Next up: faq-accordion.