## Respond to form events

You can use JavaScript to respond to user interactions on your form, reveal additional form fields, submit a form, and much more.

### Help users fill in additional form controls

Imagine that you built a survey form. After a user selects one option,
you want to show an additional `<input>` to ask a specific question related to the selection.
How can you only show the relevant `<input>` element?
<iframe allow="camera; clipboard-read; clipboard-write; encrypted-media; geolocation; microphone; midi;" loading="lazy" src="https://codepen.io/web-dot-dev/embed/8e1e7a38790c75c267a978efa1d8e937?height=350&amp;theme-id=light&amp;default-tab=result&amp;editable=true" style="height: 100%; width: 100%; border: 0;" title="Pen 8e1e7a38790c75c267a978efa1d8e937 by web-dot-dev on Codepen"></iframe>

You can use JavaScript to reveal an `<input>` only when the associated `<input type="radio">` is currently selected.

```javascript
if (event.target.checked) {
    // show additional field
} else {
   // hide additional field
}
```

> [!CAUTION]
> **Caution:** Make sure your form is still usable [if JavaScript isn't available](https://kryogenix.org/code/browser/everyonehasjs.html). The core experience should be the same for all users, with JavaScript used only as an enhancement.

Let's look at the
[JavaScript code](https://codepen.io/web-dot-dev/pen/8e1e7a38790c75c267a978efa1d8e937?editors=0010) for the demo.
Have you noticed the `aria-controls`, and `aria-expanded` attributes?
Use these
[ARIA attributes](https://developer.mozilla.org/docs/Web/Accessibility/ARIA)
to help screen reader users understand when an additional form control is shown or hidden.

> [!CAUTION]
> **Caution:** The `aria-expanded` attribute for `<input type="radio">` isn't [correctly announced by every screen reader](https://accessibility.blog.gov.uk/2021/09/21/an-update-on-the-accessibility-of-conditionally-revealed-questions/).

### Ensure users can submit a form without leaving a page

Imagine you have a comment form. When a reader adds a comment and submits the form,
it would be ideal if they could immediately see the comment without a page refresh.

To achieve this, listen to the `onsubmit` event, then use `event.preventDefault()` to prevent the default behavior,
and send the `FormData` using the [Fetch API](https://developer.mozilla.org/docs/Web/API/Fetch_API).


Browser Support

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- ![Safari: 10.1.](data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' xmlns:xlink='http://www.w3.org/1999/xlink' width='24' height='24' viewBox='195 190 135 135'%3E%3Cdefs%3E%3ClinearGradient id='s-a' x1='132.6' x2='134.4' y1='111.7' y2='-105.3' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-color='%23d2d2d2' /%3E%3Cstop offset='.5' stop-color='%23f2f2f2' /%3E%3Cstop offset='1' stop-color='%23fff' /%3E%3C/linearGradient%3E%3ClinearGradient id='s-b' gradientUnits='userSpaceOnUse' /%3E%3ClinearGradient id='s-c' x1='65.4' x2='67.4' y1='115.7' y2='17.1' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-color='%23005ad5' /%3E%3Cstop offset='.2' stop-color='%230875f0' /%3E%3Cstop offset='.3' stop-color='%23218cee' /%3E%3Cstop offset='.6' stop-color='%2327a5f3' /%3E%3Cstop offset='.8' stop-color='%2325aaf2' /%3E%3Cstop offset='1' stop-color='%2321aaef' /%3E%3C/linearGradient%3E%3ClinearGradient id='s-d' x1='158.7' x2='176.3' y1='96.7' y2='79.5' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-color='%23c72e24' /%3E%3Cstop offset='1' stop-color='%23fd3b2f' /%3E%3C/linearGradient%3E%3CradialGradient id='s-i' cx='-69.9' cy='69.3' r='54' gradientTransform='matrix(.9 -.01 .04 2.72 -9 -120)' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-color='%2324a5f3' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%231e8ceb' /%3E%3C/radialGradient%3E%3CradialGradient id='s-j' cx='109.3' cy='13.8' r='93.1' gradientTransform='matrix(-.02 1.1 -1.04 -.02 137 -115)' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%235488d6' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%235d96eb' /%3E%3C/radialGradient%3E%3C/defs%3E%3Crect width='220' height='220' x='22' y='-107' fill='url(%23s-a)' ry='49' transform='matrix(.57 0 0 .57 187 256)' /%3E%3Cg transform='translate(194 190)'%3E%3Ccircle cx='67.8' cy='67.7' fill='url(%23s-c)' paint-order='stroke fill markers' r='54' /%3E%3Ccircle cx='-69.9' cy='69.3' fill='url(%23s-i)' transform='translate(138 -2)' r='54' /%3E%3C/g%3E%3Cellipse cx='120' cy='14.2' fill='url(%23s-j)' rx='93.1' ry='93.7' transform='matrix(.58 0 0 .58 192 250)' /%3E%3Cg transform='matrix(.58 0 0 .57 197 182)'%3E%3Cpath fill='%23cac7c8' d='M46 192h1l72-48-7-9-66 57Z' /%3E%3Cpath fill='%23fbfffc' d='M46 191v1l66-57-7-9-59 65Z' /%3E%3Cpath fill='url(%23s-d)' d='m119 144-7-9 66-57-59 66Z' /%3E%3Cpath fill='%23fb645c' d='m105 126 7 9 66-57-1-1-72 49Z' /%3E%3C/g%3E%3Cpath stroke='%23fff' stroke-linecap='round' stroke-miterlimit='1' stroke-width='1.3' d='m287 278 3-2m-12-17 8-2m-8-3h4m-4-13 8 2m-8 3h4m-1-13 7 3m-4-11 7 4m-2-11 6 6m0-12 6 7m1-11 4 6m4-10 3 7m5-9 2 7m15-7-1 7m10-5-3 7m11-4-4 7m11-2-5 6m16 7-7 4m10 4-7 3m10 6-8 1m8 16-8-2m5 10-7-3m4 11-7-4m2 11-6-5m0 11-5-6m-2 11-4-7m-4 11-3-8m-6 10-1-8m-16 8 2-8m-10 5 3-7m-11 4 4-7m-11 2 5-6m-8 3 3-3m4 8 2-3m5 8 2-4m6 7 1-4m8 5v-4m8 4v-4m9 3-1-4m9 1-2-4m9 0-2-4m9-2-3-3m8-4-3-2m8-5-4-2m7-6-4-1m5-8h-4m4-8h-4m3-9-4 1m1-9-4 2m-1-9-3 2m-2-9-3 3m-4-8-2 3m-5-8-2 4m-6-6-1 3m-8-5v4m-8-4v4m-9-2 1 3m-9 0 2 3m-9 1 2 3m-9 2 3 3m-8 4 3 2m-8 5 4 2m-7 6 4 1m-4 25 4-1m-2 5 7-3m-6 7 4-2m-2 6 7-4m-13-21h8m41-41v-8m0 99v-8m49-42h-8' transform='translate(-65 8)' /%3E%3C/svg%3E)

[Source](https://developer.mozilla.org/docs/Web/API/Window/fetch)

<br />

> [!NOTE]
> **Note:** `FormData` is a set of key/value pairs representing form fields and their values. You can add all your form fields to the `FormData` object, or only send some form fields and their values.

<iframe allow="camera; clipboard-read; clipboard-write; encrypted-media; geolocation; microphone; midi;" loading="lazy" src="https://codepen.io/web-dot-dev/embed/b499a861d7b1fee74464a690cf0ff97e?height=400&amp;theme-id=light&amp;default-tab=js%2Cresult&amp;editable=true" style="height: 100%; width: 100%; border: 0;" title="Pen b499a861d7b1fee74464a690cf0ff97e by web-dot-dev on Codepen"></iframe>

Your backend script can check if a `POST` request appears to be from the browser
(using the `action` attribute of a form element, where `method="post"`) or from JavaScript,
such as a `fetch()` request.

```javascript
if (req.xhr || req.headers.accept.indexOf('json') !== -1) {
    // return JSON
} else {
    // return HTML
}
```

Always notify screen reader users about dynamic content changes.
Add an element with the `aria-live="polite"` attribute to your HTML,
and update the content of the element after a change.
For example, update the text to 'Your comment was successfully posted', after a user submits a comment.

Learn more about [ARIA live regions](https://developer.mozilla.org/docs/Web/Accessibility/ARIA/ARIA_Live_Regions).

## Validation with JavaScript

### Ensure error messages align with your site style and tone

The wording of default error messages differs between browsers.
How can you make sure the same message is shown to all users,
and that the message aligns with your site's [style and tone](https://developers.google.com/style/tone)?
Use the [`setCustomValidity()`](https://developer.mozilla.org/docs/Web/API/HTMLInputElement/setCustomValidity)
method of the [Constraint Validation API](https://developer.mozilla.org/docs/Web/API/Constraint_validation)
to define your own error messages.
<iframe allow="camera; clipboard-read; clipboard-write; encrypted-media; geolocation; microphone; midi;" loading="lazy" src="https://codepen.io/web-dot-dev/embed/7ea31257d7cd8fc28792c7f5cdaba97b?height=300&amp;theme-id=light&amp;default-tab=js%2Cresult&amp;editable=true" style="height: 100%; width: 100%; border: 0;" title="Pen 7ea31257d7cd8fc28792c7f5cdaba97b by web-dot-dev on Codepen"></iframe>

> [!NOTE]
> **Note:** Make sure to translate and localize your error messages if you have a multi-language website.

### Ensure users are notified about errors in real time

The built-in HTML features for form validation are great for notifying users
about invalid form fields before the data is sent to your backend.
Wouldn't it be great to notify users as soon as they leave a form field?
<iframe allow="camera; clipboard-read; clipboard-write; encrypted-media; geolocation; microphone; midi;" loading="lazy" src="https://codepen.io/web-dot-dev/embed/b7ed22a0539f9beef4dc03380f51f224?height=300&amp;theme-id=light&amp;default-tab=js%2Cresult&amp;editable=true" style="height: 100%; width: 100%; border: 0;" title="Pen b7ed22a0539f9beef4dc03380f51f224 by web-dot-dev on Codepen"></iframe>

Listen for the
[`blur`](https://developer.mozilla.org/docs/Web/API/Element/blur_event)
event which fires when an element loses focus, and use the
[`ValidityState`](https://developer.mozilla.org/docs/Web/API/ValidityState) interface to detect if a form control is invalid.

### Ensure users can see the password they entered

The text entered for `<input type="password">` is obscured by default,
to respect the privacy of users.
Help users to enter their password, by showing a `<button>` to toggle the visibility of the entered text.
<iframe allow="camera; clipboard-read; clipboard-write; encrypted-media; geolocation; microphone; midi;" loading="lazy" src="https://codepen.io/web-dot-dev/embed/bd8577c5380c436dba2788c7a2c8652a?height=300&amp;theme-id=light&amp;default-tab=result&amp;editable=true" style="height: 100%; width: 100%; border: 0;" title="Pen bd8577c5380c436dba2788c7a2c8652a by web-dot-dev on Codepen"></iframe>

[Try out the demo](https://codepen.io/web-dot-dev/pen/bd8577c5380c436dba2788c7a2c8652a). Toggle the
visibility of the entered text, by using the **Show Password** `<button>`.
How does this work? Clicking on **Show Password** ,
changes the `type` attribute of the password field from `type="password"` to `type="text"`,
and the `<button>` text changes to 'Hide Password'.

> [!WARNING]
> **Try it:** [Try to improve the **Show Password** `<button>`](https://codepen.io/web-dot-dev/pen/bd8577c5380c436dba2788c7a2c8652a). Where would you position the `<button>`? Could you use only an icon and still make sure the `<button>` is accessible?

It's important to make the **Show Password** button accessible.
Connect the `<button>` with the `<input type="password">` using the `aria-controls` attribute.

> [!CAUTION]
> **Caution:** The `aria-controls` attribute is not supported by every screen reader, but for those that do support it, it's a great enhancement.

To notify screen reader users if the password is currently shown or hidden,
use a hidden element with `aria-live="polite"`, and change its text accordingly.
It's important to enable screen reader users to know when a password is displayed and visible to someone else looking at their screen.

```html
<span class="visually-hidden" aria-live="polite">
    <!-- Dynamically change this text with JavaScript -->
</span>
```

> [!NOTE]
> **Note:** Microsoft Edge shows a built-in password reveal control for `<input type="password">`. To prevent having two reveal buttons, hide the built-in control with: `css ::-ms-reveal { display: none } ` Learn more about [customizing the password reveal button](https://docs.microsoft.com/en-us/microsoft-edge/web-platform/password-reveal).

Learn more about [implementing a show password option](https://technology.blog.gov.uk/2021/04/19/simple-things-are-complicated-making-a-show-password-option/).

## Resources

- [FormData](https://developer.mozilla.org/docs/Web/API/FormData)
- [Constraint Validation API](https://developer.mozilla.org/docs/Web/API/Constraint_validation)
- [`<input type="password">`](https://developer.mozilla.org/docs/Web/HTML/Element/input/password)