On this page
Data Serialization
Constraint Validation
Required Fields
Input Patterns
Input Types
Custom Error Messages
Custom Validation Styles
Styling Invalid Form Controls
Inline Form Validation
Getting Associated Form Controls

Form Controls

Every Shoelace component makes use of a shadow DOM to encapsulate markup, styles, and behavior. One caveat of this approach is that native <form> elements do not recognize form controls located inside a shadow root.

Shoelace solves this problem by using the formdata event, which is available in all modern browsers. This means, when a form is submitted, Shoelace form controls will automatically append their values to the FormData object that’s used to submit the form. In most cases, things will “just work.” However, if you’re using a form serialization library, it might need to be adapted to recognize Shoelace form controls.

Shoelace uses event listeners to intercept the form’s formdata and submit events. This allows it to inject data and trigger validation as necessary. If you’re also attaching an event listener to the form, you must attach it after Shoelace form controls are connected to the DOM, otherwise your logic will run before Shoelace has a chance to inject form data and validate form controls.

Data Serialization

Serialization is just a fancy word for collecting form data. If you’re relying on standard form submissions, e.g. <form action="...">, you can probably skip this section. However, most modern apps use the Fetch API or a library such as axios to submit forms using JavaScript.

The FormData interface offers a standard way to serialize forms in the browser. You can create a FormData object from any <form> element like this.

const form = document.querySelector('form');
const data = new FormData(form);

// All form control data is available in a FormData object

However, some folks find FormData tricky to work with or they need to pass a JSON payload to their server. To accommodate this, Shoelace offers a serialization utility that gathers form data and returns a simple JavaScript object instead.

import { serialize } from '@teamshares/shoelace/dist/utilities/form.js';

const form = document.querySelector('form');
const data = serialize(form);

// All form control data is available in a plain object

This results in an object with name/value pairs that map to each form control. If more than one form control shares the same name, the values will be passed as an array, e.g. { name: ['value1', 'value2'] }.

Constraint Validation

Client-side validation can be enabled through the browser’s Constraint Validation API for Shoelace form controls. You can activate it using attributes such as required, pattern, minlength, maxlength, etc. Shoelace implements many of the same attributes as native form controls, but check the documentation for a list of supported properties for each component.

If you don’t want to use client-side validation, you can suppress this behavior by adding novalidate to the surrounding <form> element.

If this syntax looks unfamiliar, don’t worry! Most of what you’re learning on this page is platform knowledge that applies to regular form controls, too.

Client-side validation can be used to improve the UX of forms, but it is not a replacement for server-side validation. You should always validate and sanitize user input on the server!

Required Fields

To make a field required, use the required attribute. Required fields will automatically receive a * after their labels. This is configurable through the --sl-input-required-content custom property.

The form will not be submitted if a required field is incomplete.

<form class="input-validation-required">
  <sl-input name="name" label="Name" required></sl-input>
  <br />
  <sl-select label="Favorite Animal" clearable required>
    <sl-option value="birds">Birds</sl-option>
    <sl-option value="cats">Cats</sl-option>
    <sl-option value="dogs">Dogs</sl-option>
    <sl-option value="other">Other</sl-option>
  </sl-select>
  <br />
  <sl-textarea name="comment" label="Comment" required></sl-textarea>
  <br />
  <sl-checkbox required>Check me before submitting</sl-checkbox>
  <br /><br />
  <sl-button type="submit" variant="primary">Submit</sl-button>
</form>

<script type="module">
  const form = document.querySelector('.input-validation-required');

  // Wait for controls to be defined before attaching form listeners
  await Promise.all([
    customElements.whenDefined('sl-button'),
    customElements.whenDefined('sl-checkbox'),
    customElements.whenDefined('sl-input'),
    customElements.whenDefined('sl-option'),
    customElements.whenDefined('sl-select'),
    customElements.whenDefined('sl-textarea')
  ]).then(() => {
    form.addEventListener('submit', event => {
      event.preventDefault();
      alert('All fields are valid!');
    });
  });
</script>

Input Patterns

To restrict a value to a specific pattern, use the pattern attribute. This example only allows the letters A-Z, so the form will not submit if a number or symbol is entered. This only works with <sl-input> elements.

<form class="input-validation-pattern">
  <sl-input name="letters" required label="Letters" pattern="[A-Za-z]+"></sl-input>
  <br />
  <sl-button type="submit" variant="primary">Submit</sl-button>
  <sl-button type="reset" variant="default">Reset</sl-button>
</form>

<script type="module">
  const form = document.querySelector('.input-validation-pattern');

  // Wait for controls to be defined before attaching form listeners
  await Promise.all([
    customElements.whenDefined('sl-button'),
    customElements.whenDefined('sl-input')
  ]).then(() => {
    form.addEventListener('submit', event => {
      event.preventDefault();
      alert('All fields are valid!');
    });
  });
</script>

Input Types

Some input types will automatically trigger constraints, such as email and url.

<form class="input-validation-type">
  <sl-input type="email" label="Email" placeholder="you@example.com" required></sl-input>
  <br />
  <sl-input type="url" label="URL" placeholder="https://example.com/" required></sl-input>
  <br />
  <sl-button type="submit" variant="primary">Submit</sl-button>
  <sl-button type="reset" variant="default">Reset</sl-button>
</form>

<script type="module">
  const form = document.querySelector('.input-validation-type');

  // Wait for controls to be defined before attaching form listeners
  await Promise.all([
    customElements.whenDefined('sl-button'),
    customElements.whenDefined('sl-input')
  ]).then(() => {
    form.addEventListener('submit', event => {
      event.preventDefault();
      alert('All fields are valid!');
    });
  });
</script>

Custom Error Messages

To create a custom validation error, pass a non-empty string to the setCustomValidity() method. This will override any existing validation constraints. The form will not be submitted when a custom validity is set and the browser will show a validation error when the containing form is submitted. To make the input valid again, call setCustomValidity() again with an empty string.

<form class="input-validation-custom">
  <sl-input label="Type “shoelace”" required></sl-input>
  <br />
  <sl-button type="submit" variant="primary">Submit</sl-button>
  <sl-button type="reset" variant="default">Reset</sl-button>
</form>

<script type="module">
  const form = document.querySelector('.input-validation-custom');
  const input = form.querySelector('sl-input');

  // Wait for controls to be defined before attaching form listeners
  await Promise.all([
    customElements.whenDefined('sl-button'),
    customElements.whenDefined('sl-input')
  ]).then(() => {
    form.addEventListener('submit', event => {
      event.preventDefault();
      alert('All fields are valid!');
    });

    input.addEventListener('sl-input', () => {
      if (input.value === 'shoelace') {
        input.setCustomValidity('');
      } else {
        input.setCustomValidity("Hey, you're supposed to type 'shoelace' before submitting this!");
      }
    });
  });
</script>

Custom validation can be applied to any form control that supports the setCustomValidity() method. It is not limited to inputs and textareas.

Custom Validation Styles

Due to the many ways form controls are used, Shoelace doesn’t provide out of the box validation styles for form controls as part of its default theme. Instead, the following attributes will be applied to reflect a control’s validity as users interact with it. You can use them to create custom styles for any of the validation states you’re interested in.

data-required - the form control is required
data-optional - the form control is optional
data-invalid - the form control is currently invalid
data-valid - the form control is currently valid
data-user-invalid - the form control is currently invalid and the user has interacted with it
data-user-valid - the form control is currently valid and the user has interacted with it

These attributes map to the browser’s built-in pseudo classes for validation: :required, :optional, :invalid, :valid, and the proposed :user-invalid and :user-valid.

In the future, data attributes will be replaced with custom pseudo classes such as :--valid and :--invalid. Shoelace is using data attributes as a workaround until browsers support custom states through ElementInternals.states.

Styling Invalid Form Controls

You can target validity using any of the aforementioned data attributes, but it’s usually preferable to target data-user-invalid and data-user-valid since they get applied only after a user interaction such as typing or submitting. This prevents empty form controls from appearing invalid immediately, which often results in a poor user experience.

This example demonstrates custom validation styles using data-user-invalid and data-user-valid. Try Typing in the fields to see how validity changes with user input.

<form class="validity-styles">
  <sl-input
    name="name"
    label="Name"
    help-text="What would you like people to call you?"
    autocomplete="off"
    required
  ></sl-input>

  <sl-select name="animal" label="Favorite Animal" help-text="Select the best option." clearable required>
    <sl-option value="birds">Birds</sl-option>
    <sl-option value="cats">Cats</sl-option>
    <sl-option value="dogs">Dogs</sl-option>
    <sl-option value="other">Other</sl-option>
  </sl-select>

  <sl-checkbox value="accept" required>Accept terms and conditions</sl-checkbox>

  <sl-button type="submit" variant="primary">Submit</sl-button>
  <sl-button type="reset" variant="default">Reset</sl-button>
</form>

<script type="module">
  const form = document.querySelector('.validity-styles');

  // Wait for controls to be defined before attaching form listeners
  await Promise.all([
    customElements.whenDefined('sl-button'),
    customElements.whenDefined('sl-checkbox'),
    customElements.whenDefined('sl-input'),
    customElements.whenDefined('sl-option'),
    customElements.whenDefined('sl-select')
  ]).then(() => {
    form.addEventListener('submit', event => {
      event.preventDefault();
      alert('All fields are valid!');
    });
  });
</script>

<style>
  .validity-styles sl-input,
  .validity-styles sl-select,
  .validity-styles sl-checkbox {
    display: block;
    margin-bottom: var(--sl-spacing-medium);
  }

  /* user invalid styles */
  .validity-styles sl-input[data-user-invalid]::part(base),
  .validity-styles sl-select[data-user-invalid]::part(combobox),
  .validity-styles sl-checkbox[data-user-invalid]::part(control) {
    border-color: var(--sl-color-danger-600);
  }

  .validity-styles [data-user-invalid]::part(form-control-label),
  .validity-styles [data-user-invalid]::part(form-control-help-text),
  .validity-styles sl-checkbox[data-user-invalid]::part(label) {
    color: var(--sl-color-danger-700);
  }

  .validity-styles sl-checkbox[data-user-invalid]::part(control) {
    outline: none;
  }

  .validity-styles sl-input:focus-within[data-user-invalid]::part(base),
  .validity-styles sl-select:focus-within[data-user-invalid]::part(combobox),
  .validity-styles sl-checkbox:focus-within[data-user-invalid]::part(control) {
    border-color: var(--sl-color-danger-600);
    box-shadow: 0 0 0 var(--sl-focus-ring-width) var(--sl-color-danger-300);
  }

  /* User valid styles */
  .validity-styles sl-input[data-user-valid]::part(base),
  .validity-styles sl-select[data-user-valid]::part(combobox),
  .validity-styles sl-checkbox[data-user-valid]::part(control) {
    border-color: var(--sl-color-success-600);
  }

  .validity-styles [data-user-valid]::part(form-control-label),
  .validity-styles [data-user-valid]::part(form-control-help-text),
  .validity-styles sl-checkbox[data-user-valid]::part(label) {
    color: var(--sl-color-success-700);
  }

  .validity-styles sl-checkbox[data-user-valid]::part(control) {
    background-color: var(--sl-color-success-600);
    outline: none;
  }

  .validity-styles sl-input:focus-within[data-user-valid]::part(base),
  .validity-styles sl-select:focus-within[data-user-valid]::part(combobox),
  .validity-styles sl-checkbox:focus-within[data-user-valid]::part(control) {
    border-color: var(--sl-color-success-600);
    box-shadow: 0 0 0 var(--sl-focus-ring-width) var(--sl-color-success-300);
  }
</style>

Inline Form Validation

By default, Shoelace form controls use the browser’s tooltip-style error messages. No mechanism is provided to show errors inline, as there are too many opinions on how that would work when combined with native form controls and other custom elements. You can, however, implement your own solution using the following technique.

To disable the browser’s error messages, you need to cancel the sl-invalid event. Then you can apply your own inline validation errors. This example demonstrates a primitive way to do this.

<form class="inline-validation">
  <sl-input
    name="name"
    label="Name"
    help-text="What would you like people to call you?"
    autocomplete="off"
    required
  ></sl-input>

  <div id="name-error" aria-live="polite" hidden></div>

  <sl-button type="submit" variant="primary">Submit</sl-button>
  <sl-button type="reset" variant="default">Reset</sl-button>
</form>

<script type="module">
  const form = document.querySelector('.inline-validation');
  const nameError = document.querySelector('#name-error');

  // Wait for controls to be defined before attaching form listeners
  await Promise.all([
    customElements.whenDefined('sl-button'),
    customElements.whenDefined('sl-input')
  ]).then(() => {
    // A form control is invalid
    form.addEventListener(
      'sl-invalid',
      event => {
        // Suppress the browser's constraint validation message
        event.preventDefault();

        nameError.textContent = `Error: ${event.target.validationMessage}`;
        nameError.hidden = false;

        event.target.focus();
      },
      { capture: true } // you must use capture since sl-invalid doesn't bubble!
    );

    // Handle form submit
    form.addEventListener('submit', event => {
      event.preventDefault();
      nameError.hidden = true;
      nameError.textContent = '';
      setTimeout(() => alert('All fields are valid'), 50);
    });

    // Handle form reset
    form.addEventListener('reset', event => {
      nameError.hidden = true;
      nameError.textContent = '';
    });
  });
</script>

<style>
  #name-error {
    font-size: var(--sl-input-help-text-font-size-medium);
    color: var(--sl-color-danger-700);
  }

  #name-error ~ sl-button {
    margin-top: var(--sl-spacing-medium);
  }

  .inline-validation sl-input {
    display: block;
  }

  /* user invalid styles */
  .inline-validation sl-input[data-user-invalid]::part(base) {
    border-color: var(--sl-color-danger-600);
  }

  .inline-validation [data-user-invalid]::part(form-control-label),
  .inline-validation [data-user-invalid]::part(form-control-help-text) {
    color: var(--sl-color-danger-700);
  }

  .inline-validation sl-input:focus-within[data-user-invalid]::part(base) {
    border-color: var(--sl-color-danger-600);
    box-shadow: 0 0 0 var(--sl-focus-ring-width) var(--sl-color-danger-300);
  }

  /* User valid styles */
  .inline-validation sl-input[data-user-valid]::part(base) {
    border-color: var(--sl-color-success-600);
  }

  .inline-validation [data-user-valid]::part(form-control-label),
  .inline-validation [data-user-valid]::part(form-control-help-text) {
    color: var(--sl-color-success-700);
  }

  .inline-validation sl-input:focus-within[data-user-valid]::part(base) {
    border-color: var(--sl-color-success-600);
    box-shadow: 0 0 0 var(--sl-focus-ring-width) var(--sl-color-success-300);
  }
</style>

This example is meant to demonstrate the concept of providing your own error messages inline. It is not intended to scale to more complex forms. Users who want this functionality are encouraged to build a more appropriate validation solution using the techniques shown below. Depending on how you implement this feature, custom error messages may affect the accessibility of your form controls.

Getting Associated Form Controls

At this time, using HTMLFormElement.elements will not return Shoelace form controls because the browser is unaware of their status as custom element form controls. Fortunately, Shoelace provides an elements() function that does something very similar. However, instead of returning an HTMLFormControlsCollection, it returns an array of HTML and Shoelace form controls in the order they appear in the DOM.

import { getFormControls } from '@teamshares/shoelace/dist/utilities/form.js';

const form = document.querySelector('#my-form');
const formControls = getFormControls(form);

console.log(formControls); // e.g. [input, sl-input, ...]

You probably don’t need this function! If you’re gathering form data for submission, you probably want to use Data Serialization instead.