Form Element Mixin

A mixin to associate a Custom Element with a form: https://web.dev/articles/more-capable-form-controls

Usage

To install run:

npm install form-element-mixin

Then import the mixin:

import {FormElementMixin} from './node_modules/@dannymoerkerke/form-element-mixin/src/FormElementMixin';

Then pass the element you want to extend to the FormElementMixin factory function:

class MyInput extends FormElementMixin(HTMLElement) {
  constructor() {
    super();
    const shadowRoot = this.attachShadow({mode: 'open'});

    shadowRoot.innerHTML = `
      <input>
    `;
  }

  connectedCallback() {
    this.inputNode = this.shadowRoot.querySelector('input');
  }
}

customElements.define('my-input', MyInput);

Note that you need to set the internal input to the inputNode property.

Add the HTML tag:

<my-element
  type="text"
  name="my-input"
  required
  data-valuemissing="This field is required"
></my-element>

Form Validation

The Custom Element can participate in form validation by setting constraints using the standard attributes used for native form elements (required, minlength, maxlength, min, max, pattern etc.)

Custom error messages can be displayed by setting the error messages on the element with data attributes (see below) and the native error message can be hidden so the error can be shown in another HTML element the user chooses.

To do this, set an event handler for the invalid event and get the error message from the element's validationMessage property:

const errorMessage = document.querySelector('.error-message');

myInput.addEventListener('invalid', (e) => {
  console.log('invalid', customInput.validationMessage);

  errorMessage.textContent = customInput.validationMessage;
});

When custom-error-message is set the element will get an invalid attribute when it's invalid which can be used to display the error message with CSS:

.error-message {
  display: none;
}

my-input[invalid] ~ .error-message {
  display: block;
}

Attributes

type: type of the element, can be any type that is valid for <input>
name: name of the element
placeholder: placeholder for the element
disabled: whether the element is disabled
required: whether the element is required
validate-on-change: whether the element should be validated on change, meaning when it's interacted with and the blur event fires
custom-error-message: shows a custom error message instead of the native error, enables styling of the error message
data-valuemissing: error message when the element is required and has no value
data-typemismatch: error message when the syntax of the data is not correct, for example when type is email but the value is not a valid email address
data-tooshort: error message when the element has a minlength attribute and the length of the value is less than the value of this attribute
data-toolong: error message when the element has a maxlength attribute and the length of the value is greater than the value of this attribute
data-rangeunderflow: error message when the element has a min attribute and the numeric value of the field is less than the value of this attribute, works with type="number"
data-rangeoverflow: error message when the element has a max attribute and the numeric value of the field is greater than the value of this attribute, works with type="number"
data-patternMismatch: error message when the element has a pattern attribute but the data of the field does not follow this pattern

Testing

Run npm test.

Browser support