2.5.3 • Public • Published


A collection of Custom Elements helpers for the v1 spec.

This documentation is in alphabetical order. If you want a basic understanding of how things work, skip the Attributes & Elements chapters. It's most important to know how BaseController and defineCustomElement works. Everything else is optional.


Attributes are packages of properties & methods that you can mix in on your custom elements controllers.

Important: Usage on native HTML elements, e.g. <img> or <a>, is not possible due to browser support restrictions.


The media attribute adds media query support to your custom element.


  • media - string - Value of the media attribute
  • matchesMedia - boolean - True when the current viewport matches the media query


  • whenMediaMatches() - Returns a Promise that resolves once the media query first is matched
  • whenMediaUnmatches() - Returns a Promise that resolves once the media query first is unmatched
  • watchMedia(match, unmatch) - Takes two callbacks that are called when the media query matches and unmatches, respectively

Example usage

The following example will mixin the media attribute.

The init().render().bind() cycle from the base controller will only start once the viewport matches the (min-width: 768px) media query (because of the resolve() override).

import { defineCustomElement, BaseController, AttrMedia as media } from 'custom-elements-helpers';
defineCustomElement('foo-bar', {
    attributes: [ media ],
    controller: class extends BaseController {
        resolve() {
            return Promise.all([
<foo-bar media="(min-width: 768px)">

Touch Hover

The touch-hover attribute adds touch support for hover interactions. It adds is-touch and is-hover classes to your custom element so that you can style your animations accordingly.


  • touchHover - Returns auto when touch-hover is set. Else, returns false.


  • enableTouchHover() - Call this method inside the bind method of your custom element

This binds some methods to touchstart & click, so that it works like this:

On non-touch devices, a hover is a hover and a click is a click.

On touch devices, a first tap adds the is-touch class to the element. From this moment on you know that the current device is a touch device. The first tap also adds the is-hover class. A second tap removed the is-hover class.

It also makes sure the touch device's native :hover handling gets blocked properly so you can handle it like you wish.

Example usage

import { defineCustomElement, BaseController, AttrTouchHover as touchHover } from 'custom-elements-helpers';
defineCustomElement('foo-bar', {
    attributes: [ touchHover ],
    controller: class extends BaseController {
        bind() {
            return this;
<foo-bar class="foo-bar" touch-hover>
  <span class="foo-bar__show-on-hover">
.foo-bar {
  display: inline-block;
  position: relative;
.foo-bar__show-on-hover {
  display: none;
  position: absolute;
  left: 0;
  top: 120%;
  width: 100%;
.foo-bar:not(.is-touch):hover .foo-bar__show-on-hover, .foo-bar__show-on-hover {
    display: block;


Currently, there is only one controller, conveniently named BaseController.


It's important to extend from this controller if you create an element using defineCustomElement from the utils.

A BaseController instance is a standardized wrapper for your custom element logic. It lays out a consistent blueprint across developers & projects that streamlines the lifecycle of your controller.

Lifecycle: Birth

With Custom Elements, your controller gets triggered by your browser automatically when the element you defined gets attached to the DOM. This is what happens next:

Step 1: Resolve

The resolve method should return a Promise.
It allows you to wait for something, preload something, … If you want your controller to kick in immediately, return a resolved Promise, by using return Promise.resolve(true).

The default behavior is to wait until document.readyState === 'complete'.

Once the promise has resolved, your custom element will have the is-resolved class attached. You can use this to style your element differently when it's in a loading state.

Step 2: Init

Once your custom element instance is resolved, the init method is called. You can do your initial setup here.

Step 3: Render

After your initial setup, the render method is called. You can assume a DOM-ready and configured environment here. this.el has a reference to the custom element DOM node so go wild (if you have to).

Step 4: Bind

With everything rendered, now's the time to attach your event listeners, using this.on and this.once. Read more below.

Working with events

There are four methods on the BaseController that will help you tremendously when working with events and custom elements. When you use these methods, you make sure that event listeners that you've added are removed when the custom element is no longer in the DOM etc.

You use on(name, handler, target = null, options = false) when you want to listen for an event. target falls back to this.el. You can also pass a selector in the name (e.g. click .button). When you do this, your handler gets a second argument pointing to the target.

Some examples:

this.on('mouseenter', (e) => {
  console.log('Mouse entered the custom element');
this.on('click .button', (e, target) => {
  console.log('Click on', button.textContent');
this.on('mousemove', (e) => {
  console.log('Mouse move somewhere on body');
}, document.body);

The once() method has the same footprint but will remove itself after one event.

These event listeners can also be removed. Use off(name, target = null) to remove an event listener registered through on. target falls back to this.el.

Keep in mind that all targets for the same event should be removed separately. If you have a click handler on both this.el and document.body, you would have to call:'click'); // Falls back to `this.el`'click', document.body);

A custom element can also emit events to communicate to other parts of your website. Using emit(name, data = {}, options = {}) you can easily pass around data.

The data object will be passed on to your handler as e.detail. The options object can have bubbles and cancelable as specified in the spec.

An example:

// A setter on your `foo-bar` element
set current(to) {
  this._current = to;
  this.emit('foo-bar:current', { current: this._current });
// In another custom element its `bind()` method
this.on('foo-bar:current', (e) => {
  console.log(`Current changed to ${e.detail.current}`);
}, document.body);

Lifecycle: Death

When an element gets detached from the DOM, the BaseController goes through some housekeeping methods.

Step 1: Destroy

Remove the is-resolved class from the element.

Step 2: Unbind

Remove all event listeners that you registered through on or once.



Wrap AJAX form around a working <form> to have it send over AJAX.
Form submits are supported over GET, POST and JSONP (with the jsonp attribute).

The AJAX form submits all form data to the form's action, using the form's method.

By default, it looks for an element with class js-ajax-form-success to show when the submit was successful. When a form submit is successful, the form will be removed (by default). Likewise, it looks for an element with class js-ajax-form-error to show an error message.

Override the default success & error handlers by overriding onSuccess(res) and onError(err).


  • Client-side form validation
  • Handling server-side form validation
  • Per-field error messages


  • jsonp - boolean - Needs JSONP for AJAX handling (e.g. createsend)

Example usage

import { defineCustomElement, ajaxForm } from 'custom-elements-helpers';
defineCustomElement('my-ajax-form', {
    attributes: ajaxForm.attributes,
    controller: class extends ajaxForm.controller {
        onSuccess(res) {
            // Custom success handler
        onError(err) {
            // Custom error handler
    <form action="/subscribe" method="POST">
        <input type="email" name="email" />
        <input type="submit" value="Subscribe" />

Key Trigger

You can use key trigger to bind a key to an anchor. It'll look for the href attribute on itself or on the first child that has one and trigger the click event on it.


  • key - integer - Key code of the key you want to listen for

Example usage

import { defineCustomElement, keyTrigger } from 'custom-elements-helpers';
defineCustomElement('my-key-trigger', keyTrigger);
<my-key-trigger key="37">
    <a href="/foo">Previous</a>
<my-key-trigger key="39">
    <a href="/bar">Next</a>

Smooth State

Smooth State will listen for internal clicks and asynchronously fetch the contents and swap them, allowing you to transition between pages while not losing browser state.

Warning: this has quite a few known bugs. Use with caution (and test in Safari)


There are four public hooks where you can trigger animations. Each hook should return the transition object it gets passed in as an argument. You can make modifications to this object & they will adapt the transition accordingly.

  • onBefore() - Before the page is fetched
  • onStart() - While the page fetch is running, but before it is ready
  • onReady() - When the page fetch is ready, but before it is rendered
  • onAfter() - After the fetched page is rendered

Example usage

This is kind of complex to demo, example will be added later.



This exports the defineCustomElement(tag, options = {}) method. Use this instead of customElements.define to register your custom elements.

The tag is the name of your custom element. The spec defined this to need a hypen. If in doubt, use the mr-prefix.

The options object is where the magic happens. Currently, a attributes and controller key is supported. controller is a class that extends from BaseController (see chapter Controllers).

attributes is an array of attributes that you want to support on your custom element. This removes repeating-yourself getters and setters into an easy to read syntax. You can also mix in more extensive behaviors from this library (see chapter Attributes). If you pass a mix-in it should provide a static attachTo method. Some examples below:

import { AttrMedia as media } from 'custom-elements-helpers';
defineCustomElement('foo-bar', {
  attributes: [
    { attribute: 'intattr', type: 'int' },
    { attribute: 'boolattr', type: 'boolean' },
    media // We can do this because it exposes .attachTo

The type int parsed this.intattr as an integer, so it's an actual number value.
The type boolean makes sure this.boolattr is always boolean.
The type string is the default.


These methods are currently scoped for internal use only.

Quick heads up:
parse parses an event name like click .button
getPath is a cross-browser equivalent for e.path (This should be made public)


parseHTML(html, selector = null) takes a string of HTML and returns an object { title, content, meta }.

The title is the document title.
The content is the DOM node(s) matching your selector. If no selector is given, the whole parsed body will be returned.
The meta key holds an array of { name, property, content } meta tags. This matches with the <meta name="foo" property="bar" content="baz">. The property attribute is only used by OpenGraph, AFAIK. Other meta tags will only have the content attribute. The name viewport is blacklisted, to avoid accidentaly setting or removing the viewport metatag.

renderNodes(content, container) takes a DOM node content and renders its children into container. This removes existing content from container.

cleanNodes(nodes, selector) takes a DOM node nodes and filters its child nodes against selector. If the child node matches selector, it gets removed from nodes. Keep in mind that this works on the reference itself so it will change the original DOM node.


The custom-elements.js polyfill from Polymer / is included too. At the moment, it's best to copy paste it from the repo into your public folder. Suggestions on how to improve this are welcome.




npm i custom-elements-helpers

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  • pieterbeulque
  • yvesvanbroekhoven