HTML Element Plus

2022 - This is still actively maintained, PRs and issues are welcome if you use it and encounter any issues.

The bits of Custom Elements I keep reimplementing in one handy class.

I have extended HTMLElement as HTMLElementPlus to contain the bits of functionality I keep reimplementing.

• Features
• Example
• Example as Mixins for other Web Component Libraries

Example

import HTMLElementPlus from 'https://unpkg.com/html-element-plus/html-element-plus.js';
import html from 'https://unpkg.com/html-element-plus/noop.js';

class MyElement extends HTMLElementPlus {
	constructor () {
		super();
	}

	// Set the HTML for the template
	static get templateHTML() {
		return html`<style>
			:host pre {
				background-color: #333;
				color: white;
				display: inline-block;
				tab-size: 2;
				-moz-tab-size: 2;
				-webkit-tab-size: 2;
			}
		</style>
		<link href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/monokai.min.css" rel="stylesheet" >
		<slot></slot><pre><code ref="code"></code></pre>`;
	}

	allAttributesChangedCallback() {
		if (!this.shadowRoot) {
			this.attachShadow({mode: 'open'});
			this.shadowRoot.appendChild(this.templateContent);
		}
	}

	static defaultAttributeValue (name) {
		if (name === 'lang') return null;
	}

	static get observedAttributes() {
		return ['lang'];
	}
}

customElements.define('my-element', MyElement);

Example Using Mixins

/* Pick one or more of the following features:


allAttributesChanged:
* Adds a callback for when all the attributes have been parsed.
* Also provides optional methods for Attribute parsing and providing default values

	static get observedAttributes() {}
	static defaultAttributeValue (name) {}
	static parseAttributeValue (name, value) {}

emitEvent
* Provides el.emitEvent(name, details) for quick event firing

templateHelper
* Provides management for templates and automatically running ShadyCSS
* Set `static get templateHTML` to define your html, use el.templateContent to get it out

refs
* Access defined elements in the shadow dom by using the `ref="foo"` attribute.
* Read it later using el.refs.foo 

*/
import {
	allAttributesChanged,
	refs
} from 'https://unpkg.com/html-element-plus/mixins.js';

class MyElement extends refs(allAttributesChanged(HTMLElement)) {
	constructor () {
		super();
	}

	allAttributesChangedCallback() {
		if (!this.shadowRoot) {
			this.attachShadow({mode: 'open'});
			this.shadowRoot.innerHTML = '<div ref="test" id="test-div"></div>';
		}
	}

	static get observedAttributes() {
		return ['one', 'two'];
	}

	static defaultAttributeValue (name) {
		if (name === 'one') return this.parseAttributeValue('one', '1');
		if (name === 'two') return this.parseAttributeValue('two', '2');
	}

	static parseAttributeValue (name, value) {
		return parseInt(value);
	}
}

Features

These are the current features.

Collected Attribute Change Callbacks

Provides a callback when all attributes have been parsed, rather than one-by-one. allAttributesChangedCallback useful for waiting to handle all at once.

allAttributesChangedCallback gets called with an object with parsed attributes.

The parser can be set by setting the function static parseAttributeValue(name, value) in the class.

Default attribute values can be provided by setting the static defaultAttributeValue(name) function, so you can provide sensible fallback values.

This also gets called during the constructor if there are no attributes listed.

Query the shadow dom by reference

E.g. an element in the shadow dom: <span ref="foobar"></span> can be queried using this.refs.foobar;

Easy event firing.

Fire an event using this.emitEvent('event-name', {foo: 'bar'});

This can be listed for using, el.addEventListener;

Automatic Template Creation

Setting templateHTML in your class to return a string of you template contents will build and populate a template. It will automatically run shadyDOM if you are using the polyfill. E.g.