heresy

React like Custom Elements via V1 API builtin extends.

The following example is live in Code Pen.

import {define, html, render} from 'heresy';

class MyButton extends HTMLButtonElement {

  // the only mandatory static field
  static get tagName() { return 'button'; }

  // optional but mandatory for transpiled code
  static get name() { return 'MyButton'; }

  // (optional) intercepts some attribute (any value)
  set props(props) { this._props = props; }
  get props() { return this._props; }

  // (optional) render once connected
  connectedCallback() { this.render(); }

  // (optional) populate this button content
  //            (kinda useless with void elements such img, input, ...)
  render() {
    // this.html or this.svg are provided automatically
    this.html`Click ${this.props.name}!`;
  }
}

// define the custom element (class name mandatory too)
define(MyButton);

// populate some node
render(document.body, () => html`<MyButton props=${{name: 'Magic'}} />`);

setTimeout(() => console.log(document.body.innerHTML));
// <button is='mybutton-heresy'>Click Magic!</button>

Compatibility

The test page uses, and describes, few techniques to address all browsers, from IE9 to latest evergreen.

The following list describes the heresy's compatibility break down:

• IE9 and IE10 might need an Object.freeze patch, to avoid breaking on frozen template literals when passed to polyfilled WeakMaps. The patch checks for the existence of WeakMap, hence it's completely safe for any modern browser, including IE11.
• old Edge and all IE might need a Custom Elements polyfill upfront. In this case the famous document-register-element would be the suggested choice, since it patches built in right away too.
• Safari and WebKit have an understandable but pretty stubborn position regarding built-in elements, so that a 1K polyfill is needed, in case you target Safari and WebKit too.

Broader compatibility in a nutshell

<script>
  // Patch for IE9 and IE10 (browsers with no WeakMap)
  // frozen template literals cannot be addressed by common WeakMap polyfills
  // this patch avoid Object.freeze to break when template literals are passed to WeakMaps
  this.WeakMap||!function(O,f){f=O.freeze;O.freeze=function(o){return 'raw' in o?o:f(o)}}(Object);
</script>
<script>
  // Patch for all IE, Edge, and older browsers without customElements
  // completely ignored/irrelevant for any other modern browser
  // https://github.com/WebReflection/document-register-element
  this.customElements||document.write(
    '<script src="https://unpkg.com/document-register-element"><\x2fscript>'
  );
</script>
<script defer src="https://unpkg.com/@ungap/custom-elements-builtin">/*
  1K Patch for Safari/WebKit
  https://github.com/ungap/custom-elements-builtin
*/</script>

Concept

Custom Elements builtin are likely the best thing we have to build components the way we want to.

Instead of using a non standard indirection as JSX is, we can use the power of domtagger, the hyperHTML and lighterhtml tag engine, to replace once any <DefinedElement /> with, or without nested nodes.

The Custom Elements V1 API provides enough primitives to intercepts any sort of attribute (i.e. the props in the example), but also react on events such connectedCallback or disconnectedCallback and attributeChangedCallback.

Mixed up with builtin extends in a way that any component is a real thing on the DOM, instead of a facade of itself, herey makes creation of simple to complex App a no-brainer: define the content through this.html or this.svg and that's it.

When any class is defined, it's not just necessarily a useless HTMLElement, it can be pretty much any kind of element.

The following example is live in Code Pen.

import {define, html, render} from 'heresy';

// a div
define(class Div extends HTMLDivElement {
  static get tagName() { return 'div'; }
})

// a paragraph
define(class P extends HTMLParagraphElement {
  static get tagName() { return 'p'; }
})

// a h1
define(class H1 extends HTMLHeadingElement {
  static get tagName() { return 'h1'; }
})

render(document.body, () => html`
  <Div>
    <H1>Hello there</H1>
    <P>This is how custom elements look via heresy.</P>
    <P>Isn't this awesome?</P>
  </Div>
`);

Goals

• declared elements are the instance you'd expect (no virtual, no facade)
• declared elements can be of any kind (table, tr, select, option, ...)
• any attribute change, or node lifecycle, can be tracked via Custom Elements V1 API (no componentDidMount and friends)
• no redundant dom nodes, no ghost fragments, a clean as possible output

CSS - How to query or style this heresy

Every builtin extend will have a -heresy suffix to ensure both that the Custom Element can be registered, but also grant a common pattern to reach components.

*[is$='-heresy']:hover {
  opacity: .8;
}

tag[is='specific-heresy'] {
  display: block;
}