custom-elements-hmr-polyfill

100% standard-compliant polyfill to allow WebComponent re-definition at runtime (used for HMR)

At the time of the creation of this readme, the API customElements.define(...) doesn't allow to re-define a Web Component with the same tag name but a different implementation. This limitation made it impossible to do Hot Module Replacement (HMR) with standard Web Components. - until today.

Live Demo

• Codesandbox

How to install it?

• npm install custom-elements-hmr-polyfill

What does it do?

This polyfill overrides the native browser API customElement.define and enables re-definition of Web Components at runtime.

Most simple integration

import { applyPolyfill } from 'custom-elements-hmr-polyfill';

// custom-elements-hmr-polyfill
applyPolyfill();

//  reset the root to trigger rerender after the HMR event
if (document.body) {
    requestAnimationFrame(() => {
        document.body.innerHTML = '';
        document.body.innerHTML = '<root-app></root-app>';
    });
}

export class RootApp extends HTMLElement {
    private name = 'I am RootApp';
    connectedCallback() {
        this.innerHTML = `
      <div style="background-color:green">${this.name}</div>
    `;
    }
}

// PS! customElements.define can be called more then once when running pollyfil
// it need to do this to activate new instance
customElements.define('root-app', RootApp);

Browser Support

This polyfill requires support of the following browser API's (natively).

• Proxy
• MutationObserver
• customElements

Limitations

---

1:

attributeChangedCallback returned namespace will be null Not sure how namespaces works for attributes atm...

---

2:

Since HMR polyfill wraps your class in a proxy, you need to use customElements.get('my-element') to get the class thats sent to customElements.define().

Sample under will not work, since your class isn't really sent to customELements.define, a proxy handler is.

class MyElement extends HTMLElement {}
window.customElements.define('my-element', MyElement);
new MyElement();

Solution:

class MyElement extends HTMLElement {}
window.customElements.define('my-element', MyElement);
const MyElementHMR = customElements.get('my-element');
new MyElementHMR();

Or just use document.createElement('my-element')

---

3:
Some elements dont like the deep patching we do, like lit-element.

Solution: You can try with (window as any).HMR_SKIP_DEEP_PATCH = true to skip some of this.

---

4:
Atm I return empty array on observedAttributes in the proxy so browser dont get them. This might create issues for some libs, if it does we can maybe add a option to return a array and just overlook some elements might be called 2 times

---

5:

Some elements just do a lot and have own state containers / instance variables I cant do much about.

Try and load these before this polyfill, you really dont need these to be redefined..

--

For reference see: W3C/WhatWG standard limitation of Web Component re-definition.

Distribution formats

The bundled npm package contains the following formats:

• IIFE (.iife.js)
• AMD (.amd.js)
• Common JS (.cjs.js)
• ES Module (.mjs)
• SystemJS (.system.js)
• UMD (.umd.js)

You can find single file outputs in dist/custom-elements-hmr-polyfill.*, i.e. dist/custom-elements-hmr-polyfill.iife.js.

Furthermore, multiple file outputs are available in dist/*/**/*.js. i.e. dist/AMD/**/*.js.

• AMD
• CommonJS
• ES6
• ES2015
• ESNext
• System
• UMD

Advanced: How to start the sample code (of this repo)?

• npm run bootstrap
• npm start

How to transpile and bundle this polyfill on your own?

• npm run bootstrap
• npm build