Angular 4 and Polymer 3 web component integration

I haven't found a detailed explanation on how to integrate ES6 style web component modules (here based on Polymer 3) into an Angular 4+ app. So I document it here.

Integration scenario

Imagine different teams developing web components running their own release cycle. So...

• you want to integrate a web component hosted by a third party at a given URL / at runtime.
• you explicitly don't want to use self-hosted NPM packages (although you could given the solution provided here).

Polyfill the Angular App

You need to polyfill older browsers that are not (yet) web components compliant: This is the steps which caused a lot of issues for me because I wanted to support IE11 along with ever-green browsers. Theoretically there are two strategies on how to polyfill:

1. Polymer-driven polyfilling: outside the angular app in index.html. Bootstrap Angular app second.
2. Angular-driven polyfilling: let Angular bootstrap first, i.e. let angular do the polyfilling: Bootstrap web components second.

It turns out that (2) is IMHO the better way to go. (1) failed because of current issues in webcomponentsjs.

So, in order to polyfill just follow these steps:

 npm i @webcomponents/webcomponentsjs --save

in polyfills.ts add the following line after the zone polyfill:

import '@webcomponents/webcomponentsjs/bundles/webcomponents-sd-ce-pf.js';

ES6 module loader for older browsers

Since older browsers do not understand the JS module syntax, you need to have an adapter that enables these browsers to load such modules. I use a code snippet from polyserve which you can find under src/assets/esm-amd-loader.min.js. You need to include it in your index.html

  <script id="esm-amd-loader" src="assets/esm-amd-loader.min.js"></script>

Note: This script node has an id which will be relevant later in the process.

Module loader service

You can only load modules after the angular app bootstrapped completely. Since the JS module loading process depends on the platform, i.e. older browsers don't understand js modules, you unfortunately need a few lines of additional bootstrapping code. The module-loader.service.ts will handle this loading process. It will add html script tags after the esm-amd-loader script into the HTML head. You need to call the module-loader service during initialization of your app.component.ts:

ngOnInit(): void {
    this.webComponentLoader.load('https://localhost:8000/custom-polymer-element.js');
  }

Starting the app

First you need to host a custom element under https://localhost:8000/custom-polymer-element.js for this to work:

 npm i -g polymer-cli@1.7.2
 mkdir custom-polymer-element && cd custom-polymer-element
 polymer init polymer-3-element
 polymer serve --protocol https/1.1

Then you can install and start the app

 npm install
 npm start

Resources

• Using Web Components with Angular
• Polyserve Project
• Webcomponentsjs Project
• How to host es6 custom module in static HTML

Known issues

• Accept self-signed dev SSL certificates from the polymer dev server otherwise HTTP requests from your Angular app will be cancelled.
• Chrome initially blocks requests from http://localhost:4200 (angular hosting) to https://localhost:8000 (polymer hosting). You can use a Chrome CORS plugin to work around that for demo purposes. IE doesn't care about CORS.