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A case of Web Component: Revising wc-blink

Posted on  by Tommy Ku

This post is more than 2 years old, it may contain outdated information

On 2015, I wrote <wc-blink> (check it out on GitHub), a custom element that mimics the notorious and obsolete native element <blink> (except that it has a display: block instead of display: inline now that I think about it).

If your browser supports custom component and HTML import, you’ll see this blinking! (and appreciate the fact that it’s now obsolete)
<link rel="import" href="" async />

<wc-blink>If your browser supports custom component and HTML import, you'll see this blinking! (and appreciate the fact that it's now obsolete)</wc-blink>

It was absoluely unnecessary to make it a web component because there exists a CSS-only polyfill that receives wide browser support (it’s just CSS) and requires no JavaScript or custom element support.

This element was written to explore the then-v0 customer element standard. Even today, it still sound like the future to come, where you can plug and play any HTML element without having to convert them from front-end frameworks such as React, Vue.js or AngularJS.

When I was reading Mikeal’s post on web component, I was still full of hope even though I jumped into developing in React a few months before and never turned back. Web component is still at it’s infancy in terms of adoption and browser supports while component-based front-end frameworks are quickly fulfilling its promise - except the interoperatbility part.

<wc-blink> was originally written in custom element v0, without the ES2015 vibe of JavaScript class. Yet the general idea was the same: first you attach a shadowDOM to the element housing your content, then fill it up with lifecycle callbacks such as attachedCallback() or attributeChangedCallback() which reassembles React’s componentDidMount() and a high level shouldComponentUpdate() + render() + componentDidUpdate().

<!-- Commit: df3fcb41c880cb6762c609d7657cc0fd833d9134 -->
<!-- wc-blink.html -->
    /* Credit goes to google for this part
       Search "html blink" on google and see */
    @-webkit-keyframes blink {80%{opacity:0.0}}
    @keyframes blink {80%{opacity:0.0}}
    :host, * {
      -webkit-animation:blink 1s steps(1,end) 0s infinite;
      animation:blink 1s steps(1,end) 0s infinite;

  (function() {
    // Refers to the "importer", which is index.html
    var thatDoc = document;

    // Refers to the "importee", which is src/hello-world.html
    var thisDoc =  (thatDoc._currentScript || thatDoc.currentScript).ownerDocument;

    // Gets content from <template>
    var template = thisDoc.querySelector('template').content;

    // Creates an object based in the HTML Element prototype
    var element = Object.create(HTMLElement.prototype);

    // Fires when an instance of the element is created
    element.createdCallback = function() {
      var shadowRoot = this.createShadowRoot();
      shadowRoot.appendChild(document.importNode(template, true));

    // Fires when an instance was inserted into the document
    element.attachedCallback = function() {};

    // Fires when an instance was removed from the document
    element.detachedCallback = function() {};

    // Fires when an attribute was added, removed, or updated
    element.attributeChangedCallback = function(attr, oldVal, newVal) {};

    // Registers <hello-world> in the main document
    window.WcBlink = thatDoc.registerElement('wc-blink', {
        prototype: element

Of course, this is not pretty. Despite <template> reassembles the convenient JSX where we define the HTML structure of the “shadowDOM” (it isn’t exactly shadowDOM in React’s case despite some degree of encapusulation).

Nevertheless, custom component v0 <wc-blink> has a much lighter footprint compared to even the simpliest React app. All you need is a wc-blink.html, and a line of <link rel='import' href='wc-blink.html'>. No npm install (I used git submodule add). Bundling isn’t necessary until you have multiple HTML elements to import depending on how much extra lines of <link> you can tolerate in your HTML file.

<wc-blink> was built as an exercise to see how the custom element standard works. Judging from the amount of code I need to write by myself to create an element as simple as <blink>, it wasn’t really attractive back then. I did love the part that I could import an element and expect it’ll work like a charm, it was just that betting on a web standard that was in drafting stage was rather reckless.

With such a simple component, I haven’t even got to partial shadowDOM update (where I really hoped there was something like React’s VirtualDOM) or inter-component communication. As the web component standard is less opinionated than frameworks like React, it gives you lots of ropes to hang yourself, ending up writing overly coupled components, unnecessary DOM updates and weirdly behaving components due to lifecycle issue (like things not initializing/firing properly).

My worry came true. Custom element v0 was removed and v1 was introduced relying on ES2015. document.registerElement was no longer in the web standard and we should expect one day no browser would support it (Firefox never supported v0 in the first place 😐, nor v1 at the time of writing)

In general, my v0 and v1 implementations of <wc-blink> are quite similar. This revision only makes use of ES2015 class, and replaced <content> with <slot> to give users a better idea where the dynamic content go.

<!-- Commit: 3f8fa76675c15177baf9ccb81dcbb9b6831effcd -->
<!-- wc-blink-element.html -->
<template id='wc-blink'>
    /* Credit goes to google for this part
       Search "html blink" on google and see */
    @keyframes blink {80%{opacity:0.0}}
    :host, * {
      animation:blink 1s steps(1,end) 0s infinite;
  <slot name='content'></slot>

<script type='text/javascript'>
  // hack to get reference to this document even if calling constructors somewhere else
  const thisDoc = document.currentScript.ownerDocument;

  class WcBlink extends HTMLElement {
    constructor() {

      const template = thisDoc.getElementById('wc-blink')

      const shadowRoot = this.attachShadow({ mode: 'open' })

      this.blinkStep = 0;
      this.blinkInterval = setInterval(function() {
      }.bind(this), 200);

    connectedCallback() {
      const content = document.createElement('div');
      content.setAttribute('slot', 'content');

      content.setAttribute('slot', 'content');
      this.childNodes.forEach((child) => content.appendChild(child));

    updateDisplay() {
      const content = this.querySelector('div[slot=content]'); = (this.blinkStep == 3) ? 0 :; = (this.blinkStep == 4) ? 1 :;

      this.blinkStep = this.blinkStep % 5;

  customElements.define('wc-blink', WcBlink);

Even in 2018, tutorials on the topic are hard to find. Web component is much less opinionated than frameworks and therefore each tutorial author has their own way of doing things. For example, to get content <template> from template some decided to use await fetch('template.html') while some referenced the link[rel="import"] to get the template document when importing the component into other documents. At the end, I figured out a way to get a hold of the reference to ownerDocument even after importing the component from other documents.

You may have also noticed blickStep and updateDisplay, which are unnnecessary given that there’s a blinking animation applied to the shadowDOM. Although the selector can select the element within the shadowDOM (verified using color: red !important;), animation simply doesn’t fire, perhaps owing to browser bug as it works on my mobile browser but not on desktop.

Although adoption of web component is still low to date and there seems to be a larger community for Polymer, I still think that going through all the hassles to write a web component-powered app less attractive than writing a React app in a production setting.

With that being said, I may still have some fun writing web components when I am offline.

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About the author

Profile pic of Tommy Ku

Tommy Ku, a Hong Kong-based Software Engineer experienced developing PHP and Java-based web solutions and passionate in Web technology.

Also a hobbyist digital and film photographer.