HTML - Custom Tag / Element


custom is the possibility create custom element

By adding them in a shadow DOM (to scope the DOM selection) and using templates element, you are creating web components

Example: Words Count

HTML Element

count word analytics element.

  • An HTML text area to change the number of words.
<textarea cols=40 rows=4>Word Count example</textarea>
  • The HTML with a custom element defined with the is attribute. The custom element will count the words of the parent node
  • The definition of the custom element is in a class and in a new sub dom (shadow)
// Create a class for the element
class WordCount extends HTMLElement {
  constructor() {
    // Always call super first in constructor

    // count words in element's parent element
    const wcParent = this.previousElementSibling;

    function countWords(node){
      const text = node.value || node.innerText || node.textContent;
      return text.split(/\s+/g).length;

    const count = `Words: ${countWords(wcParent)}`;

    // Create a shadow root
    const shadow = this.attachShadow({mode: 'open'});

    // Create text node and add word count to it
    const text = document.createElement('p');
    text.textContent = count;

    // Append it to the shadow root

    // Update count at interval
    setInterval(function() {

      const count = `Words: ${countWords(wcParent)}`;
      text.textContent = count;
    }, 5000);

// Define the new element
customElements.define('word-count', WordCount);
  • The result

Extends HTML Element

This example shows how to extends an actual HTML element. It 's the same previous count word example but that extends the p element

The difference with the previous example are:

  • A custom element that extends an actual HTML element should be defined with the is attribute and not as element
<p is="word-count"></p>
  • The definition of the custom element extends declaratively the HTMLParagraphElement and no more the HTMLElement
// Create a class for the element
class WordCount extends HTMLParagraphElement {
  // ...
  • When defining the new element, the extension should be declared in the extends option property.
customElements.define('word-count', WordCount, { extends: 'p' });
  • The result

Ref: Taken from custom element

Documentation / Reference

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