simonw/single-file-web-component.html

## single-file-web-component.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Single File Web Component</title>
    <style>
      body {
        background-color: #eee;
        font-family: Helvetica, sans-serif;
      }
      h1 {
        color: blue;
        background-color: pink;
      }
    </style>
  </head>
  <body>
    <template id="single-file">
      <style>
        /*
        These styles affect only content inside the shadow DOM.

        Styles in the outside document mostly do not affect these,
        with the  exception of inheritable styles such as color,
        font and line-height.
      */
        h1 {
          color: red;
        }
      </style>

      <h1>Hello world (web component)</h1>
      <!--
      This code still works if you remove the type="module" parameter.

      Using that parameter enables features such as 'import ... from'

      More importantly it stops variables inside the script tag from
      leaking out to the global scope - if you remove type="module"
      from here then the HelloWorld class becomes visible.
      -->
      <script type="module">
        class HelloWorld extends HTMLElement {
          constructor() {
            /*
              If you remove the call to super() here Firefox shows an error:
              "Uncaught ReferenceError: must call super constructor before
              using 'this' in derived class constructor'"
            */
            super();
            const template = document.getElementById("single-file");
            /*
              mode: 'open' means you are allowed to access
              document.querySelector('hello-world').shadowRoot to get
              at the DOM inside. Set to 'closed' and the .shadowRoot
              property will return null.
            */
            this.attachShadow({ mode: "open" }).appendChild(
              template.content.cloneNode(true)
            );
            /*
              template.content is a 'DocumentFragment' array.

              template.content.cloneNode() without the true performs
              a shallow clone, which provides a empty DocumentFragment
              array.

              template.content.cloneNode(true) provides one with 6 items
            */
          }
          connectedCallback() {
            // https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements#using_the_lifecycle_callbacks
            console.log("Why hello there 👋");
          }
        }

        customElements.define("hello-world", HelloWorld);
      </script>
    </template>

    <h1>This is not a web component</h1>

    <hello-world></hello-world>

    <script>
      const sf = document.getElementById("single-file");
      /*
        Before executing this line, sf.content.lastElementChild
        is the <script type="module"> element hidden away inside
        the <template> - we remove it from the template here and
        append it to the document.body, causing it to execute in
        the main document and activate the <hello-world> tag.
      */
      document.body.appendChild(sf.content.lastElementChild);
    </script>
  </body>
</html>
	<!DOCTYPE html>
	<html lang="en">
	<head>
	<meta charset="UTF-8" />
	<title>Single File Web Component</title>
	<style>
	body {
	background-color: #eee;
	font-family: Helvetica, sans-serif;
	}
	h1 {
	color: blue;
	background-color: pink;
	}
	</style>
	</head>
	<body>
	<template id="single-file">
	<style>
	/*
	These styles affect only content inside the shadow DOM.

	Styles in the outside document mostly do not affect these,
	with the exception of inheritable styles such as color,
	font and line-height.
	*/
	h1 {
	color: red;
	}
	</style>

	<h1>Hello world (web component)</h1>
	<!--
	This code still works if you remove the type="module" parameter.

	Using that parameter enables features such as 'import ... from'

	More importantly it stops variables inside the script tag from
	leaking out to the global scope - if you remove type="module"
	from here then the HelloWorld class becomes visible.
	-->
	<script type="module">
	class HelloWorld extends HTMLElement {
	constructor() {
	/*
	If you remove the call to super() here Firefox shows an error:
	"Uncaught ReferenceError: must call super constructor before
	using 'this' in derived class constructor'"
	*/
	super();
	const template = document.getElementById("single-file");
	/*
	mode: 'open' means you are allowed to access
	document.querySelector('hello-world').shadowRoot to get
	at the DOM inside. Set to 'closed' and the .shadowRoot
	property will return null.
	*/
	this.attachShadow({ mode: "open" }).appendChild(
	template.content.cloneNode(true)
	);
	/*
	template.content is a 'DocumentFragment' array.

	template.content.cloneNode() without the true performs
	a shallow clone, which provides a empty DocumentFragment
	array.

	template.content.cloneNode(true) provides one with 6 items
	*/
	}
	connectedCallback() {
	// https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements#using_the_lifecycle_callbacks
	console.log("Why hello there 👋");
	}
	}

	customElements.define("hello-world", HelloWorld);
	</script>
	</template>

	<h1>This is not a web component</h1>

	<hello-world></hello-world>

	<script>
	const sf = document.getElementById("single-file");
	/*
	Before executing this line, sf.content.lastElementChild
	is the <script type="module"> element hidden away inside
	the <template> - we remove it from the template here and
	append it to the document.body, causing it to execute in
	the main document and activate the <hello-world> tag.
	*/
	document.body.appendChild(sf.content.lastElementChild);
	</script>
	</body>
	</html>