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Custom elements contain their own semantics, behaviors, markup and can be shared across frameworks and browsers.

class MyComponent extends HTMLElement {
  connectedCallback() {
    this.innerHTML = `<h1>Hello world</h1>`;
  }
}
    
customElements.define('my-component', MyComponent);

See the Pen
Custom elements demo by Caleb Williams (@calebdwilliams)
on CodePen.

In this example, we define , our very own HTML element. Admittedly, it doesn’t do much, however this is the basic building block of a custom element. All custom elements must in some way extend an HTMLElement in order to be registered with the browser.

Custom elements exist without third-party frameworks and the browser vendors are dedicated to the continued backward compatibility of the spec, all but guaranteeing that components written according to the specifications will not suffer from breaking API changes. What’s more, these components can generally be used out-of-the-box with today’s most popular frameworks, including Angular, React, Vue, and others with minimal effort.

Shadow DOM

The shadow DOM is an encapsulated version of the DOM. This allows authors to effectively isolate DOM fragments from one another, including anything that could be used as a CSS selector and the styles associated with them. Generally, any content inside of the document’s scope is referred to as the light DOM, and anything inside a shadow root is referred to as the shadow DOM.

When using the light DOM, an element can be selected by using document.querySelector('selector') or by targeting any element’s children by using element.querySelector('selector'); in the same way, a shadow root’s children can be targeted by calling shadowRoot.querySelector where shadowRoot is a reference to the document fragment — the difference being that the shadow root’s children will not be select-able from the light DOM. For example, If we have a shadow root with a inside of it, calling shadowRoot.querySelector('button') would return our button, but no invocation of the document’s query selector will return that element because it belongs to a different DocumentOrShadowRoot instance. Style selectors work in the same way.

In this respect, the shadow DOM works sort of like an where the content is cut off from the rest of the document; however, when we create a shadow root, we still have total control over that part of our page, but scoped to a context. This is what we call encapsulation.

If you’ve ever written a component that reuses the same id or relies on either CSS-in-JS tools or CSS naming strategies (like BEM), shadow DOM has the potential to improve your developer experience.

Imagine the following scenario:

<div>
  <div id="example">
    <!-- Pseudo-code used to designate a shadow root -->
    <#shadow-root>
      <style>
      button {
        background: tomato;
        color: white;
      }
      </style>
      <button id="button">This will use the CSS background tomato</button>
    </#shadow-root>
  </div>
  <button id="button">Not tomato</button>
</div>

Aside from the pseudo-code of (which is used here to demarcate the shadow boundary which has no HTML element), the HTML is fully valid. To attach a shadow root to the node above, we would run something like:

const shadowRoot = document.getElementById('example').attachShadow({ mode: 'open' });
shadowRoot.innerHTML = `<style>
button {
  color: tomato;
}
</style>
<button id="button">This will use the CSS color tomato <slot></slot></button>`;

A shadow root can also include content from its containing document by using the element. Using a slot will drop user content from the outer document at a designated spot in your shadow root.

See the Pen
Shadow DOM style encapsulation demo by Caleb Williams (@calebdwilliams)
on CodePen.

HTML templates

The aptly-named HTML element allows us to stamp out re-usable templates of code inside a normal HTML flow that won’t be immediately rendered, but can be used at a later time.

<template id="book-template">
  <li><span class="title"></span> &mdash; <span class="author"></span></li>
</template>

<ul id="books"></ul>

The example above wouldn’t render any content until a script has consumed the template, instantiated the code and told the browser what to do with it.

const fragment = document.getElementById('book-template');
const books = [
  { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald' },
  { title: 'A Farewell to Arms', author: 'Ernest Hemingway' },
  { title: 'Catch 22', author: 'Joseph Heller' }
];

books.forEach(book => {
  // Create an instance of the template content
  const instance = document.importNode(fragment.content, true);
  // Add relevant content to the template
  instance.querySelector('.title').innerHTML = book.title;
  instance.querySelector('.author').innerHTML = book.author;
  // Append the instance ot the DOM
  document.getElementById('books').appendChild(instance);
});

Notice that this example creates a template () without any other Web Components technology, illustrating again that the three technologies in the stack can be used independently or collectively.

Ostensibly, the consumer of a service that utilizes the template API could write a template of any shape or structure that could be created at a later time. Another page on a site might use the same service, but structure the template this way:

<template id="book-template">
  <li><span class="author"></span>'s classic novel <span class="title"></span></li>
</template>

<ul id="books"></ul>

See the Pen
Template example by Caleb Williams (@calebdwilliams)
on CodePen.

That wraps up our introduction to Web Components

As web development continues to become more and more complicated, it will begin to make sense for developers like us to begin deferring more and more development to the web platform itself which has continued to mature. The Web Components specifications are a set of low-level APIs that will continue to grow and evolve as our needs as developers evolve.

In the next article, we will take a deeper look at the HTML templates part of this. Then, we’ll follow that up with a discussion of custom elements and shadow DOM. Finally, we’ll wrap it all up by looking at higher-level tooling and incorporation with today’s popular libraries and frameworks.

Article Series:

1. An Introduction to Web Components (This post)
2. Crafting Reusable HTML Templates (Coming soon!)
3. Creating a Custom Element from Scratch (Coming soon!)
4. Encapsulating Style and Structure with Shadow DOM (Coming soon!)
5. Advanced Tooling for Web Components (Coming soon!)

The post An Introduction to Web Components appeared first on CSS-Tricks.