Web Components: The Basics

What Are Web Components #

You have likely heard of web components in passing, maybe seen an example of one or two, and wondered if it's a solution you could actually use at scale. I'm here to tell you that you can, but not without some caveats. While today's examples will be simple for the sake of clarity, I'll also be discussing how web components have evolved over time and where we stand today.

"Web components" is an umbrella term for a set of technologies, mainly APIs, that allow you to create custom elements, which you can then use within your web applications. The goal is to make reusable code with encapsulated functionality, all while using standards.

Building a Simple Web Component #

If you were to create a web component right now, the steps would look something like this:

  • Create a class.
  • Register a new custom element. This is where you create the name of your element.
  • Attach a shadow DOM to that custom element. This is where you'd add event listeners and maybe some child elements.
  • Optionally, define an HTML template with <template> or <slot>, to prepare it to be reused somewhere else.
  • Place your custom element in your HTML.

Today we'll be creating a quick counter as a web component. To add some visual effect I've also added an SVG that slowly fills to the top as the counter approaches 10 and, empties as the counter decreases to 0. I also want the maximum range to be 0 to 10.

Main Building Blocks #

There are three core blocks of web components: custom elements, shadow DOM, and HTML templates.

Custom Elements #

Custom elements are a set of JavaScript APIs that allow you to define your custom element as well as its behavior. The main controller of customs elements is an object called CustomElementRegistry, available to us in the DOM as customElements.

For example, if you were to create a custom element, you'd call its define method like this: customElements.define(). The define() method accepts a DOMString for the name, a class object for its behavior, and an optional "options" object that allows you to extend your element.

To build our counter, the first thing we need to do is define our custom element in our HTML:

<my-counter></my-counter>

In our JavaScript, we we'll want to write out the HTML that we want this custom element to correspond to, and we'll assign it to a variable for future reference. We can create this element using good ole document.createElement() (don't forget the backticks):

const template = document.createElement("template");
template.innerHTML = `
<style>
* {
font-size: 150%;
text-align: center;
}

span {
width: 4rem;
display: inline-block;
}

button {
width: 4rem;
height: 4rem;
border: none;
border-radius: 10px;
background-color: royalblue;
color: white;
}
</style>
<button id="dec">-</button>
<span id="count"></span>
<button id="inc">+</button>
`
;

Now we'll create a class in JavaScript that extends HTMLElement. We'll call it MyCounter. And inside of that class, we'll first need to write a constructor that calls super(). This is done to establish the correct prototype chain and it always needs to be the first thing inside the class.

class MyCounter extends HTMLElement {
constructor() {
super();
// Do other things here!
}
}

Then we'll define our custom element with customElements.define(), giving it a string for the name and the class we created above as the second argument.

const template = document.createElement("template");
// [..]
// Our template-y stuff up here

class MyCounter extends HTMLElement {
constructor() {
super();
// Do other things here!
}
}

customElements.define("my-counter", MyCounter);

Shadow DOM #

Shadow DOM is a set of JavaScript APIs that allow you to encapsulate your code. It's rendered separately from the main document DOM, so you never have to worry about collision with other parts of the document.

Here we are attaching the shadow root to our element, and we do this from inside the constructor. While we're at it, we'll also set our initial count to 0:

class MyCounter extends HTMLElement {
constructor() {
super();
this.count = 0;
this.attachShadow({ mode: "open" });
}
}

We previously defined our template at the top of the file and assigned it to a variable called template. We then attached shadow root to our element. Now, we can have shadow root clone the template to create the element's internal shadow DOM structure.

We'll do all of this from within the constructor for now:

class MyCounter extends HTMLElement {
constructor() {
super();
this.count = 0;
this.attachShadow({ mode: "open" });

this.shadowRoot.appendChild(template.content.cloneNode(true));
}
}

We can also add some styling to the shadow root, but we won't be doing that here. Instead, let's add in some event handlers for when the user clicks the + or - buttons:

class MyCounter extends HTMLElement {
constructor() {
super();
this.count = 0;
this.attachShadow({ mode: "open" });
this.shadowRoot.getElementById("inc").onclick = () => this.inc();
this.shadowRoot.getElementById("dec").onclick = () => this.dec();
this.update(this.count);
this.shadowRoot.appendChild(template.content.cloneNode(true));
}
}

We'll create these functions outside of the constructor, but still inside of the class, because we need to be able to access the scope:

class MyCounter extends HTMLElement {
constructor() {
super();
this.count = 0;
this.attachShadow({ mode: "open" });
this.shadowRoot.getElementById("inc").onclick = () => this.inc();
this.shadowRoot.getElementById("dec").onclick = () => this.dec();
this.update(this.count);
this.shadowRoot.appendChild(template.content.cloneNode(true));
}

inc() {
// We want the max to be 10
if(this.count === 10) {
return;
} else {
this.update(++this.count);
}
}

dec() {
// We want the min to be 0
if(this.count === 0) {
return;
} else {
this.update(--this.count);
}
}

update(count) {
this.shadowRoot.getElementById("count").innerHTML = count;
}
}

I also would like to add an effect of filling an SVG and emptying it based on the clicks. Let's add a fun little SVG to the HTML, just below our custom element:

<my-counter></my-counter>

<div>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" width="200" height="200">
<linearGradient id="lg" x1="0.5" y1="1" x2="0.5" y2="0">
<stop offset="0%" stop-opacity="1" stop-color="royalblue"/>
<stop offset="40%" stop-opacity="1" id="secondStop" stop-color="royalblue"/>
<stop offset="40%" stop-opacity="0" id="thirdStop" stop-color="royalblue"/>
<stop offset="100%" stop-opacity="0" stop-color="royalblue"/>
</linearGradient>
<circle cx="50" cy="50" r="45" fill="url(#lg)" stroke="lightblue" stroke-width="5"/>
</svg>
</div>

Back to our JS, we'll need to make sure the SVG gets updated with every click. Let's add some basic DOM manipulation to the update function:

class MyCounter extends HTMLElement {
constructor() {
super();
this.count = 0;
this.attachShadow({ mode: "open" });
}

// [..]

update(count) {
this.shadowRoot.getElementById("count").innerHTML = count;

// A bit hacky, but use the count to create the string
const percentage = count + '0%';
const secondStop = document.getElementById("secondStop");
const thirdStop = document.getElementById("thirdStop");

secondStop.setAttribute('offset', percentage);
thirdStop.setAttribute('offset', percentage);
}
}

See the Pen Web Component Counter Fill by Rachel (@kahboom) on CodePen.

Now we have a fully functioning counter web component with a cool SVG animation. It doesn't do anything particularly special, but at least we've got some of the basics down.

HTML Templates #

HTML templates allow for you to write markup templates for your custom element that you can later reuse with the <template> and <slot> elements. They do not get displayed in the rendered page.

We didn't use templates in our example, but we just as easily could have by defining the template within the HTML:

<template id="my-counter">
<style>
[..]
</style>
<button id="dec">-</button>
<span id="count"></span>
<button id="inc">+</button>
</template>

It will not appear in the rendered page until we select it with JS and append it to the DOM, or shadow root, during runtime:

let template = document.getElementById('my-counter');
let templateContent = template.content;
const shadowRoot = this.attachShadow({mode: 'open'}).appendChild(templateContent.cloneNode(true));

Slots #

Although there is less browser support for slots than templates, I still think they are valuable enough that you should understand the basics.

This is a slot: <slot></slot>

Slots are meant to be used with templates, though technically it's possible to use it without them. The purpose of slots is to provide a placeholder for consumers, or application developers, that are using the component, to replace content within it. The consumer adds slottable elements they specify inside of the provided slots. Elements can vary anywhere from <p> to <h2>.

When the browser renders the document, it composes the template and the slottable elements that the consumer has provided. In other words, two different DOM trees will be composed together.

Slots can have names. We'll want to use this name attribute to make it easier to reference our slots when someone wants to consume them. These names will not be rendered in the DOM, but provide a kind of internal reference.

Drawing on our previous template example, here's what it would look like if we provided slots that could be overridden:

<template id="my-counter">
<style>
[..]
</style>
<h1>Some Counter</h1>
<slot name="decrease">-</slot>
<slot name="count"></slot>
<slot name="increase">+</slot>
</template>

Then, the consumer application would provide slottable elements for it like this (removing any id for the sake of clarity):

<my-counter>
<button slot="decrease">Subtract</button>
<span slot="count"></span>
<button slot="increase">Add</button>
</my-counter>

This would compose the following:

<my-counter>
<style>
[..]
</style>
<h1>Some Counter</h1>
<button>Subtract</button>
<span></span>
<button>Add</button>
</my-counter>

Slots also allow you to specify default values to be used when the consumer does not provide a slottable element with a matching name. Our template had provided the text inside of the buttons as "+" and "-", and we had provided the text "Subtract" and "Add" in our consumer application. If we had not done that, it would have defaulted to the strings ("+" and "-") originally provided in the template.

Lifecycle Callbacks of Web Components #

Something we haven't discussed yet are the lifecycle methods that you get within a custom element's class definition. Each of these fire at different points in the element's lifecycle:

  • connectedCallback - Gets invoked when the custom element is attached to the DOM.
  • disconnectedCallback - Invoked when the custom element is removed from the DOM.
  • adoptedCallback - Invoked when the custom element is moved to a new document.
  • attributeChangedCallback - Invoked when the custom element's attributes change (e.g. added, removed).

We can test some of these callbacks by adding a simple log to our custom element example:

class MyCounter extends HTMLElement {
constructor() {
super();
this.count = 0;
this.attachShadow({ mode: "open" });
}

connectedCallback() {
console.log('Custom element has been attached to the DOM!');
}

disconnectedCallback() {
console.log('Custom element has been removed from the DOM!');
}
}

attributeChangedCallback can be quite useful since we can act on attributes individually.

Build Tools and Preparing for Production #

The question of whether or not you will need a transpiler or bundler is subjective. You have some options, like Parcel, Rollup, Webpack, etc. but also consider just using ES6's import/export.

In your HTML, make sure to set type to 'module':

<script src='/my-counter.js' type='module'></script>

By adding a package.json you could publish the module to npm, even if it's not a Node.js library. Just make sure to list the files that pertain to your component:

{
"name": "my-counter",
"version": "1.0.0",
"description": "My custom counter web component",
"repository": {
"type": "git",
"url": "https://www.github.com/example/my-counter.git"
},
"files": [
"my-counter.css",
"my-counter.html",
"my-counter.js"
]
}

What About TypeScript + Web Components? #

Using TypeScript is possible with web components, but the implementation varies depending on what build tool you are using, if you're using one at all. Typically, this isn't very different than if you were using TypeScript for any other project, so I recommend you check the documentation for your specific build tool.

Template Rendering for Web Components #

When it comes to template rendering, you have a variety of options that use very different approaches. It's possible to leave template rendering to a framework like React, Angular, or Vue. Of course, you can also use a library that is specifically for templating, such as lit-html.

Browser Compatibility of Web Components #

As recently as Fall 2018, the only browsers that had full support for web components were Chrome and Safari. Although custom elements were a quick and easy polyfill, shadow DOM was far more complicated and difficult to use without native support. It really wasn't until late 2019 / early 2020 that all major browsers adopted support for Web Components. With the release of Chromium-based Edge 76, web components are now supported by most modern browsers. If you do need to support legacy browsers, you'll need to include polyfills.

Styling Web Components #

One of the benefits of using shadow DOM is that our elements are isolated. As such, we won't need to use prefixed CSS variable names like --my-counter-background. Instead, we can simply use --background().

Probably the easiest way to make our host components easy for a consumer to use their own styles is by creating CSS variables and assigning them a default value through the :host selector:

:host {
--background-color: royalblue;
}
#my-counter {
background-color: var(--background-color);
}

It then becomes easy to override these CSS variables from the consumer side, because all they would need to use is select the custom element name:

my-counter {
--background-color: purple;
}

When creating components, we can also style our slots using the ::slotted() selector:

<template id="my-counter">
<style>
::slotted(button) {
background-color: orange;
}
</style>
<h1>Some Counter</h1>
<slot name="decrease">-</slot>
<slot name="count"></slot>
<slot name="increase">+</slot>
</template>

The argument of the ::slotted() function should be the element that the consuming application is providing. In our case from before, that was a <button>. We need to use a top-level element, because a nested one will not work with ::slotted().

Though this component probably isn't the best example for it, we can also use ::part to reference any specified element within the shadow tree:

my-counter::part(action) {
color: navy;
}
<template id="my-counter">
<style>
:host {
display: flex;
}
</style>
<button id="dec" part="action">-</button>
<span id="count"></span>
<button id="inc" part="action">+</button>
</template>

Testing Web Components #

It's always a good idea to unit test your components. Polymer has a TDD testing tool called web-component-tester that works with Mocha and Chai out of the box, but you can also use Karma as well. Depending on your setup, it might also be a good idea to write some integration tests to ensure there is no issue rendering custom HTML, especially if you're using a SPA framework.

Drawbacks #

Though I won't go into all of the drawbacks of using web components, there are a few you should probably know about:

  • Polyfills. As I previously mentioned, if you need to support legacy browsers you'll need to include polyfills. They can be a bit chunky, and you'll also face some limitations on things like CSS encapsulation.
  • They don't work without JS. This may or may not be an issue for you, but it's kind of a big one if it is.
  • Issues with accessibility. There is no encapsulation of Aria declarations, and you have to re-add them for custom elements.
  • Custom CSS pseudo selectors don't work with web components.
  • They're not a drop-in replacement for frameworks. They don't support data binding out of the box, and you're responsible for architecting an application in a way that best makes sense for you. You probably won't be able to just drag and drop components into an existing application without writing some code.

Final Thoughts #

Those that have been working with web components for a long time now are all too familiar with some of the shortcomings they have been plagued with in the past. But even today, web components still have a long way to go, and not just in terms of changing misperceptions surrounding the technology. With that being said, it's a very exciting set of technologies that are certainly worth working with and learning about. After all, if we don't all push for a better modern web, who will?