Creating a first ready-to-use custom element

The idea of this post, in addition to learn something new, is to be a quick introduction to custom elements for people who have never tried it and to show how to create a custom element really easily without diving deep into more advanced notions.

The example element will be a subscription form (input + label + alert). If you wanna jump directly to the code, head up to the GitHub repository.

Definition and registration

While the HTML reference defines a plenty of tags (<p>, <input>, <section>…), you can create custom ones (such as <subscribe-form> or <yo-lo>) thanks to the web components API. It allows to have a more modular and reusable code like with modern frameworks components (Vue, React…). They behave like standard HTML tags and are defined like this:

class SubscribeForm extends HTMLElement {
	/* definition, attributes, methods... */
}

window.customElements.define('subscribe-form', SubscribeForm)

A custom element is just a JavaScript class which is defined inside the window custom elements registry.

Content and behaviour

In a nutshell, the custom element <subscription-form> is a labelled input, a button and an alert box (see the markup in the code below).

Inside the constructor() {} object, you have to call super() (required by the custom elements spec). Then, you basically do what you want. In this example, I start by retrieving attributes (with fallbacks in case they are not provided) and then I define the markup for the tag:

constructor() {
  super()

  this.name = this.getAttribute('name') || 'email'
  this.label = this.getAttribute('label') || 'Email'
  this.type = 'email'
  this.placeholder = this.getAttribute('placeholder') || ''

  this.innerHTML = `
    <form>
      <label for="${this.name}">${this.label}</label>
      <input type="${this.type}" required aria-required="true" id="${this.name}" placeholder="${this.placeholder}">
      <input type="submit" value="Subscribe" />
    </form>
    <div role="alert" hidden class="success">
      <p>You successfuly subscribed!</p>
      <button role="button" aria-label="Close">
        &times;
      </button>
    </div>
  `
}

Note: this refers to the custom element itself.

Then the custom element has to be call in the HTML page with its attributes (don’t forget to link the JavaScript file). Note that custom elements must contain a dash and can’t be self closing.

<subscribe-form required="true" name="email" label="Your email" placeholder="eg: john@doe.com"></subscribe-form>

So the cycle is complete: styling doesn’t require extra knowledge. In fact, cutom elements can be targeted with a simple CSS tag selector: subscribe-form {color: blue}.

To have a preview of what the code does, simply clone the GitHub repository and open index.html in a browser (see the browser support).

Going further

As expected, this component only scratches the surface of custom elements capabilities. Indeed, everything here is done inside the constructor() object and I played neither with the Shadow DOM nor with lifecycle callbacks which are powerful features to build custom elements. But this code already provides the ability to build reusable, autonomous and flexible components through a website.

Here are links about custom elements that I found useful while writing this article: