Wiederverwendbare HTML-Komponenten - Wiederverwenden von Kopf- und Fußzeilen auf einer Website

Stellen Sie sich vor, Sie erstellen eine Website für einen Kunden, einen kleinen Tante-Emma-Laden mit nur zwei Seiten.

Das ist nicht viel. Wenn Sie mit der Arbeit an der Zielseite fertig sind und mit der Kontaktseite beginnen, erstellen Sie einfach eine neue HTML-Datei und kopieren den gesamten Code von der ersten Seite.

Die Kopf- und Fußzeile sehen bereits gut aus, und Sie müssen lediglich den Rest des Inhalts ändern.

Aber was ist, wenn Ihr Kunde 10 Seiten möchte? Oder 20? Und sie fordern geringfügige Änderungen an Kopf- und Fußzeile während der Entwicklung an.

Plötzlich muss jede noch so kleine Änderung in all diesen Dateien wiederholt werden.

Dies ist eines der Hauptprobleme, die Dinge wie React oder Handlebars.js lösen: Jeder Code, insbesondere strukturelle Dinge wie Kopf- oder Fußzeilen, kann einmal geschrieben und während eines Projekts problemlos wiederverwendet werden.

Bis vor kurzem war es nicht möglich, Komponenten in Vanille-HTML und JavaScript zu verwenden. Mit der Einführung von Webkomponenten ist es jedoch möglich, wiederverwendbare Komponenten zu erstellen, ohne Dinge wie Reagieren zu verwenden.

Was sind Webkomponenten?

Webkomponenten sind eine Sammlung verschiedener Technologien, mit denen Sie benutzerdefinierte HTML-Elemente erstellen können.

Diese Technologien sind:

  • HTML-Vorlagen : Fragmente von HTML-Markups mit Elementen, die erst gerendert werden, wenn sie mit JavaScript an die Seite angehängt werden.
  • Benutzerdefinierte Elemente : Weit verbreitete JavaScript-APIs, mit denen Sie neue DOM-Elemente erstellen können. Sobald Sie ein benutzerdefiniertes Element mit diesen APIs erstellt und registriert haben, können Sie es ähnlich wie eine React-Komponente verwenden.
  • Schatten-DOM : Ein kleineres, gekapseltes DOM, das vom Haupt-DOM isoliert und separat gerendert wird. Alle Stile und Skripte, die Sie für Ihre benutzerdefinierten Komponenten im Schatten-DOM erstellen, wirken sich nicht auf andere Elemente im Haupt-DOM aus.

Wir werden uns im Verlauf des Tutorials etwas eingehender mit diesen Themen befassen.

Verwendung von HTML-Vorlagen

Der erste Teil des Puzzles besteht darin, zu lernen, wie HTML-Vorlagen verwendet werden, um wiederverwendbare HTML-Markdowns zu erstellen.

Schauen wir uns ein einfaches Beispiel für eine Begrüßungsnachricht an:

Hello, World!

And all who inhabit it

Wenn Sie sich die Seite ansehen, ist weder die

Original text


oder

To actually render the welcome message, you'll need to use a bit of JavaScript:

const template = document.getElementById('welcome-msg'); document.body.appendChild(template.content); 

Even though this is a pretty simple example, you can already see how using templates makes it easy to reuse code throughout a page.

The main issue is that, at least with the current example, the welcome message code is mixed in with the rest of the page's content. If you want to change the welcome message later, you'll need to change the code across multiple files.

Instead, you can pull the HTML template into the JavaScript file, so any page the JavaScript is included in will render the welcome message:

const template = document.createElement('template'); template.innerHTML = ` 

Hello, World!

And all who inhabit it

` document.body.appendChild(template.content);

Now that everything's in the JavaScript file, you don't need to create a element – you could just as easily create a or .

However, elements can be paired with a element, which allows you to do things like change the text for elements within the . It's a bit outside the scope of this tutorial, so you can read more about elements over on MDN.

How to create custom elements

One thing you might have noticed with HTML templates is that it can be tricky to insert your code in the right place. The earlier welcome message example was just appended to the page.

If there was content already on the page, say, a banner image, the welcome message would appear below it.

As a custom element, your welcome message might look like this:

And you can put it wherever you want on the page.

With that in mind, let's take a look at custom elements and create our own React-like header and footer elements.

Setup

For a portfolio site, you might have some boilerplate code that looks like this:

* { margin: 0; padding: 0; box-sizing: border-box; } html, body { height: 100%; } body { color: #333; font-family: sans-serif; display: flex; flex-direction: column; } main { flex: 1 0 auto; } 

Each page will have the same header and footer, so it makes sense to create a custom element for each of those.

Let's start with the header.

Define a custom element

First, create a directory called components and inside that directory, create a new file called header.js with the following code:

class Header extends HTMLElement { constructor() { super(); } } 

It's just a simple ES5 Class declaring your custom Header component, with the constructor method and special super keyword. You can read more about those on MDN.

By extending the generic HTMLElement class, you can create any kind of element you want. It's also possible to extend specific elements like HTMLParagraphElement.

Register your custom element

Before you can start using your custom element, you'll need to register it with the customElements.define() method:

class Header extends HTMLElement { constructor() { super(); } } customElements.define('header-component', Header); 

This method takes at least two arguments.

The first is a DOMString you'll use when adding the component to the page, in this case, .

The next is the component's class that you created earlier, here, the Header class.

The optional third argument describes which existing HTML element your custom element inherits properties from, for example, {extends: 'p'}. But we won't be using this feature in this tutorial.

Use lifecycle callbacks to add the header to the page

There are four special lifecycle callbacks for custom elements that we can use to append header markdown to the page: connectedCallback, attributeChangeCallback, disconnectedCallback, and adoptedCallback.

Of these callbacks, connectedCallback is one of the most commonly used. connectedCallback runs each time your custom element is inserted into the DOM.

You can read more about the other callbacks here.

For our simple example, connectedCallback is enough to add a header to the page:

class Header extends HTMLElement { constructor() { super(); } connectedCallback() { this.innerHTML = `  nav { height: 40px; display: flex; align-items: center; justify-content: center; background-color: #0a0a23; } ul li { list-style: none; display: inline; } a { font-weight: 700; margin: 0 25px; color: #fff; text-decoration: none; } a:hover { padding-bottom: 5px; box-shadow: inset 0 -2px 0 0 #fff; }    
  • About
  • Work
  • Contact
`; } } customElements.define('header-component', Header);

Then in index.html, add the components/header.js script and just above the element:

And your reusable header component should be rendered to the page:

Now adding a header to the page is as easy as adding a tag pointing to components/header.js, and adding wherever you want.

Note that, since the header and its styling are being inserted into the main DOM directly, it's possible to style it in the styles.css file.

But if you look at the header styles included in connectedCallback, they're quite general, and can affect other styling on the page.

For example, if we add the following footer component:

class Footer extends HTMLElement { constructor() { super(); } connectedCallback() { this.innerHTML = `  footer { height: 60px; padding: 0 10px; list-style: none; display: flex; justify-content: space-between; align-items: center; background-color: #dfdfe2; } ul li { list-style: none; display: inline; } a { margin: 0 15px; color: inherit; text-decoration: none; } a:hover { padding-bottom: 5px; box-shadow: inset 0 -2px 0 0 #333; } .social-row { font-size: 20px; } .social-row li a { margin: 0 15px; }   
  • About
  • Work
  • Contact
`; } } customElements.define('footer-component', Footer);

Here's what the page would look like:

The styling from the footer component overrides the styling for the header, changing the color of the links. That's expected behavior for CSS, but it would be nice if each component's styling was scoped to that component, and wouldn't affect other things on the page.

Well, that's exactly where the Shadow DOM shines. Or shades? Anyway, the Shadow DOM can do that.

How to use the Shadow DOM with custom elements

The Shadow DOM acts as a separate, smaller instance of the main DOM. Rather than act as a copy of the main DOM, the Shadow DOM is more like a subtree just for your custom element. Anything added to a Shadow DOM, especially styles, are scoped that particular custom element.

In a way, it's like using const and let rather than var.

Let's start by refactoring the header component:

const headerTemplate = document.createElement('template'); headerTemplate.innerHTML = `  nav { height: 40px; display: flex; align-items: center; justify-content: center; background-color: #0a0a23; } ul li { list-style: none; display: inline; } a { font-weight: 700; margin: 0 25px; color: #fff; text-decoration: none; } a:hover { padding-bottom: 5px; box-shadow: inset 0 -2px 0 0 #fff; }    
  • About
  • Work
  • Contact
`; class Header extends HTMLElement { constructor() { super(); } connectedCallback() { } } customElements.define('header-component', Header);

The first thing you need to do is to use the .attachShadow() method to attach a shadow root to your custom header component element. In connectedCallback, add the following code:

... class Header extends HTMLElement { constructor() { super(); } connectedCallback() { const shadowRoot = this.attachShadow({ mode: 'closed' }); } } customElements.define('header-component', Header); 

Notice that we're passing an object to .attachShadow() with an option, mode: 'closed'. This just means that the header component's shadow DOM is inaccessible from external JavaScript.

If you'd like to manipulate the header component's shadow DOM later with JavaScript outside the components/header.js file, just change the option to mode: 'open'.

Finally, append shadowRoot to the page with the .appendChild() method:

... class Header extends HTMLElement { constructor() { super(); } connectedCallback() { const shadowRoot = this.attachShadow({ mode: 'closed' }); shadowRoot.appendChild(headerTemplate.content); } } customElements.define('header-component', Header); 

And now, since the header component's styles are encapsulated in its Shadow DOM, the page should look like this:

Here's what the final code looks like across all files after refactoring the footer component:

* { margin: 0; padding: 0; box-sizing: border-box; } html, body { height: 100%; } body { color: #333; font-family: sans-serif; display: flex; flex-direction: column; } main { flex: 1 0 auto; } 
const headerTemplate = document.createElement('template'); headerTemplate.innerHTML = `  nav { height: 40px; display: flex; align-items: center; justify-content: center; background-color: #0a0a23; } ul li { list-style: none; display: inline; } a { font-weight: 700; margin: 0 25px; color: #fff; text-decoration: none; } a:hover { padding-bottom: 5px; box-shadow: inset 0 -2px 0 0 #fff; }    
  • About
  • Work
  • Contact
`; class Header extends HTMLElement { constructor() { super(); } connectedCallback() { const shadowRoot = this.attachShadow({ mode: 'closed' }); shadowRoot.appendChild(headerTemplate.content); } } customElements.define('header-component', Header);
const footerTemplate = document.createElement('template'); footerTemplate.innerHTML = `  footer { height: 60px; padding: 0 10px; list-style: none; display: flex; flex-shrink: 0; justify-content: space-between; align-items: center; background-color: #dfdfe2; } ul li { list-style: none; display: inline; } a { margin: 0 15px; color: inherit; text-decoration: none; } a:hover { padding-bottom: 5px; box-shadow: inset 0 -2px 0 0 #333; } .social-row { font-size: 20px; } .social-row li a { margin: 0 15px; }   
  • About
  • Work
  • Contact
`; class Footer extends HTMLElement { constructor() { super(); } connectedCallback() { const shadowRoot = this.attachShadow({ mode: 'closed' }); shadowRoot.appendChild(footerTemplate.content); } } customElements.define('footer-component', Footer);

In closing

We've covered a lot here, and you might have already decided just to use React or Handlebars.js instead.

Those are both great options!

Still, for a smaller project where you'll only need a few reusable components, a whole library or templating language might be overkill.

Hopefully now you have the confidence to create your own reusable HTML components. Now get out there and create something great (and reusable).