Creating a Multiselect Component as a Web Component

Update 12.05.2016: A companion article, "How to Make Accessible Web Components," addresses this article's limitations. Please read both for a complete understanding. This article benefited from peer review by Ryan Lewis. Thanks to SitePoint's peer reviewers!

Modern web applications demand efficient component management. Handling large amounts of HTML, JavaScript, and CSS necessitates breaking down applications into reusable, encapsulated components to prevent style conflicts and script interference. Traditionally, component code is scattered across multiple files. Furthermore, complex markup with numerous divs and spans hinders readability and maintainability. Web Components, a W3C standard, offer a solution. This tutorial explains Web Components and demonstrates building a multiselect widget.

Key Takeaways:

<li>Web Components streamline development by encapsulating markup, styles, and scripts, improving maintainability and reducing conflicts. <li>This tutorial builds a reusable multiselect widget using custom elements, shadow DOM, and HTML templates. <li>The component is natively supported by modern browsers, with polyfills ensuring broader compatibility. <li>Features include customizable placeholders, dynamic updates, and custom event handling. <li>Extending Web Components using frameworks like Polymer and X-Tag is also discussed.

Understanding Web Components:

Web Components address the challenges mentioned above. They allow linking a single HTML file containing a component's implementation and using it on a page via a custom HTML element. This enhances encapsulation and improves code expressiveness. The core specifications are:

<li> Custom Elements: Define custom HTML elements for components. <li> HTML Templates: Define the component's markup. <li> Shadow DOM: Encapsulates the component's internal structure, isolating it from the page's styles and scripts. <li> HTML Imports: (largely superseded) Facilitated including components into pages.

Let's build a production-ready multiselect component. A demo and source code are available on GitHub.

Building a Production-Ready Multiselect Component:

Our multiselect widget will have the following markup structure:

<x-multiselect placeholder="Select Item">
  <li value="1" selected>Item 1</li>
  <li value="2">Item 2</li>
  <li value="3" selected>Item 3</li>
</x-multiselect>

The <x-multiselect> element uses a placeholder attribute. List items (<li>) have value and selected attributes. The component exposes a selectedItems() method returning an array of selected values ([1, 3] in this example). It also fires a change event on selection changes. The goal is full modern browser compatibility.

Template (multiselect.html):

The component's HTML, CSS, and JavaScript reside in multiselect.html. An HTML template defines the component's structure:

<template id="multiselectTemplate">
  <style>
    /* component styles */
  </style>
  <div class="multiselect">
    <div class="multiselect-field"></div>
    <div class="multiselect-popup">
      <ul class="multiselect-list">
        <content select="li"></content>
      </ul>
    </div>
  </div>
</template>

The <content select="li"> element inserts <li> elements from the host element into the shadow DOM.

Component Creation:

The JavaScript registers the custom element:

// 1. Find template
var ownerDocument = document.currentScript.ownerDocument;
var template = ownerDocument.querySelector('#multiselectTemplate');

// 2. Create component prototype
var multiselectPrototype = Object.create(HTMLElement.prototype);

// 3. Define createdCallback
multiselectPrototype.createdCallback = function() {
  var root = this.createShadowRoot();
  var content = document.importNode(template.content, true);
  root.appendChild(content);
};

// 4. Register custom element
document.registerElement('x-multiselect', { prototype: multiselectPrototype });

This involves finding the template, creating a prototype inheriting from HTMLElement.prototype, defining createdCallback to create the shadow DOM, and registering the custom element.

(The remainder of the component's implementation, including rendering, event handling, attribute handling, and styling, would follow here. Due to length constraints, it's omitted but is available in the linked GitHub repository.)

Browser Support and Polyfills:

Web Components have excellent support in modern browsers, but polyfills like webcomponentsjs ensure compatibility across all major browsers. The demo page demonstrates usage and addresses potential polyfill-related issues (like handling document.currentScript variations and shadow DOM styling).

Polymer and X-Tag:

Frameworks like Polymer and X-Tag simplify Web Component development, offering additional features and reducing boilerplate code.

Conclusion:

Web Components significantly advance web development. This tutorial showcases building a functional multiselect component, highlighting the power and benefits of this technology. Remember to consult the linked "How to Make Accessible Web Components" article for accessibility best practices.

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