What are the best practices for using the HTML Shadow DOM API in web components?

2 Use custom properties for style customization

Web components offer the benefit of exposing style hooks for consumers of your component to customize its appearance through custom properties, also known as CSS variables. These are declared in the shadow root and can be overridden by the host document. When utilizing custom properties, it's important to use a consistent naming convention and document them for the consumers of your web component. The :host and :host-context pseudo-classes should be used to apply styles based on the state or context of your component, and hard-coding colors, fonts, or other values should be avoided to prevent clashing with the host document's style.

3 Use ARIA attributes for accessibility

A common challenge of web components is that they can create accessibility issues for users who rely on assistive technologies, such as screen readers or keyboard navigation. This is because the shadow root hides the internal structure and semantics of your component from the browser's accessibility tree. To counter this, you should use ARIA attributes, which are a set of attributes that provide additional information about the role, state, and properties of your component. For effective use of these attributes, you should specify the type of your component with the role attribute, provide a descriptive name for it with the aria-label or aria-labelledby attributes, hide elements that are not relevant or visible to the user with the aria-hidden attribute, and indicate its state with the aria-expanded, aria-selected, aria-checked, or other attributes.

4 Use custom events for communication

Another challenge of web components is that they can create communication barriers between the component and the host document, as well as between nested components due to the shadow root isolating the component from the global event system and creating its own event context. To overcome this challenge, custom events can be used. These are a type of events that you can create and dispatch from your component, allowing you to pass data, bubble up or down the DOM tree, and cancel the default behavior. To use custom events effectively, you should use a descriptive and unique name for them and document them for consumers of your web component. Additionally, the detail property should be used to pass data along with your custom events and validate it before using it. Furthermore, the composed and bubbles properties should be used to control how your custom events propagate through the shadow boundary and the host document. Finally, make sure to utilize the preventDefault and stopPropagation methods to handle your custom events and prevent conflicts or interference.

5 Use lifecycle callbacks for initialization and cleanup

The last best practice for using the Shadow DOM API in web components is to use the lifecycle callbacks, which are a set of methods that are invoked when your component is created, connected, disconnected, adopted, or updated. These callbacks allow you to perform initialization and cleanup tasks such as creating the shadow root, attaching event listeners, fetching data, or removing references. To use them effectively, you should define the initial properties and state of your component in the constructor but avoid manipulating the DOM or accessing attributes. In connectedCallback, create the shadow root, render the template, add event listeners, or fetch data. In disconnectedCallback remove event listeners, cancel requests, or release resources. The adoptedCallback handles the case when your component is moved to a new document and attributeChangedCallback should be used to react to changes in the attributes of your component and update the DOM or state accordingly.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?