Web Components: create reusable HTML elements

In the landscape of modern web development, the quest for increased modularity and better maintainability inevitably leads developers toward innovative methods. Web Components embody a native and standardized response to this need, offering the ability to create custom HTML elements and reusable elements that integrate easily into any web environment. Thanks to them, the code becomes clearer, more structured, and adapts smoothly to the growing demands of contemporary projects, which can sometimes be complex.

These components are not limited to a simple form of HTML extension; they bring together technologies such as Custom Elements, Shadow DOM, and HTML Templates, all under one roof. Together, they provide effective encapsulation of both style and behavior of components, ensuring interoperability without conflict with the rest of the page. This evolution addresses several challenges related to reusability, modularity, and isolation of functionalities, which are essential in front-end development in 2025.

Throughout this article, various facets of web components will be explored, from the simple definition of a custom element to mastering advanced techniques like the Shadow DOM and extending native HTML elements. The methodical approach adopts a technical angle, enriched with concrete and tailored examples to provide a clear and pragmatic understanding of this essential technology of the modern web.

Key points to remember:

• Native modularity: Web Components facilitate the creation of independent and reusable blocks, reducing code duplication.
• Encapsulation: The Shadow DOM ensures that the style and behavior of a component do not interfere with those of the page.
• Interopérability: These components work with or without a framework, ensuring flexible integration.
• Extensibility: Ability to define custom elements or extend existing native HTML elements.
• Managed lifecycle: Custom element reactions allow for efficient management of component states and modifications.

Defining and understanding the basics of Custom Elements to create reusable HTML elements

At the core of Web Components, Custom Elements provide the capability to define new HTML tags with custom behavior and presentation. This means that the developer can design specific elements tailored to a wide variety of functional needs while maintaining a syntax and usage similar to traditional HTML tags.

The definition of a custom element relies on a JavaScript class that inherits from HTMLElement. This extension allows the component to benefit from all standard DOM properties, methods, and events while integrating its own functionalities. For example, creating a <app-drawer> component that acts like a sliding panel can include both specific properties like open and disabled, and methods modulating its behavior like toggleDrawer().

Here’s a methodical overview of the fundamental steps:

1. Creating the extended class: You must call super() in the constructor to respect the DOM rules and ensure the component operates correctly.
2. Defining properties: Properties like open or disabled are linked to getters/setters that reflect their state through HTML attributes, allowing for declarative use in the DOM.
3. Registering the component: Using the method customElements.define('component-name', ComponentClass), you inform the browser of the existence of this new element, which it can then respond to by parsing the page and upgrading the encountered instances.

It is important to note the naming constraint specific to custom elements: their name must contain a hyphen (“-”) to avoid conflicts with native HTML elements. This rule is essential to ensure their proper functioning and maintain backward compatibility.

Furthermore, custom elements possess a lifecycle that allows them to manage their existence in the DOM through several methods called “custom element reactions”:

• constructor: initialization of the component.
• connectedCallback: triggered when the element is inserted into the DOM, ideal for deferred initialization.
• disconnectedCallback: called when the element is removed from the DOM, useful for cleanup.
• attributeChangedCallback: allows reacting to changes in observed attributes.
• adoptedCallback: triggered when the element is moved between documents.

This approach offers rigorous control over the component’s behavior and state, significantly improving the robustness of web applications. Several popular frameworks today rely on these native features as a foundation.

Encapsulating style and structure with the Shadow DOM for advanced modularity

The Shadow DOM is a key technology of Web Components, providing strict encapsulation of the DOM structure and CSS style within a custom component. Unlike traditional methods where styles can leak or be overridden by global rules, the Shadow DOM creates a completely isolated DOM subtree.

When creating a component, you attach a Shadow root using the attachShadow method, typically within the constructor:

<!– wp:code {"content":"

nthis.attachShadow({ mode: 'open' });n

“} –>

this.attachShadow({ mode: 'open' });

The “open” mode allows access to the Shadow DOM from the outside (via element.shadowRoot), which is useful for development and testing. The “closed” mode disables this access, enhancing encapsulation, although it is less commonly used in practice.

The content of the component, made up of its HTML and CSS, is then injected into this Shadow DOM, ensuring that the style will not be affected by external conflicts nor will it unwittingly modify the global page. For example, a decorated button or a custom panel can have its own design without affecting the rest of the site.

The concrete advantages are numerous:

• Total isolation of styles: The CSS applied within the Shadow DOM is scoped (limited) only to this element.
• Protection against global conflicts: Global CSS rules do not directly apply to elements in the Shadow DOM.
• Better organization: The internal HTML structure is hidden, allowing the developer to expose only what is necessary via slots.
• Interopérability: The Shadow DOM operates natively in all modern browsers, promoting a consistent user experience.

The concept of slot complements this encapsulation to include custom content in specific parts of the component while preserving isolation. This mechanism allows interface designers to offer configurable content areas.

Finally, the Shadow DOM, combined with HTML Templates, allows for the efficient creation of encapsulated components that can be activated or cloned at will, offering unprecedented modularity in a native context.

HTML Templates and encapsulation for performant and flexible web components

HTML Templates are another cornerstone of Web Components, allowing the definition of inactive DOM fragments during page loading that can be cloned and dynamically inserted into the DOM tree. This optimizes the creation and reuse of complex structures.

A template is declared using the <template> tag: its content is not immediately rendered by the browser. By activating the content at will using JavaScript, an independent instance is reloaded, avoiding issues from manual code duplication and enhancing performance.

Templates are often combined with Custom Elements. When a component is instantiated, it clones its internal template into its Shadow DOM. This process offers structural consistency and style isolation, allowing for the development of complex components without compromising readability and maintenance.

Here’s a typical illustration:

<!– wp:code {"content":"

n<template id="custom-template">n  <style>n    p { color: blue; }n  </style>n  <p>Stylized encapsulated content</p>n</template>n

“} –>

<template id="custom-template">
  <style>
    p { color: blue; }
  </style>
  <p>Stylized encapsulated content</p>
</template>

In the component script:

<!– wp:code {"content":"

nconst tmpl = document.getElementById('custom-template');nconst shadow = this.attachShadow({ mode: 'open' });nshadow.appendChild(tmpl.content.cloneNode(true));n

“} –>

const tmpl = document.getElementById('custom-template');
const shadow = this.attachShadow({ mode: 'open' });
shadow.appendChild(tmpl.content.cloneNode(true));

This method ensures that each instance of the component will have its own copy of the structure and style. It promotes performance through reuse without rewriting, a determining factor in 2025 for managing complex application interfaces smoothly.

The content management defined in the template allows for a complete and autonomous component with its own internal styles and behaviors, while giving developers the ability to expose a clear and efficient API.

Extending or enhancing native HTML elements with Web Components for maximum interoperability

In addition to creating completely new elements, Web Components allow for extending native HTML elements like <button> or <img> to enhance their functionalities without altering their basic behavior. This technique, called built-in custom elements, ensures maximum compatibility and a smooth learning curve.

When a component extends a native element, the class must inherit from the corresponding DOM class, for example HTMLButtonElement for a button. Registration via customElements.define must then specify the extended element using the extends option. This method also allows the end-user to use this component declaratively in HTML, through the syntax <button is="fancy-button">.

The advantages of this approach:

• Preserved native functionalities: The element retains its standard behavior, essential for accessibility and performance.
• Easy customization: Adding methods or advanced visual effects without rewriting the element. For example, a button can display a “ripple” animation on each click.
• Interopérability with third-party libraries: The component remains compatible with external APIs and frameworks.

Here’s an excerpt illustrating a custom button:

<!– wp:code {"content":"

nclass FancyButton extends HTMLButtonElement {n  constructor() {n    super();n    this.addEventListener('click', e => this.createRipple(e.offsetX, e.offsetY));n  }n  n  createRipple(x, y) {n    // Animation ripple implementationn  }n}nncustomElements.define('fancy-button', FancyButton, { extends: 'button' });n

“} –>

class FancyButton extends HTMLButtonElement {
  constructor() {
    super();
    this.addEventListener('click', e => this.createRipple(e.offsetX, e.offsetY));
  }
  
  createRipple(x, y) {
    // Animation ripple implementation
  }
}

customElements.define('fancy-button', FancyButton, { extends: 'button' });

This approach is favored in 2025 to progressively enhance interfaces without major breaks, ensuring a natural transition to an architecture based on reusable components.

Best practices, tools, and perspectives for mastering the creation of reusable Web Components

The creation of effective web components also relies on a series of technical and methodological best practices, essential for avoiding common pitfalls and optimizing robustness.

Among the recommendations, one can include:

• Reflection between properties and attributes: Ensure by design the synchronization between HTML attributes and JavaScript properties for consistent usage in the DOM and scripts.
• Rigorous lifecycle management: Fully leverage callbacks to manage resources, avoid memory leaks particularly through cleanup in disconnectedCallback.
• Using the Shadow DOM for style: Limit CSS side effects by encapsulating styles, but accept that users may apply global styling via external selectors.
• Polyfills and cross-browser support: By 2025, while most browsers are compatible with Custom Elements v1, using appropriate polyfills ensures better accessibility on remaining environments.
• Tests and continuous integration: Establish unit tests and integration checks to verify isolated behavior and integration into the overall application.

Here’s a synthetic table of the main methods of the lifecycle of Custom Elements:

Name	When called	Main utility
constructor	Upon creation or upgrade of the element	Initialization, configuration
connectedCallback	When the element is inserted into the DOM	Loading resources, rendering
disconnectedCallback	When the element is removed from the DOM	Releasing resources, cleanup
attributeChangedCallback	Modification of an observed attribute	Reactivity to changes
adoptedCallback	When the element is moved between documents	Contextual update

/* Function toggleDetails(event): Manages the opening or closing of detailed sections with “Learn more” buttons. Also updates the aria-expanded attribute for accessibility. */ function toggleDetails(event) { const button = event.currentTarget; const contentId = button.getAttribute(‘aria-controls’); const content = document.getElementById(contentId); if (!content) return; const isHidden = content.hasAttribute(‘hidden’); if (isHidden) { content.removeAttribute(‘hidden’); button.setAttribute(‘aria-expanded’, ‘true’); button.textContent = ‘Collapse’; } else { content.setAttribute(‘hidden’, ”); button.setAttribute(‘aria-expanded’, ‘false’); button.textContent = ‘Learn more’; } }

Finally, it is crucial to adopt a progressive approach, combining progressive enhancement techniques to ensure compatibility in the varied environments commonly encountered. Mastery of Web Components also opens the door to better integration within modern ecosystems, fostering effective collaboration between front-end and back-end developers.

Relevant reuse, modularity, and ease of maintenance brought by web components thus remain a major lever for the quality and longevity of applications in 2025 and beyond.

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What is a Web Component?

A Web Component is a custom HTML element defined by the developer, combining encapsulated structure, style, and behavior, usable like a traditional HTML tag.

How does the Shadow DOM work?

The Shadow DOM creates an isolated DOM subtree within a custom element, allowing for style and structure encapsulation, avoiding any conflict with the main page.

Can Web Components be used with frameworks like React or Vue?

Yes, Web Components are interoperable and can seamlessly integrate into applications using React, Vue, Angular, or other frameworks.

What is the difference between a custom element and an extended native element?

A custom element creates a new HTML tag, while an extended native element enhances an existing HTML tag by extending its DOM class and functionalities.

Are Web Components compatible with all browsers?

Most modern browsers support Web Components via Custom Elements v1 and Shadow DOM, with polyfills available to ensure optimal compatibility.