Vue.js 3: Understanding Custom Elements and Web Components

Anton Ioffe - December 23rd 2023 - 10 minutes read

As senior developers seek to harness the full potential of modern web development, Vue.js 3 stands at the forefront of revolutionizing how we approach component-based architecture. In this deep dive, we'll unlock the synergistic power of Vue.js 3 and Web Components, guiding you through the creation, management, and best practices of Custom Elements that elegantly bridge native browser capabilities with Vue's reactive ecosystem. Prepare to elevate your development experience as we explore the seamless integration of communication patterns, advanced state management, and sophisticated styling techniques that will redefine the boundaries of your web applications. Join us on this journey to master the art of crafting highly encapsulated and interoperable web components that promise to streamline your projects and dazzle your users.

The Intersection of Vue.js 3 and Custom Elements

Vue.js 3 strengthens the synergy between component-based frameworks and the native web platform's component model through the embrace of the Custom Elements API. This vital part of the Web Components suite enables developers to craft new HTML tags with insulated styling and functionality. In Vue.js 3, the creation of custom elements is natural, harnessing Vue's component architecture with the reactivity model neatly integrating within the Custom Elements lifecycle. This integration affords a streamlined development experience that harmonizes with the web's native extensibility.

The framework's reactivity system, intrinsic to Vue.js 3, aligns gracefully with the lifecycle of custom elements. Vue-based custom elements respond to data mutations with comparable responsiveness and elegance to standard Vue components. This integration ensures that property updates of a custom element trigger corresponding view updates sans superfluous code. The reactivity model extends naturally to custom elements, offering a straightforward state management approach within the web components paradigm.

// Define a Vue component
const myVueComponent = {
  data() {
    return {
      message: 'Hello from Custom Element!'
  template: `<div>{{ message }}</div>`

// Define a custom element using Vue.js's defineCustomElement
const myCustomElement = Vue.defineCustomElement(myVueComponent);

// Register the custom element with the browser
customElements.define('my-custom-element', myCustomElement);

// Usage in HTML
// <my-custom-element></my-custom-element>

A key feature is the integration of Vue.js 3's component lifecycle hooks with the Custom Elements API's lifecycle callbacks. These hooks allow developers to engage with essential phases in a component's lifespan, facilitating creation, mounting, updating, and unmounting. The correlation between Vue.js 3's lifecycle and the Custom Elements API empowers developers to control the custom elements’ behavior intuitively and robustly.

Vue.js 3 facilitates the adoption of the Custom Elements API without compromising the advanced capabilities of the framework. While custom elements derived from Vue.js 3 are standard HTML elements in essence, they can still harness Vue-specific innovations like the powerful reactivity system and component-based architecture. This union of standards compliance and Vue.js enhancements expedites the creation of versatile custom elements that fully leverage Vue.js's progressive features.

In integrating with custom elements, Vue.js 3 not only introduces an additional component definition approach; it also partakes in a unified vision for the future of web development. It encourages developers to extend their scope beyond the framework norms, aiming for an interoperable and cohesive web environment. With the provision of necessary tools for crafting interactive and dynamic interfaces, Vue.js 3 positions itself as an avant-garde facilitator, marrying web extensibility with unique Vue-powered components.

Creating and Registering Custom Elements in Vue.js

Creating custom elements in Vue.js 3 is accomplished through the defineCustomElement method, which encapsulates a Vue component into a format that can be consumed as a native HTML element. The method takes a configuration object similar to that of the defineComponent function but goes further by returning a custom element constructor that extends HTMLElement. Defining a custom element involves outlining the usual Vue component options, such as props, emits, template, and uniquely for custom elements, styles to be injected into their shadow DOM. Here's an example of defining a Vue custom element:

const MyVueElement = defineCustomElement({
    props: {
        message: String
    emits: ['click'],
    template: '<button @click="$emit(\'click\')">{{ message }}</button>',

    // Setup styles to be scoped within the custom element's shadow DOM.
    styles: ['button { color: blue; }']

// Register the custom element globally.
// Now '<my-vue-element>' can be used anywhere in your HTML files.
customElements.define('my-vue-element', MyVueElement);

When registering custom elements, the approach should be thoughtful. Registering too eagerly might pollute the global scope, while registering too lazily could lead to elements not being defined when they're needed. The timing and scope of registration are critical: globally for common elements reused across the application, and locally for more specialized components that are utilized in specific regions. Performance-wise, consider externalizing Vue from your built bundle if these custom elements are to be used within a host application that also uses Vue. This prevents duplication of the Vue library and saves on memory overhead.

A common mistake is to forget that despite their Vue-like capabilities, these custom elements do not integrate directly with Vue's reactivity system after they are registered. It's important to handle property updates deliberately:

document.querySelector('my-vue-element').setAttribute('message', 'New message');

As opposed to Vue components that automatically react to data changes. Developers may need to implement the attributeChangedCallback in their custom element's class to handle property updates reactively.

To enhance usability, it is advisable to provide an entry point in your Vue custom element library that exports a convenience function to register all of your custom elements at once. This provides users with the ability to import and register components easily while still allowing for on-demand individual imports. Here is how you can provide both the convenience function and individual registration options:

import { MyVueElementConfig } from './MyVueElementConfig';

// Define the custom element.
const MyVueButton = defineCustomElement(MyVueElementConfig);

export function registerAllCustomElements() {
    customElements.define('my-vue-button', MyVueButton);
    // More elements can be added here.

// Individual export for specific on-demand registration
export { MyVueButton };

A final contemplation for developers: how will you orchestrate the lifecycle of your Vue.js custom elements so they flourish in the varied garden of frameworks and applications? Will these components bloom as self-contained islands of functionality, or will they intertwine with greater ecosystems through event listeners and callbacks? Devise strategies that harmonize with their environmental context while preserving their autonomy.

Communication Patterns: Props, Events, and Slots

In managing data flow to custom elements via props, remember that strings alone aren't adequate for complex data types. Vue adheres to web standards by preferring to set values as DOM properties instead of attributes wherever possible, especially important for non-stringifiable data such as objects or arrays. In cases where a custom element does not declare a DOM property, you can ensure Vue sets a value as a property using the .prop modifier:

  <my-element v-bind:some-object.prop="myObject"></my-element>

Events provide a way for custom elements to communicate back to Vue. When using this.$emit or emit from the setup function, they generate native CustomEvent instances. Developers should note that Vue packages additional event data into the CustomEvent object's detail property as an array, unlike the typical singular value used in standard JavaScript event handling:

  <my-element @my-event="handleEvent"></my-element>

export default {
  methods: {
    handleEvent(event) {
      // Access the payload from event.detail

Common missteps include attempting to use Vue's v-slot directive with custom elements, which is incompatible. The correct approach involves using standard HTML slot attributes for content distribution into custom element slots:

    <div slot="header">This is a header slot content.</div>
    <div slot="footer">This is a footer slot content.</div>

However, employing native slot syntax means relinquishing Vue's scoped slots feature and its just-in-time rendering. Native slots necessitate immediate content rendering, which may lead to unnecessary resource usage, as slots need to be filled during the initial rendering even if they are not immediately displayed.

The upgrade in reactivity from Vue to Custom Elements highlights a substantial challenge. Simple prop modifications don't suffice as the Custom Element's independent rendering logic can overlook changes. This necessitates a deliberate strategy to manage reactive updates, mandating a careful approach to syncing the internal state of the component with the external interface, ensuring the reactivity is maintained:

  <my-element :some-prop="dynamicValue"></my-element>

export default {
  data() {
    return {
      dynamicValue: 'Original Value'
  mounted() {
    setTimeout(() => {
      this.dynamicValue = 'Updated Value';
    }, 2000);

Advanced State Management and Lifecycle Hooks

Vue.js's reactivity system is renowned for its efficiency and ease of understanding, an experience it brings to the world of Custom Elements. Within the Custom Elements environment, Vue leverages its reactivity primitives like watchEffect and computed to observe and respond to data changes. When used inside Custom Elements, watchEffect provides a straightforward way to perform side effects in response to reactive state changes, and it cleans up after itself when the component unmounts. This is particularly useful within the connectedCallback to set up any side effects that need to clean up on disconnection. On the other hand, computed properties inside Custom Elements maintain their lazy nature, recalculating only when their dependencies change, thereby ensuring a performant approach to deriving state.

The Composition API presents an interesting avenue for managing state within Custom Elements, thanks to its function-based API and fine-grained reactivity control. You can create encapsulated reactive states and operations using Vue's ref, reactive, computed, and watch APIs, significantly enhancing the custom element's modularity and reusability. The Composition API's setup function is invoked just once, coinciding with the element's connectedCallback, aligning neatly with the Custom Element's lifecycle. However, this requires careful orchestration to manage cleanup and avoid memory leaks, which can be achieved using lifecycle hooks such as disconnectedCallback.

Performance considerations diverge when comparing these approaches. watchEffect is eager and runs immediately, which can be suboptimal if the setup code is expensive or if there are many watchEffect instances in a single element. The Composition API, while granular, can introduce added complexity if not managed properly. It opens up potential for unintended side effects if reactivity is wrongly orchestrated across different lifecycle hooks.

On the readability scale, the Composition API scores high due to its organized structure and clear separation of concerns. However, it tends to be verbose, particularly in setups with extensive business logic. In contrast, watchEffect aims for succinctness, which could come at the cost of clarity when side effects become complex or multiple reactive sources are involved.

Avoiding common mistakes involves understanding the lifecycle of a Vue.js powered Custom Element. For instance, attempting to access the element's shadow DOM in the constructor will fail because it's not yet attached. The correct approach is to use connectedCallback. Also, when using watchEffect, a common mistake is forgetting to perform cleanup of side effects, which can be avoided by utilizing the cleanup function provided as an argument to the watchEffect callback. It prompts you to wonder, how might you balance the adaptation of Vue's reactivity system with the imperative nature of custom elements to harness the best of both paradigms?

Styling and Theming Vue.js Web Components

When dealing with the styling and theming of Vue.js Web Components, developers need to consider encapsulation and isolation to ensure styles are not inadvertently affecting other elements on the page. Scoped styles in Vue Single File Components (SFCs) provide an immediate solution. This is achieved by appending unique data attributes to component elements and corresponding styles, resulting in a form of style encapsulation akin to Shadow DOM without the full isolation.

However, when a Vue component is converted to a Web Component using defineCustomElement, developers can opt to take advantage of the Shadow DOM's style encapsulation features. This is particularly useful for creating truly isolated components. To facilitate this, Vue allows for scoped CSS in combination with Shadow DOM. This ensures that styles defined within the component do not leak out, and external styles do not interfere with the component's presentation. It's vital to remember that while the Shadow DOM provides strong encapsulation, it has its complexity, such as requiring custom strategies for theming and style overrides, which can be challenging in a large-scale application where design consistency is key.

CSS-in-JS libraries present an alternative approach, enabling developers to define styles within JavaScript. While this technique can offer a higher degree of control and dynamism, aligning it with the Web Component model requires careful consideration. Mixing Vue with CSS-in-JS inside of Web Components can lead to performance concerns, as styles may be duplicated or injected at runtime. Nevertheless, adopting CSS-in-JS can bring improved modularity and reusability to your components, making theming more dynamic, as styles become a part of the component's logic.

The best practice for styling Vue.js Web Components revolves around the principle of least surprise. It entails using the standard style tags and CSS files while leveraging the modular capabilities of Vue's SFCs to ensure styles are predictably scoped and applied. As part of this, developers should carefully manage component styles to avoid conflicts and provide clear theming hooks via custom properties (CSS variables), which can effortlessly pierce the Shadow DOM boundary when needed.

Developers must watch out for common pitfalls such as overreliance on global styles or inadvertently creating specificity wars due to unscoped CSS. Moreover, while Shadow DOM provides excellent style encapsulation, it may pose barriers for CSS frameworks that rely on global styles unless they are specifically designed to penetrate shadow boundaries. It is essential to weigh the pros and cons of each styling method, balancing between encapsulation, theming flexibility, and performance, to cultivate scalable and maintainable Web Components using Vue.js.


In this article about Vue.js 3 and Custom Elements, developers are introduced to the seamless integration of these technologies and the benefits it brings to modern web development. The article covers topics such as creating and registering custom elements in Vue.js, communication patterns using props, events, and slots, advanced state management, and styling and theming Vue.js web components. The key takeaways from the article include the power of Vue.js 3's reactivity system in custom elements, the importance of thoughtful registration of custom elements, and the various approaches to styling and theming web components. As a challenging technical task, the reader is prompted to consider how they would balance the adaptation of Vue's reactivity system with the imperative nature of custom elements to harness the best of both paradigms.

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