components-web

Web Components

Component Structure

Stencil components are plain ES6/TypeScript classes with some decorator metadata:

import { Component, Prop, h } from "@stencil/core";

@Component({
  tag: "core-component",
  styleUrl: "core-component.css",
})
export class Component {
  @Prop() first: string;
  @Prop() last: string;

  render() {
    return (
      <div>
        Hello world, my name is {this.first} {this.last}
      </div>
    );
  }
}

Use it just like any other HTML element:

<core-component first="Stencil" last="JS"></core-component>

codesandbox.io example

Naming Components

When creating new components, use the core- prefix. e.g. core-button, core-icon, etc are valid names.

Custom Elements must contain kebab-case names so they do not collide with existing html elements.

API

The API for Stencil closely mirrors the API for Custom Elements v1.

Components

Decorator	Description
@Component()	Indicate a class is a Stencil component.
@Prop()	Creates a property that will exist on the element and be data-bound to this component.
@State()	Creates a local state variable that will not be placed on the element.
@Method()	Expose specific methods to be publicly accessible.

Component Example

Expand a detailed component example

@Component({
  tag: "core-component",
  styleUrls: {
    ios: "component.ios.css",
    md: "component.md.css",
    wp: "component.wp.css",
  },
})
export class Component {
  /**
   * 1. Own Properties
   * Always set the type if a default value has not
   * been set. If a default value is being set, then type
   * is already inferred. List the own properties in
   * alphabetical order. Note that because these properties
   * do not have the @Prop() decorator, they will not be exposed
   * publicly on the host element, but only used internally.
   */
  num: number;
  someText = "default";

  /**
   * 2. Reference to host HTML element.
   * Inlined decorator
   */
  @Element() el: HTMLElement;

  /**
   * 3. State() variables
   * Inlined decorator, alphabetical order.
   */
  @State() isValidated: boolean;
  @State() status = 0;

  /**
   * 4. Public Property API
   * Inlined decorator, alphabetical order. These are
   * different than "own properties" in that public props
   * are exposed as properties and attributes on the host element.
   * Requires JSDocs for public API documentation.
   */
  @Prop() content: string;
  @Prop() enabled: boolean;
  @Prop() menuId: string;
  @Prop() type = "overlay";

  /**
   * Prop lifecycle events SHOULD go just behind the Prop they listen to.
   * This makes sense since both statements are strongly connected.
   * - If renaming the instance variable name you must also update the name in @Watch()
   * - Code is easier to follow and maintain.
   */
  @Prop() swipeEnabled = true;

  @Watch("swipeEnabled")
  swipeEnabledChanged(newSwipeEnabled: boolean, oldSwipeEnabled: boolean) {
    this.updateState();
  }

  /**
   * 5. Events section
   * Inlined decorator, alphabetical order.
   * Requires JSDocs for public API documentation.
   */
  @Event() coreClose: EventEmitter;
  @Event() coreDrag: EventEmitter;
  @Event() coreOpen: EventEmitter;

  /**
   * 6. Component lifecycle events
   * Ordered by their natural call order, for example
   * WillLoad should go before DidLoad.
   */
  componentWillLoad() {}
  componentDidLoad() {}
  componentWillUpdate() {}
  componentDidUpdate() {}
  componentDidUnload() {}

  /**
   * 7. Listeners
   * It is ok to place them in a different location
   * if makes more sense in the context. Recommend
   * starting a listener method with "on".
   * Always use two lines.
   */
  @Listen("click", { enabled: false })
  onClick(ev: UIEvent) {
    console.log("hi!");
  }

  /**
   * 8. Public methods API
   * These methods are exposed on the host element.
   * Always use two lines.
   * Public Methods must be async.
   * Requires JSDocs for public API documentation.
   */
  @Method()
  async open(): Promise<boolean> {
    // ...
    return true;
  }

  @Method()
  async close(): Promise<void> {
    // ...
  }

  /**
   * 9. Local methods
   * Internal business logic. These methods cannot be
   * called from the host element.
   */
  prepareAnimation(): Promise<void> {
    // ...
  }

  updateState() {
    // ...
  }

  /**
   * 10. render() function
   * Always the last one in the class.
   */
  render() {
    return (
      <Host
        attribute="navigation"
        side={this.isRightSide ? "right" : "left"}
        type={this.type}
        class={{
          "core-is-animating": this.isAnimating,
        }}
      >
        <div class="menu-inner page-inner">
          <slot />
        </div>
      </Host>
    );
  }
}

Browser Support

Web Components, specifically Custom Elements, are natively supported in Chrome and Safari and are coming to both Edge and Firefox. A dynamic polyfill loader is already included in order to only load the polyfills for the browsers that are missing specific features.

• Chrome (and all Chromium based browsers)
• Safari
• Edge
• Firefox
• IE 11

Polyfills

Stencil includes a subset of the core-js polyfills for old browsers like IE 11, fetch and conditionally downloads the Custom Elements v1 only when it's needed.

Internet Explorer 11

Browsers that do not support native ESM (at the moment, only IE11 and older) will download a subset of core-js.

Expand IE11 polyfill details

This subset is generated using the core-js-builder tool with the following configuration:

require("core-js-builder")({
  targets: "ie 11",
  modules: [
    "es",
    "web.url",
    "web.url.to-json",
    "web.url-search-params",
    "web.dom-collections.for-each",
  ],
  blacklist: [
    "es.math",
    "es.date",
    "es.symbol",
    "es.array-buffer",
    "es.data-view",
    "es.typed-array",
    "es.reflect",
    "es.promise",
  ],
});

In addition, the following set of polyfills are also included:

• Promise
• fetch()
• CSS variables: Stencil implemented their own CSS variables polyfill that integrates into the runtime.

All browsers

Some modern browsers like Edge do not include native support for Web Components. In that case, we conditionally load the Custom Elements v1 polyfill.