Component Decorator

@Component() is a decorator that designates a TypeScript class as a Stencil component. Every Stencil component gets transformed into a web component at build time.

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
  // additional options
})
export class TodoList {
  // implementation omitted
}

Component Options

The @Component() decorator takes one argument, an object literal containing configuration options for the component. This allows each component to be individually configured to suit the unique needs of each project.

Each option, its type, and whether it's required is described below.

tag

Required

Type: string

Details:
This value sets the name of the custom element that Stencil will generate. To adhere to the HTML spec, the tag name must contain a dash ('-').

Ideally, the tag name is a globally unique value. Having a globally unique value helps prevent naming collisions with the global CustomElementsRegistry, where all custom elements are defined. It's recommended to choose a unique prefix for all your components within the same collection.

Example:

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
})
export class TodoList {
  // implementation omitted
}

After compilation, the component defined in TodoList can be used in HTML or another TSX file:

<!-- Here we use the component in an HTML file -->
<todo-list></todo-list>

{/* Here we use the component in a TSX file */}
<todo-list></todo-list>

assetsDirs

Optional

Type: string[]

Details:
assetsDirs is an array of relative paths from the component to a directory containing the static files (assets) the component requires.

Example:
Below is an example project's directory structure containing an example component and assets directory.

src/
└── components/
    ├── assets/
    │   └── sunset.jpg
    └── todo-list.tsx

Below, the todo-list component will correctly load the sunset.jpg image from the assets/ directory, using Stencil's getAssetPath().

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

@Component({
  tag: 'todo-list',
  // 1. assetsDirs lists the 'assets' directory as a relative (sibling)
  // directory
  assetsDirs: ['assets']
})
export class TodoList {
  image = "sunset.jpg";

  render() {
    // 2. the asset path is retrieved relative to the asset base path to use in
    // the <img> tag
    const imageSrc = getAssetPath(`./assets/${this.image}`);
    return <img src={imageSrc} />
  }
}

In the example above, the following allows todo-list to display the provided asset:

1. The TodoList's @Component() decorator has the assetsDirs property, and lists the file's sibling directory, assets/. This will copy the assets directory over to the distribution directory.
2. Stencil's getAssetPath() is used to retrieve the path to the image to be used in the <img> tag

For more information on configuring assets, please see Stencil's Assets Guide

formAssociated

Optional

Type: boolean

Default: false

If true the component will be form-associated, allowing you to take advantage of the ElementInternals API to enable your Stencil component to participate in forms.

A minimal form-associated Stencil component could look like this:

import { Component } from '@stencil/core';

@Component({
  tag: 'form-associated',
  formAssociated: true
})
export class FormAssociated {
  render() {
    return <span>form associated!</span>
  }
}

See the documentation for form-associated components for more info and examples.

scoped

Optional

Type: boolean

Default: false

Details:
If true, the component will use scoped stylesheets.

Scoped CSS is an alternative to using the native shadow DOM style encapsulation. It appends a data attribute to your styles to make them unique and thereby scope them to your component. It does not, however, prevent styles from the light DOM from seeping into your component.

To use the native shadow DOM, see the configuration for shadow.

This option cannot be set to true if shadow is enabled.

Example:

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
  scoped: true
})
export class TodoList {
  // implementation omitted
}

shadow

Optional

Type: boolean | { delegatesFocus: boolean }

Default: false

Details:
If true, the component will use native Shadow DOM encapsulation. It will fall back to scoped if the browser does not support shadow-dom natively.

delegatesFocus is a property that provides focus to the first focusable entry in a component using Shadow DOM. If an object literal containing delegatesFocus is provided, the component will use native Shadow DOM encapsulation, regardless of the value assigned to delegatesFocus.

When delegatesFocus is set to true, the component will have delegatesFocus: true added to its shadow DOM.

When delegatesFocus is true and a non-focusable part of the component is clicked:

• the first focusable part of the component is given focus
• the component receives any available focus styling

If shadow is set to false, the component will not use native shadow DOM encapsulation.

This option cannot be set to enabled if scoped is enabled.

Example 1:

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
  shadow: true
})
export class TodoList {
  // implementation omitted
}

Example 2:

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
  shadow: { 
    delegatesFocus: true,
  }
})
export class TodoList {
  // implementation omitted
}

styleUrl

Optional

Type: string

Details:
Relative URL to an external stylesheet containing styles to apply to your component. Out of the box, Stencil will only process CSS files (files ending with .css). Support for additional CSS variants, like Sass, can be added via a plugin.

Example:
Below is an example project's directory structure containing an example component and stylesheet.

src/
└── components/
    ├── todo-list.css
    └── todo-list.tsx

By setting styleUrl, Stencil will apply the todo-list.css stylesheet to the todo-list component:

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
  styleUrl: './todo-list.css',
})
export class TodoList {
  // implementation omitted
}

styleUrls

Optional

Type: string[] | { [modeName: string]: string | string[]; }

Details:
A list of relative URLs to external stylesheets containing styles to apply to your component.

Alternatively, an object can be provided that maps a named "mode" to one or more stylesheets.

Out of the box, Stencil will only process CSS files (ending with .css). Support for additional CSS variants, like Sass, can be added via a plugin.

Example:
Below is an example project's directory structure containing an example component and stylesheet.

src/
└── components/
    ├── todo-list-1.css
    ├── todo-list-2.css
    └── todo-list.tsx

By setting styleUrls, Stencil will apply both stylesheets to the todo-list component:

Using an array of styles

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
  styleUrls: ['./todo-list-1.css', './todo-list-2.css']
})
export class TodoList {
  // implementation omitted
}

Using modes

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
  styleUrls: {
     ios: 'todo-list-1.ios.scss',
     md: 'todo-list-2.md.scss',
  }
})
export class TodoList {
  // implementation omitted
}

styles

Optional

Type: string | { [modeName: string]: any }

Details:
A string that contains inlined CSS instead of using an external stylesheet. The performance characteristics of this feature are the same as using an external stylesheet.

When using styles, only CSS is permitted. See styleUrl if you need more advanced features.

Example:

import { Component } from '@stencil/core';

@Component({
  tag: 'todo-list',
  styles: 'div { background-color: #fff }'
})
export class TodoList {
  // implementation omitted
}

Embedding or Nesting Components

Components can be composed easily by adding the HTML tag to the JSX code. Since the components are just HTML tags, nothing needs to be imported to use a Stencil component within another Stencil component.

Here's an example of using a component within another component:

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

@Component({
  tag: 'my-embedded-component'
})
export class MyEmbeddedComponent {
  @Prop() color: string = 'blue';

  render() {
    return (
      <div>My favorite color is {this.color}</div>
    );
  }
}

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

@Component({
  tag: 'my-parent-component'
})
export class MyParentComponent {

  render() {
    return (
      <div>
        <my-embedded-component color="red"></my-embedded-component>
      </div>
    );
  }
}

The my-parent-component includes a reference to the my-embedded-component in the render() function.