Typed Components

Web Components generated with stencil come with typing information automatically generated by the Stencil compiler. Typescript declarations provide strong guarantees when consuming components:

  • Ensuring that proper values are passed down as properties
  • Code autocompletion in modern IDEs such as Atom and VSCode
  • Events' details
  • Signature of components' methods

This public types are automatically generated by Stencil in src/component.d.ts, which allows for strong typing in JSX (just like React) and HTMLElement interfaces for each component.

Because Web Components generated by Stencil are just vanilla Web Components, they extend the HTMLElement interface. For each component a type named HTML{CamelCaseTag}Element is registered at the global scope, meaning developers DO NOT have to import them explicitly, just like HTMLElement or HTMLScriptElement are not imported.

  • ion-button => HTMLIonButtonElement
  • ion-menu-controller => HTMLIonMenuControllerElement
const button: HTMLIonButtonElement = document.queryElement('ion-button');
button.fill = 'outline';

IMPORTANT: always use the HTML{}Element interfaces in order to hold references to components.


Component properties and attributes are defined by annotating an instance variable with the @Prop() decorator:

@Prop() prop: number;

Stencil also uses the type information in order to generate the Web Component types and automatically cast the values to the specified type at runtime**:

@Prop() str1: string; // string attribute
@Prop() str2: 'md' | 'ios'; // string attribute
@Prop() str3 = 'defaultValue'; // string attribute

@Prop() number1: number; // numeric attribute
@Prop() number2 = 42; // numeric attribute

@Prop() boolean1: boolean; // boolean attribute
@Prop() boolean2 = true; // boolean attribute

Required properties

Using ! after the variable name, will mark this attribute/property as required, this way the JSX types will ensure the property is passed out.

@Prop() value!: string;