Version: v4.19

Properties

Props are custom attributes/properties exposed publicly on an HTML element. They allow developers to pass data to a component to render or otherwise use.

The Prop Decorator (`@Prop()`)

Props are declared on a component using Stencil's @Prop() decorator, like so:

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

@Component({
    tag: 'todo-list',
})
export class TodoList {
    // Second, we decorate a class member with @Prop()
    @Prop() name: string;
    
    render() {
        // Within the component's class, its props are
        // accessed via `this`. This allows us to render
        // the value passed to `todo-list`
        return <div>To-Do List Name: {this.name}</div>
    }
}

In the example above, @Prop() is placed before (decorates) the name class member, which is a string. By adding @Prop() to name, Stencil will expose name as an attribute on the element, which can be set wherever the component is used:

{/* Here we use the component in a TSX file */}
<todo-list name={"Tuesday's To-Do List"}></todo-list>

<!-- Here we use the component in an HTML file -->
<todo-list name="Tuesday's To-Do List"></todo-list>

In the example above the todo-list component is used almost identically in TSX and HTML. The only difference between the two is that in TSX, the value assigned to a prop (in this case, name) is wrapped in curly braces. In some cases however, the way props are passed to a component differs slightly between HTML and TSX.

Variable Casing

In the JavaScript ecosystem, it's common to use 'camelCase' when naming variables. The example component below has a class member, thingToDo that is camelCased.

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    // thingToDo is 'camelCased'
    @Prop() thingToDo: string;

    render() {
        return <div>{this.thingToDo}</div>;
    }
}

Since thingToDo is a prop, we can provide a value for it when we use our todo-list-item component. Providing a value to a camelCased prop like thingToDo is nearly identical in TSX and HTML.

When we use our component in a TSX file, an attribute uses camelCase:

<todo-list-item thingToDo={"Learn about Stencil Props"}></todo-list-item>

In HTML, the attribute must use 'dash-case' like so:

<todo-list-item thing-to-do="Learn about Stencil Props"></todo-list-item>

Data Flow

Props should be used to pass data down from a parent component to its child component(s).

The example below shows how a todo-list component uses three todo-list-item child components to render a ToDo list.

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

@Component({
  tag: 'todo-list',
})
export class TodoList {
  render() {
    return (
      <div>
        <h1>To-Do List Name: Stencil To Do List</h1>
        <ul> 
           {/* Below are three Stencil components that are children of `todo-list`, each representing an item on our list */}
           <todo-list-item thingToDo={"Learn about Stencil Props"}></todo-list-item>
           <todo-list-item thingToDo={"Write some Stencil Code with Props"}></todo-list-item>
           <todo-list-item thingToDo={"Dance Party"}></todo-list-item>
        </ul>
      </div>
    )
  }
}

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
  @Prop() thingToDo: string;
  
  render() {
    return <li>{this.thingToDo}</li>;
  }
}

note

Children components should not know about or reference their parent components. This allows Stencil to efficiently re-render your components. Passing a reference to a component as a prop may cause unintended side effects.

Mutability

A Prop is by default immutable from inside the component logic. Once a value is set by a user, the component cannot update it internally. For more advanced control over the mutability of a prop, please see the mutable option section of this document.

Types

Props can be a boolean, number, string, or even an Object or Array. The example below expands the todo-list-item to add a few more props with different types.

import { Component, Prop, h } from '@stencil/core';
// `MyHttpService` is an `Object` in this example
import { MyHttpService } from '../some/local/directory/MyHttpService';

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    @Prop() isComplete: boolean;
    @Prop() timesCompletedInPast: number;
    @Prop() thingToDo: string;
    @Prop() myHttpService: MyHttpService;
}

Boolean Props

A property on a Stencil component that has a type of boolean may be declared as:

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    @Prop() isComplete: boolean;
}

To use this version of todo-list-item in HTML, we pass the string "true"/"false" to the component:

<!-- Set isComplete to 'true' -->
<todo-list-item is-complete="true"></todo-list-item>
<!-- Set isComplete to 'false' -->
<todo-list-item is-complete="false"></todo-list-item>

To use this version of todo-list-item in TSX, true/false is used, surrounded by curly braces:

// Set isComplete to 'true'
<todo-list-item isComplete={true}></todo-list-item>
// Set isComplete to 'false'
<todo-list-item isComplete={false}></todo-list-item>

There are a few ways in which Stencil treats props that are of type boolean that are worth noting:

The value of a boolean prop will be false if provided the string "false" in HTML

<!-- The 'todo-list-item' component will have an isComplete value of `false` -->
<todo-list-item is-complete="false"></todo-list-item>

The value of a boolean prop will be true if provided a string that is not "false" in HTML

<!-- The 'todo-list-item' component will have an isComplete value of -->
<!-- `true` for each of the following examples -->
<todo-list-item is-complete=""></todo-list-item>
<todo-list-item is-complete="0"></todo-list-item>
<todo-list-item is-complete="False"></todo-list-item>

The value of a boolean prop will be undefined if it has no default value and one of the following applies:
1. the prop is not included when using the component
2. the prop is included when using the component, but is not given a value

<!-- Both examples using the 'todo-list-item' component will have an --> 
<!-- isComplete value of `undefined` -->
<todo-list-item></todo-list-item>
<todo-list-item is-complete></todo-list-item>

Number Props

A property on a Stencil component that has a type of number may be declared as:

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    @Prop() timesCompletedInPast: number;
}

To use this version of todo-list-item in HTML, we pass the numeric value as a string to the component:

<!-- Set timesCompletedInPast to '0' -->
<todo-list-item times-completed-in-past="0"></todo-list-item>
<!-- Set timesCompletedInPast to '23' -->
<todo-list-item times-completed-in-past="23"></todo-list-item>

To use this version of todo-list-item in TSX, a number surrounded by curly braces is passed to the component:

// Set timesCompletedInPast to '0'
<todo-list-item timesCompletedInPast={0}></todo-list-item>
// Set timesCompletedInPast to '23'
<todo-list-item timesCompletedInPast={23}></todo-list-item>

String Props

A property on a Stencil component that has a type of string may be declared as:

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    @Prop() thingToDo: string;
}

To use this version of todo-list-item in HTML, we pass the value as a string to the component:

<!-- Set thingToDo to 'Learn about Stencil Props' -->
<todo-list-item thing-to-do="Learn about Stencil Props"></todo-list-item>
<!-- Set thingToDo to 'Write some Stencil Code with Props' -->
<todo-list-item thing-to-do="Write some Stencil Code with Props"></todo-list-item>

To use this version of todo-list-item in TSX, we pass the value as a string to the component. Curly braces aren't required when providing string values to props in TSX, but are permitted:

// Set thingToDo to 'Learn about Stencil Props'
<todo-list-item thingToDo="Learn about Stencil Props"></todo-list-item>
// Set thingToDo to 'Write some Stencil Code with Props'
<todo-list-item thingToDo="Write some Stencil Code with Props"></todo-list-item>
// Set thingToDo to 'Write some Stencil Code with Props' with curly braces
<todo-list-item thingToDo={"Learn about Stencil Props"}></todo-list-item>

Object Props

A property on a Stencil component that has a type of Object may be declared as:

// TodoListItem.tsx
import { Component, Prop, h } from '@stencil/core';
import { MyHttpService } from '../path/to/MyHttpService';

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    // Use `@Prop()` to declare the `httpService` class member
    @Prop() httpService: MyHttpService;
}

// MyHttpService.ts 
export class MyHttpService {
    // This implementation intentionally left blank
}

In TypeScript, MyHttpService is both an Object and a 'type'. When using user-defined types like MyHttpService, the type must always be exported using the export keyword where it is declared. The reason for this is Stencil needs to know what type the prop httpService is when passing an instance of MyHttpService to TodoListItem from a parent component.

To set httpService in TSX, assign the property name in the custom element's tag to the desired value like so:

// TodoList.tsx
import { Component, h } from '@stencil/core';
import { MyHttpService } from '../MyHttpService';

@Component({
   tag: 'todo-list',
   styleUrl: 'todo-list.css',
   shadow: true,
})
export class ToDoList {
   private httpService = new MyHttpService();

   render() {
      return <todo-list-item httpService={this.httpService}></todo-list-item>;
   }
}

Note that the prop name is using camelCase, and the value is surrounded by curly braces.

It is not possible to set Object props via an HTML attribute like so:

<!-- this will not work -->
<todo-list-item http-service="{ /* implementation omitted */ }"></todo-list-item>

The reason for this is that Stencil will not attempt to serialize object-like strings written in HTML into a JavaScript object. Similarly, Stencil does not have any support for deserializing objects from JSON. Doing either can be expensive at runtime, and runs the risk of losing references to other nested JavaScript objects.

Instead, properties may be set via <script> tags in a project's HTML:

<script>
   document.querySelector('todo-list-item').httpService = { /* implementation omitted */ };
</script>

Array Props

A property on a Stencil component that is an Array may be declared as:

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    @Prop() itemLabels: string[];
}

To set itemLabels in TSX, assign the prop name in the custom element's tag to the desired value like so:

// TodoList.tsx
import { Component, h } from '@stencil/core';
import { MyHttpService } from '../MyHttpService';

@Component({
   tag: 'todo-list',
   styleUrl: 'todo-list.css',
   shadow: true,
})
export class ToDoList {
   private labels = ['non-urgent', 'weekend-only'];

   render() {
      return <todo-list-item itemLabels={this.labels}></todo-list-item>;
   }
}

Note that the prop name is using camelCase, and the value is surrounded by curly braces.

It is not possible to set Array props via an HTML attribute like so:

<!-- this will not work -->
<todo-list-item item-labels="['non-urgent', 'weekend-only']"></todo-list-item>

The reason for this is that Stencil will not attempt to serialize array-like strings written in HTML into a JavaScript object. Doing so can be expensive at runtime, and runs the risk of losing references to other nested JavaScript objects.

Instead, properties may be set via <script> tags in a project's HTML:

<script>
   document.querySelector('todo-list-item').itemLabels = ['non-urgent', 'weekend-only'];
</script>

Advanced Prop Types

`any` Type

TypeScript's any type is a special type that may be used to prevent type checking of a specific value. Because any is a valid type in TypeScript, Stencil props can also be given a type of any. The example below demonstrates three different ways of using props with type any:

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    // isComplete has an explicit type annotation
    // of `any`, and no default value
    @Prop() isComplete: any;
    // label has an explicit type annotation of
    // `any` with a default value of 'urgent',
    // which is a string
    @Prop() label: any = 'urgent';
    // thingToDo has no type and no default value,
    // and will be considered to be type `any` by
    // TypeScript
    @Prop() thingToDo;

    render() {
        return (
            <ul>
                <li>isComplete has a value of - {this.isComplete} - and a typeof value of "{typeof this.isComplete}"</li>
                <li>label has a value of - {this.label} - and a typeof value of "{typeof this.label}"</li>
                <li>thingToDo has a value of - {this.thingToDo} - and a typeof value of "{typeof this.thingToDo}"</li>
            </ul>
        );
    }
}

When using a Stencil prop typed as any (implicitly or explicitly), the value that is provided to a prop retains its own type information. Neither Stencil nor TypeScript will try to change the type of the prop. To demonstrate, let's use todo-list-item twice, each with different prop values:

{/* Using todo-list-item in TSX using differnt values each time */}
<todo-list-item isComplete={42} label={null} thingToDo={"Learn about any-typed props"}></todo-list-item>
<todo-list-item isComplete={"42"} label={1} thingToDo={"Learn about any-typed props"}></todo-list-item>

The following will rendered from the usage example above:

- isComplete has a value of - 42 - and a typeof value of "number" 
- label has a value of -  - and a typeof value of "object"
- thingToDo has a value of - Learn about any-typed props - and a typeof value of "string"

- isComplete has a value of - 42 - and a typeof value of "string"
- label has a value of - 1 - and a typeof value of "number"
- thingToDo has a value of - Learn about any-typed props - and a typeof value of "string"

In the first usage of todo-list-item, isComplete is provided a number value of 42, whereas in the second usage it receives a string containing "42". The types on isComplete reflect the type of the value it was provided, 'number' and 'string', respectively.

Looking at label, it is worth noting that although the prop has a default value, it does not narrow the type of label to be of type 'string'. In the first usage of todo-list-item, label is provided a value of null, whereas in the second usage it receives a number value of 1. The types of the values stored in label are correctly reported as 'object' and 'number', respectively.

Optional Types

TypeScript allows members to be marked optional by appending a ? at the end of the member's name. The example below demonstrates making each a component's props optional:

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    // completeMsg is optional, has an explicit type
    // annotation of `string`, and no default value
    @Prop() completeMsg?: string;
    // label is optional, has no explicit type
    // annotation, but does have a default value
    // of 'urgent'
    @Prop() label? = 'urgent';
    // thingToDo has no type annotation and no 
    // default value
    @Prop() thingToDo?;

    render() {
        return (
            <ul>
                <li>completeMsg has a value of - {this.completeMsg} - and a typeof value of "{typeof this.completeMsg}"</li>
                <li>label has a value of - {this.label} - and a typeof value of "{typeof this.label}"</li>
                <li>thingToDo has a value of - {this.thingToDo} - and a typeof value of "{typeof this.thingToDo}"</li>
            </ul>
        );
    }
}

When using a Stencil prop that is marked as optional, Stencil will try to infer the type of the prop if a type is not explicitly given. In the example above, Stencil is able to understand that:

completeMsg is of type string, because it has an explicit type annotation
label is of type string, because it has a default value that is of type string
thingToDo is of type any, because it has no explicit type annotation, nor default value

Because Stencil can infer the type of label, the following will fail to compile due to a type mismatch:

{/* This fails to compile with the error "Type 'number' is not assignable to type 'string'" for the label prop. */}
<todo-list-item completeMsg={"true"} label={42} thingToDo={"Learn about any-typed props"}></todo-list-item>

It is worth noting that when using a component in an HTML file, such type checking is unavailable. This is a constraint on HTML, where all values provided to attributes are of type string:

<!-- using todo-list-item in HTML -->
<todo-list-item complete-msg="42" label="null" thing-to-do="Learn about any-typed props"></todo-list-item>

renders:

- completeMsg has a value of - 42 - and a typeof value of "string"
- label has a value of - null - and a typeof value of "string"
- thingToDo has a value of - Learn about any-typed props - and a typeof value of "string"

Union Types

Stencil allows props types be union types, which allows you as the developer to combine two or more pre-existing types to create a new one. The example below shows a todo-list-item who accepts a isComplete prop that can be either a string or boolean.

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    @Prop() isComplete: string | boolean;
}

This component can be used in both HTML:

<todo-list-item is-complete="true"></todo-list-item>
<todo-list-item is-complete="false"></todo-list-item>
<todo-list-item is-complete></todo-list-item>
<todo-list-item></todo-list-item>

and TSX:

<todo-list-item isComplete={true}></todo-list-item>
<todo-list-item isComplete={false}></todo-list-item>

When using union types, the type of a component's @Prop() value can be ambiguous at runtime. In the provided example, under what circumstances does @Prop() isComplete function as a string, and when does it serve as a boolean?

When using a component in HTML, the runtime value of a @Prop() is a string whenever an attribute is set. This is a result of setting the HTML attribute for the custom element.

<!-- Since this is HTML, the value of `isComplete` in `ToDoListItem` will be a string -->

<!-- Set isComplete to "true". -->
<todo-list-item is-complete="true"></todo-list-item>
<!-- Set isComplete to "false" -->
<todo-list-item is-complete="false"></todo-list-item>
<!-- Set isComplete to "" -->
<todo-list-item is-complete></todo-list-item>

However, if an attribute is not specified, the runtime value of the property will be undefined:

<!-- Since `is-complete` is omitted, the value of `isComplete` in `ToDoListItem` will be `undefined` -->
<todo-list-item></todo-list-item>

When the attribute on a component is set using setAttribute, the runtime value of a @Prop() is always coerced to a string.

<script>
  // both of these `setAttribute` calls set the property `isComplete` to "true" (string)
  document.querySelector('todo-list-item').setAttribute('is-complete', true);
  document.querySelector('todo-list-item').setAttribute('is-complete', "true");
  // both of these `setAttribute` calls set the property `isComplete` to "false" (string)
  document.querySelector('todo-list-item').setAttribute('is-complete', false);
  document.querySelector('todo-list-item').setAttribute('is-complete', "false");
</script>

However, if the property of a custom element is directly changed, its type will match the value that was provided.

<script>
  // Set the property `isComplete` to `true` (boolean)
  document.querySelector('todo-list-item').isComplete = true;
  // Set the property `isComplete` to "true" (string)
  document.querySelector('todo-list-item').isComplete = "true";
  // Set the property `isComplete` to `false` (boolean)
  document.querySelector('todo-list-item').isComplete = false;
  // Set the property `isComplete` to "false" (string)
  document.querySelector('todo-list-item').isComplete = "false";
</script>

When using a component in TSX, a @Prop()'s type will match the value that was provided.

// Since this is TSX, the value of `isComplete` in `ToDoListItem`
// depends on the type of the value passed to the component.
//
// Set the property `isComplete` to `true` (boolean)
<todo-list-item isComplete={true}></todo-list-item>
// Set the property `isComplete` to "true" (string)
<todo-list-item isComplete={"true"}></todo-list-item>
// Set the property `isComplete` to `false` (boolean)
<todo-list-item isComplete={false}></todo-list-item>
// Set the property `isComplete` to "false" (string)
<todo-list-item isComplete={"false"}></todo-list-item>

Default Values

Stencil props can be given a default value as a fallback in the event a prop is not provided:

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

@Component({
    tag: 'component-with-some-props',
})
export class ComponentWithSomeProps {
    @Prop() aNumber = 42;
    @Prop() aString = 'defaultValue';

    render() {
        return <div>The number is {this.aNumber} and the string is {this.aString}</div>
    }
}

Regardless of if we use this component in HTML or TSX, "The number is 42 and the string is defaultValue" is displayed when no values are passed to our component:

<component-with-some-props></component-with-some-props>

The default values on a component can be overridden by specifying a value for a prop with a default value. For the example below, "The number is 7 and the string is defaultValue" is rendered. Note how the value provided to aNumber overrides the default value, but the default value of aString remains the same:

<component-with-some-props a-number="7"></component-with-some-props>

Inferring Types from Default Values

When a default value is provided, Stencil is able to infer the type of the prop from the default value:

import { Component, Prop, h } from '@stencil/core';
@Component({
    tag: 'component-with-many-props',
})
export class ComponentWithManyProps {
    // both props below are of type 'boolean'
    @Prop() boolean1: boolean;
    @Prop() boolean2 = true;

    // both props below are of type 'number'
    @Prop() number1: number;
    @Prop() number2 = 42;
    
    // both props below are of type 'string'
    @Prop() string1: string;
    @Prop() string2 = 'defaultValue';
}

Required Properties

By placing a ! after a prop name, Stencil mark that the attribute/property as required. This ensures that when the component is used in TSX, the property is used:

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

@Component({
   tag: 'todo-list-item',
})
export class ToDoListItem {
   // Note the '!' after the variable name.
   @Prop() thingToDo!: string;
}

Prop Validation

To do validation of a Prop, you can use the @Watch() decorator:

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

@Component({
   tag: 'todo-list-item',
})
export class TodoList {
  // Mark the prop as required, to make sure it is provided when we use `todo-list-item`.
  // We want stricter guarantees around the contents of the string, so we'll use `@Watch` to perform additional validation.
  @Prop() thingToDo!: string;

  @Watch('thingToDo')
  validateName(newValue: string, _oldValue: string) {
    // don't allow `thingToDo` to be the empty string  
    const isBlank = typeof newValue !== 'string' || newValue === '';
    if (isBlank) { 
        throw new Error('thingToDo is a required property and cannot be empty') 
    };
    // don't allow `thingToDo` to be a string with a length of 1
    const has2chars = typeof newValue === 'string' && newValue.length >= 2;
    if (!has2chars) {
       throw new Error('thingToDo must have a length of more than 1 character')
    };
  }
}

@Prop() Options

The @Prop() decorator accepts an optional argument to specify certain options to modify how a prop on a component behaves. @Prop()'s optional argument is an object literal containing one or more of the following fields:

export interface PropOptions {
  attribute?: string;
  mutable?: boolean;
  reflect?: boolean;
}

Attribute Name (`attribute`)

Properties and component attributes are strongly connected but not necessarily the same thing. While attributes are an HTML concept, properties are a JavaScript concept inherent to Object-Oriented Programming.

In Stencil, the @Prop() decorator applied to a property will instruct the Stencil compiler to also listen for changes in a DOM attribute.

Usually, the name of a property is the same as the attribute, but this is not always the case. Take the following component as example:

import { Component, Prop, h } from '@stencil/core';
// `MyHttpService` is an `Object` in this example
import { MyHttpService } from '../some/local/directory/MyHttpService';

@Component({
   tag: 'todo-list-item',
})
export class ToDoListItem {
   @Prop() isComplete: boolean;
   @Prop() thingToDo: string;
   @Prop() httpService: MyHttpService;
}

This component has 3 properties, but the compiler will create only 2 attributes: is-complete and thing-to-do.

<todo-list-item is-complete="false" thing-to-do="Read Attribute Naming Section of Stencil Docs"></todo-list-item>

Notice that the httpService type is not a primitive (e.g. not a number, boolean, or string). Since DOM attributes can only be strings, it does not make sense to have an associated DOM attribute called "http-service". Stencil will not attempt to serialize object-like strings written in HTML into a JavaScript object. See Object Props for guidance as to how to configure httpService.

At the same time, the isComplete & thingToDo properties follow 'camelCase' naming, but attributes are case-insensitive, so the attribute names will be is-complete & thing-to-do by default.

Fortunately, this "default" behavior can be changed using the attribute option of the @Prop() decorator:

import { Component, Prop, h } from '@stencil/core';
// `MyHttpService` is an `Object` in this example
import { MyHttpService } from '../some/local/directory/MyHttpService';

@Component({
   tag: 'todo-list-item',
})
export class ToDoListItem {
   @Prop({ attribute: 'complete' }) isComplete: boolean;
   @Prop({ attribute: 'thing' }) thingToDo: string;
   @Prop({ attribute: 'my-service' }) httpService: MyHttpService;
}

By using this option, we are being explicit about which properties have an associated DOM attribute and the name of it when using the component in HTML.

<todo-list-item complete="false" thing="Read Attribute Naming Section of Stencil Docs" my-service="{}"></todo-list-item>

Prop Mutability (`mutable`)

A Prop is by default immutable from inside the component logic. However, it's possible to explicitly allow a Prop to be mutated from inside the component, by declaring it as mutable, as in the example below:

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

@Component({
   tag: 'todo-list-item',
})
export class ToDoListItem {
   @Prop({ mutable: true }) thingToDo: string;

   componentDidLoad() {
      this.thingToDo = 'Ah! A new value!';
   }
}

Mutable Arrays and Objects

Stencil compares Props by reference in order to efficiently rerender components. Setting mutable: true on a Prop that is an object or array allows the reference to the Prop to change inside the component and trigger a render. It does not allow a mutable change to an existing object or array to trigger a render.

For example, to update an array Prop:

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

@Component({
   tag: 'my-component',
})
export class MyComponent {
   @Prop({mutable: true}) contents: string[] = [];
   timer: NodeJS.Timer;

   connectedCallback() {
      this.timer = setTimeout(() => {
         // this does not create a new array. when stencil
         // attempts to see if any of its Props have changed,
         // it sees the reference to its `contents` Prop is
         // the same, and will not trigger a render
         
         // this.contents.push('Stencil')
         
         // this does create a new array, and therefore a
         // new reference to the Prop. Stencil will pick up
         // this change and rerender
         this.contents = [...this.contents, 'Stencil'];
         // after 3 seconds, the component will re-render due
         // to the reference change in `this.contents`
      }, 3000);
   }

   disconnectedCallback() {
      if (this.timer) {
         clearTimeout(this.timer);
      }
   }

   render() {
      return <div>Hello, World! I'm {this.contents[0]}</div>;
   }
}

In the example above, updating the Prop in place using this.contents.push('Stencil') would have no effect. Stencil does not see the change to this.contents, since it looks at the reference of the Prop, and sees that it has not changed. This is done for performance reasons. If Stencil had to walk every slot of the array to determine if it changed, it would incur a performance hit. Rather, it is considered better for performance and more idiomatic to re-assign the Prop (in the example above, we use the spread operator).

The same holds for objects as well. Rather than mutating an existing object in-place, a new object should be created using the spread operator. This object will be different-by-reference and therefore will trigger a re-render:

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

export type MyContents = {name: string};

@Component({
   tag: 'my-component',
})
export class MyComponent {
   @Prop({mutable: true}) contents: MyContents;
   timer: NodeJS.Timer;

   connectedCallback() {
      this.timer = setTimeout(() => {
         // this does not create a new object. when stencil
         // attempts to see if any of its Props have changed,
         // it sees the reference to its `contents` Prop is
         // the same, and will not trigger a render

         // this.contents.name = 'Stencil';

         // this does create a new object, and therefore a
         // new reference to the Prop. Stencil will pick up
         // this change and rerender
         this.contents = {...this.contents, name: 'Stencil'};
         // after 3 seconds, the component will re-render due
         // to the reference change in `this.contents`
      }, 3000);
   }

   disconnectedCallback() {
      if (this.timer) {
         clearTimeout(this.timer);
      }
   }

   render() {
      return <div>Hello, World! I'm {this.contents.name}</div>;
   }
}

Reflect Properties Values to Attributes (`reflect`)

In some cases it may be useful to keep a Prop in sync with an attribute. In this case you can set the reflect option in the @Prop() decorator to true. When a prop is reflected, it will be rendered in the DOM as an HTML attribute.

Take the following component as example:

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

@Component({
    tag: 'todo-list-item',
})
export class ToDoListItem {
    @Prop({ reflect: false }) isComplete: boolean = false;
    @Prop({ reflect: true }) timesCompletedInPast: number = 2;
    @Prop({ reflect: true }) thingToDo: string = "Read Reflect Section of Stencil Docs";
}

The component in the example above uses default values, and can be used in HTML like so:

<!-- Example of using todo-list-item in HTML -->
<todo-list-item></todo-list-item>

When rendered in the DOM, the properties configured with reflect: true will be reflected in the DOM:

<todo-list-item times-completed-in-past="2" thing-to-do="Read Reflect Section of Stencil Docs" ></todo-list-item>

While the properties not set to "reflect", such as isComplete, are not rendered as attributes, it does not mean it's not there - the isComplete property still contains the false value as assigned:

const cmp = document.querySelector('todo-list-item');
console.log(cmp.isComplete); // it prints 'false'

The Prop Decorator (@Prop())​

Variable Casing​

Data Flow​

Mutability​

Types​

Boolean Props​

Number Props​

String Props​

Object Props​

Array Props​

Advanced Prop Types​

any Type​

Optional Types​

Union Types​

Default Values​

Inferring Types from Default Values​

Required Properties​

Prop Validation​

@Prop() Options​

Attribute Name (attribute)​

Prop Mutability (mutable)​

Mutable Arrays and Objects​

Reflect Properties Values to Attributes (reflect)​

Contents