Reactive Data

Stencil components update when props or state on a component change. For performance and simplicity, Stencil only compares references for changes, and will not re-render when data inside of an array or object changes.

Rendering methods

When a component updates because a state change (props or state change), the render() method is scheduled to run.

Watch Decorator

When a user updates a property, Watch will fire what ever method it's attached to and pass that method the new value of the prop along with the old value. Watch is useful for validating props or handling side effects. Watch decorator does not fire when a component initially loads.

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

export class LoadingIndicator {
  @Prop() activated: boolean;

  watchHandler(newValue: boolean, oldValue: boolean) {
    console.log('The new value of activated is: ', newValue);

Handling arrays and objects

To update array or object data, use the following techniques which are fast-becoming a core part of the modern JavaScript toolbox.

Updating arrays

For arrays, this means that the standard mutable array operations such as push() and unshift() won't work. Instead, non-mutable array operators such as map() and filter(), and the ES6 spread operator syntax, should be used as they return a copy of a new array.

For example, to push a new item to an array, create a new array with the existing values and the new value at the end:

// our original array
this.items = ['ionic', 'stencil', 'webcomponents'];

// update the array
this.items = [

The ...this.items syntax is a relatively new feature of JavaScript that "expands" the given object in place. Read more about the Spread operator here.

Updating an object

The spread operator should also be used to update objects. As with arrays, mutating an object will not trigger a view update in Stencil, but returning a new copy of the object will. Below is an example:

// our original object
let myCoolObject = {first: '1', second: '2'}

// update our object
myCoolObject = { ...myCoolObject, third: '3' }