What are Custom elements?

Custom elements are a Web Components standard that allows developers to define new HTML tags, extend existing ones, and create reusable web components. They enable the encapsulation of functionality into a custom HTML element, which can then be used declaratively in HTML pages or programmatically via JavaScript.

In Angular, custom elements are implemented using the @angular/elements package, which provides a bridge from Angular's component interface and change detection functionality to the built-in DOM API. This allows Angular components to be packaged as custom elements, making them usable in any HTML page, regardless of whether the page is using Angular or not.

Custom elements are defined by creating a class that extends HTMLElement and then registering that class with the browser's CustomElementRegistry using the customElements.define() method. Once registered, the custom element behaves like any other HTML element, with its own set of properties, methods, and event handling.

Angular simplifies the creation and registration of custom elements by providing the createCustomElement() function. This function takes an Angular component and an injector and returns a constructor that can be registered with the customElements.define() method. This allows the Angular component to be used as a custom element, with Angular handling data binding and change detection.

Here's a basic example of how to define a custom element in Angular:

import { Injector, createCustomElement } from '@angular/elements';
import { MyComponent } from './my.component';

// Obtain the injector
const injector: Injector = ...;

// Create a custom element for 'my-component'
co...