Angular Elements

Code reuse is a very important feature that all developers seek to implement. We always want to write a functionality once and use it reuse it when the need arises. By reusing code, developers can drastically cut development and maintenance time for software projects. This is most times achieved by creating custom elements.

Angular elements are Angular components packaged as custom elements, a web standard for defining new HTML elements in a framework-agnostic way.

Custom elements are a Web Platform feature currently supported by Chrome, Firefox, Opera, and Safari, and available in other browsers through polyfills. A custom element extends HTML by allowing you to define a tag whose content is created and controlled by JavaScript code. The browser maintains a CustomElementRegistry of defined custom elements (also called Web Components), which maps an instantiable JavaScript class to an HTML tag.

The @angular/elements package exports a createCustomElement() API that provides a bridge from Angular's component interface and change detection functionality to the built-in DOM API.

Transforming a component to a custom element makes all of the required Angular infrastructure available to the browser. Creating a custom element is simple and straightforward, and automatically connects your component-defined view with change detection and data binding, mapping Angular functionality to the corresponding native HTML equivalents.

Using custom elements

Custom elements bootstrap themselves - they start automatically when they are added to the DOM, and are automatically destroyed when removed from the DOM. Once a custom element is added to the DOM for any page, it looks and behaves like any other HTML element, and does not require any special knowledge of Angular terms or usage conventions.

Easy dynamic content in an Angular app

Transforming a component to a custom element provides an easy path to creating dynamic HTML content in your Angular app. HTML content that you add directly to the DOM in an Angular app is normally displayed without Angular processing, unless you define a dynamic component, adding your own code to connect the HTML tag to your app data and participate in change detection. With a custom element, all of that wiring is taken care of automatically.

Content-rich applications

If you have a content-rich app, custom elements let you give your content providers sophisticated Angular functionality without requiring knowledge of Angular.

How it works

Use the createCustomElement() function to convert a component into a class that can be registered with the browser as a custom element. After you register your configured class with the browser's custom-element registry, you can use the new element just like a built-in HTML element in content that you add directly into the DOM:

TypeScript Code:

`<my-popup message="Use Angular!"></my-popup>`

When your custom element is placed on a page, the browser creates an instance of the registered class and adds it to the DOM. The content is provided by the component's template, which uses Angular template syntax, and is rendered using the component and DOM data. Input properties in the component corresponding to input attributes for the element.

Transforming components to custom elements

Angular provides the createCustomElement() function for converting an Angular component, together with its dependencies, to a custom element. The function collects the component's observable properties, along with the Angular functionality the browser needs to create and destroy instances and to detect and respond to changes.

The conversion process implements the NgElementConstructor interface and creates a constructor class that is configured to produce a self-bootstrapping instance of your component.

Use a JavaScript function, customElements.define(), to register the configured constructor and its associated custom-element tag with the browser's CustomElementRegistry. When the browser encounters the tag for the registered element, it uses the constructor to create a custom-element instance.

Mapping

A custom element hosts an Angular component, providing a bridge between the data and logic defined in the component and standard DOM APIs. Component properties and logic maps directly into HTML attributes and the browser's event system.

The creation API parses the component looking for input properties and defines corresponding attributes for the custom element. It transforms the property names to make them compatible with custom elements, which do not recognize case distinctions. The resulting attribute names use dash-separated lowercase. For example, for a component with @Input('myInputProp') inputProp, the corresponding custom element defines an attribute my-input-prop.

Component outputs are dispatched as HTML Custom Events, with the name of the custom event matching the output name. For example, for a component with @Output() valueChanged = new EventEmitter(), the corresponding custom element will dispatch events with the name "valueChanged", and the emitted data will be stored on the event’s detail property. If you provide an alias, that value is used; for example, @Output('myClick') clicks = newEventEmitter<string>(); results in dispatch events with the name "myClick".

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