Capturing events from slotted content in containing web component

Question

I have a simple web component that contains a <slot>. It handles form data and inside I have UI elements that emit data change/selected events. I'm wondering how the web component can react on events emitted from slot content. Something along these lines:

<my-form-handler>
     <my-player-selector player-id="master"></my-player-selector>
     <my-player-selector player-id="challenger"></my-player-selector>
     <my-weapons-selector default="sword"></my-weapons-selector>
</my-form-handler>

I know I could write <my-form-handler ondataSelected="someFunction"> presuming the selector elements emit dataSelected events. But that would require the code to live in the containing page instead of my-form-handler.

I'm not using any framework (Vue, Angular, React), just vanilla JS.

Answer 1

You can define in your custom element <my-form-handler> a handler in the constructor() or in the connectedCallback() method.

customElements.define( 'my-form-handler', class extends HTMLElement {
    conectedCallabck() {
        this.addEventListener( 'dataSelected', dsHandler )
   
        function dsHandler( ev ) {
             console.log( ev.type, ev.target.textContent )
        }
    }
} )

Of course you could use an arrow function if you want that this refers to the custom element object:

let dsHandler = ev => {
    console.log( ev.type, ev.target.textContent )
}
this.addEventListener( 'dataSelected', dsHandler )

You can also define the handler inline:

this.addEventListener( 'dataSelected', ev => 
    console.log( ev.type, ev.target.textContent )
)

NB: if you want the event handler to be a custom element method , you'll need to use bind(this) to be sure it can still access the custom element object.

customElements.define( 'my-form-handler', class extends HTMLElement {
    conectedCallabck() {
        this.addEventListener( 'dataSelected', this.dsHandler.bind( this ) )
    }
    dsHandler( ev ) {
        console.log( 'recever: %s / emitter: %', this.localName, ev.target.textContent )
    }
} )

Alternately, you can opt to a centralized solution with the help of the handleEvent() interface:

1 Set the custom element itself as the listener:

this.addEventListener( 'dataSelected', this )

2 Implement the handleEvent() method:

handleEvent( ev ) {
    switch( ev.type ) {
        case 'dataSelected':
            console.log( 'recever: %s / emitter: %', this.localName, ev.target.textContent )
            break
   }
}
   

Below a running snippet:

customElements.define( 'my-form-handler', class extends HTMLElement {
    constructor() {
        super()
        this.addEventListener( 'dataSelected', this )
    }
    handleEvent( ev ) {
        console.log( ev.type, ev.target.textContent )
    }
} )

customElements.define( 'my-player-selector', class extends HTMLElement {
    constructor() {
        super()   
        this.addEventListener( 'click', () =>
          this.dispatchEvent( new CustomEvent( 'dataSelected', { bubbles: true } ) )
        )
    }    
} )

<my-form-handler>
     <my-player-selector player-id="master">master</my-player-selector>
     <my-player-selector player-id="challenger">challenger</my-player-selector>
     <my-weapons-selector default="">sword</my-weapons-selector>
</my-form-handler>