How to overwrite part of <template> element in custom element inheritance?

Question

I have parent Custom Element, BasicSwitcher, which has its shadow dom html template:

const template_basic_switcher = document.createElement('template');
template_basic_switcher.innerHTML = `
<style>
    @import url("https://use.fontawesome.com/releases/v5.6.3/css/all.css")
</style>

<div id="controls-container">
    <span>ON</span>
    <span>OFF</span>
</div>
`;

Now I have another Custom Element, ModeSwitcher, inheriting from BasicSwitcher. It has completely different switches.

Is there a way to overwrite just the controls-container part of the template, while still utilizing other part? the element doesn't seem to support any kind of inheritance.

Answer 1

1) Create class for you Base Custom Element with a defaut render() method.

class BasicSwitcher extends HTMLElement {
    render() {
        this.shadowRoot.innerHTML = baseTemplate      
    }
}  

2) Extend the above class for your inherited Custum Element, and redefine the render() method and use the reference to the new template.

class ModeSwicher extends BasicSwitch () {
    render() {
        this.shadowRoot.innerHTML = otherTemplate      
    }
}  

Below is a working snippet:

class BasicSwitcher extends HTMLElement {
    constructor() {
        super()
        this.attachShadow( { mode: 'open' } )
        this.render()
    }
    render() {
        this.shadowRoot.innerHTML = BS.innerHTML
    }
}
customElements.define( 'basic-switch', BasicSwitcher ) 

class ModeSwitcher extends BasicSwitcher {
    render() {
        this.shadowRoot.innerHTML = MS.innerHTML
    }
}
customElements.define( 'mode-switch', ModeSwitcher )

<template id=BS>
  <style> :host { display: inline-block ; border: 1px solid gray } </style>
  <span>Basic Switch</span>
</template>

<template id=MS>
  <style> :host { display: inline-block ; border: 1px dotted blue ; color: red } </style>
  <span>Mode Switch</span>
</template>

<basic-switch></basic-switch>

<mode-switch></mode-switch>