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

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>