Modern JS event handling mixin

handler.js

const Handler = Base => class extends Base {
  handleEvent (evt) {
    return this[`on${evt.type.replace(/^./, c => c.toUpperCase())}`](evt)
  }
}

Usage example:

class ExampleElement extends Handler(HTMLElement) {
  attachedCallback () {
    this.addEventListener('click', this)
  }
  detachedCallback () {
    this.removeEventListener('click', this)
  }
  onClick (evt) {
    console.log('clicked!')
  }
}