Helper to extend a ES6 class from multiple source classes

example.js

class A = {
  constructor() {
    this.A = true
  }
}

class B = {
  constructor() {
    this.B = true
  }
}

class C extends B = {
  constructor(...args) {
    super(...args)
    multiExtend(this, A, args)
  }
}

MultiExtend.js

const multiExtend = (targetInstance, SourceClass, ...args) => {
    const sourceInstance = new SourceClass(...args)
    const sourceDescriptors = Reflect.ownKeys(SourceClass)
        .filter(key => !['constructor', 'name'].includes(key))
        .reduce((acc, key) => ([...acc, { key, descriptor: Object.getOwnPropertyDescriptor(SourceClass, key) }]), [])
    const sourceInstanceDescriptors = Reflect.ownKeys(sourceInstance)
        .reduce((acc, key) => ([...acc, { key, descriptor: Object.getOwnPropertyDescriptor(sourceInstance, key) }]), [])

    sourceDescriptors.concat(sourceInstanceDescriptors).forEach(({ key, descriptor }) => {
        Object.defineProperty(targetInstance, key, descriptor)
    })
}