Smoosh two object's methods together

method-compose.js

const { keys, assign } = Object

const concern = {
  doSideEffectyThing () {
    this.property1 = 'Hip, hip!'
  }
}

const instance = {
  someProp: 'Foo',
  doSideEffectyThing (name) {
    this.property2 = `Horay! ${ name }`
  }
}

const isFunc = (candidate) => (candidate instanceof Function)

const methodCompose = (concern, instance) => {
  return keys(concern).reduce((newInstance, prop) => {
    if (!isFunc(concern[prop])) {
      throw new Error('methodCompose() composes only methods')
    }
    if (!isFunc(instance[prop])) return newInstance
    
    newInstance[prop] = (...args) => {
      concern[prop].apply(newInstance, args)
      instance[prop].apply(newInstance, args) 
    }
    
    return newInstance
  }, assign({}, instance))
}

const another = methodCompose(concern, instance)

another.doSideEffectyThing('Me')

// [object Object] {
//   doSideEffectyThing () {
//     ... 
//   },
//   property1: "Hip, hip!",
//   property2: "Horay! Me",
//   someProp: "Foo"
// }