Hyperapp V2 computed properties

computed.js

/*

Implements computed properties for Hyperapp#v2

It's not recomended approach to use 
unless you really know what you're doing

Usage examples:

// action is triggered whenever state.property updated*
const action = state => {
  // some compute code
}

app({
  init: {
    property: true
  }
  subscriptions: state => [
    computed(state, 'property')(action)
  ],
  //...
})

* please notice, you have to double Object.assign to invoke 'property' setter,
const propUpdate = (state, value) => 
  Object.assign({}, Object.assign(state, { property: value }))

*/

const effect = ({ target, prop, action }, dispatch) => {
  let value = target[prop]
  Object.defineProperty(target, prop, {
    get: () => value,
    set: update => {
      value = update
      setTimeout(() => dispatch([action, value]), 0)
    }
  })
  return () => {}
}

const computed = (target, prop) => action =>
  ({ target, prop, action, effect })