const getProp = prop => object => object[prop]
function KeyedCollection (Iterable, prop){
  if (!(this instanceof KeyedCollection)) {
    return new KeyedCollection(Iterable, prop);
  }
  if(!prop){
    throw new Error('must supply a prop')
  }
  const it_target = []
  const selectPropFrom = getProp(prop)
  //Functor.map(obj=>[getProp(prop)(obj),obj])
  for (item of Iterable){
    it_target.push([String(selectPropFrom(item)),item])
  }

  this.selectPropFrom = selectPropFrom
  this.prop = prop;
  this.map = new Map(
    it_target
  )
}

KeyedCollection.prototype.get = function(key){
  return this.map.get(String(key))//mutates!
}
KeyedCollection.prototype.has = function(key){
  return this.map.has(String(key))
}
KeyedCollection.prototype.size = function(){
  return this.map.size()
}
KeyedCollection.prototype.clear = function(){
  return this.map.clear()
}
KeyedCollection.prototype.delete = function(key){
  return this.map.delete(key)//mutates!
}

KeyedCollection.prototype[Symbol.iterator] = function* (){
  for (let [k,v] of this.map){
    yield v
  }
}

KeyedCollection.prototype.addFromArray = function(ArrayOfObjects){
  for (obj of ArrayOfObjects){
    this.map.set(this.selectPropFrom(obj),obj)
  }
  return this;
}

//another attempt to save an iteration.... [...KeyedCollection.toReact(Component)]
KeyedCollection.prototype.toReact = function *(Component){
  for (let [key,v] of this.map){
    yield Component(Object.assign(v,{key}))//is key getting assigned in the React way here?
    //have to actually try this out...
  }
}