transactions.js

let users = []

// This could be our action, a generator
let createUser = function* (params) {
  // This is an optimistic update
  yield { id: 'cid0', ...params }
  
  // This simulates some async behavior to go do something
  yield Promise.resolve({ id: 0, name: params.name })
}

// This could be our store. It is a pure function
let addUser = function (users, params) {
  return users.concat(params)
}

function dispatch(state, params) {
  console.log(addUser(state, params))
}

function push(params) {
  // The same state is sent to all `yields`, so
  // optimistic updates will be blown away by later calls
  let state = users

  for (let body of createUser({ name: 'Bob' })) {
    if (typeof body.then === 'function') {
      body.then(result => dispatch(state, result))
    } else {
      dispatch(state, body)
    }
  }
  
}

push({ name: 'bob' })