State Machine with Async Transitions

index.js

// Async Generator for implementing State Machines with asynchronous transitions.
// 
// Try online: https://jsfiddle.net/ecf1bho9/

console.clear()

async function* state() {
  let state = 0
  let action
  while (true) {
    action = yield state
    
    switch (action) {
        
      case 'ADD':
        state++
        break
        
      case 'SUB':
        state--
        break
        
      case 'RESET':
        state = 0
        break
        
      case 'FETCH':
        const res = await fetch('https://jsonplaceholder.typicode.com/posts')
        const data = await res.json()
        state = data.length
        break
        
    }
  }
}

const get = state()

get.next().then(({ value }) => value).then(console.log)         // 0
get.next('FETCH').then(({ value }) => value).then(console.log)  // 100
get.next('ADD').then(({ value }) => value).then(console.log)    // 101
get.next('ADD').then(({ value }) => value).then(console.log)    // 102
get.next('RESET').then(({ value }) => value).then(console.log)  // 0
get.next().then(({ value }) => value).then(console.log);        // 0