41 async-await style in generator

co.js

function apiReq(result, delay) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(result)
    }, delay*1000)
  })
}

// the generator function version of async-await programming scheme
// function* -> async
// yield -> await
function* routine() {
  console.log('a')
  const a = yield apiReq(5, 1)
  console.log('b')
  const b = yield apiReq(4 + a, 2)
  console.log('c')
  const c = yield apiReq(3 + b, 1)
  console.log('result')
  return c
}

// co - the function runner that can follow the promise chain yielded, and execute async procedures in sequence
function co(generator) {
  // run once
  function tick(curValue) {
    const res = generator.next(curValue) // the parameter will become result of a yield expression
    let { value, done } = res
    if (!(value instanceof Promise)) {
      value = Promise.resolve(value)
    }

    return value.then((resolvedValue) => {
      if (!done) {
        return tick(resolvedValue)
      } else {
        return resolvedValue
      }
    })
  }
  return tick(undefined)
}

const result = co(routine())
result.then(res => {
  console.log(res)
})