Simple co implementation that works with only promises. It Let's you write sync code using generators and promises.

co-simple.js

function co(genInstance) {
  const success = v => step(genInstance, {value: v, failed: false})
  const failure = err => step(genInstance, {error: err, failed: true})

  if ('function' === typeof genInstance) genInstance = genInstance()

  // kick start generator
  const retVal = genInstance.next()
  if (retVal.done) return retVal.value
  else return retVal.value.then(success, failure)

  function step(g, o) {
    let retVal

    if (!o.failed) retVal = g.next(o.value)
    else retVal = g.throw(o.error)

    if (retVal.done) return retVal.value
    else retVal.value.then(success, failure)
  }
}

test1.js

function *a() {
  var v = yield new Promise((r) => setTimeout(() => {
  	console.log('resolving promise 1 with "dude"')
    r('dude')
  }, 2000))
  try {
    var a = yield new Promise(() => {
      var msg = `last value was ${v}`
      console.log('rejecting promise with error. Msg is "', msg, '"')
	    throw new Error(msg)
    })
  } catch(e) {
    console.error(e)
    console.log(`a is ${a}. It should be undefined`)
    var p = yield Promise.resolve('man')
  }
  console.log('p is', p)
  try {
    var r = yield Promise.reject('i am a rejected promise')
    console.log('this will never be reached')
  } catch (e) {
    console.error(e)
  }
  var m = yield new Promise(r => setTimeout(() => r(8888), 4000))
  console.log(`m is ${m}`)
}

test2.js

function *a() {}

test3.js

function *b() {
  return yield Promise.resolve(10)
}