Async generator from generator

example.js

const makeAsyncGenerator = require('./make-async-generator')

const f = makeAsyncGenerator(function * () {
  yield 'foo'
  const value = yield Promise.resolve('bar') // yield promises to await them
  yield value
  try {
    yield Promise.reject('baz')
  } catch (value) {
    yield value
  }
})

for await (const value of f()) {
  console.log(value)
}

make-async-generator.js

'use strict'

function step (key, value) {
  try {
    var cursor = this.iterator[key](value)
  } catch (error) {
    return Promise.reject(error)
  }
  var then
  return cursor.done || (value = cursor.value) == null || typeof (then = value.then) !== 'function'
    ? Promise.resolve(cursor)
    : then.call(value, this.next, this.throw)
}

function AsyncIterator (syncIterator) {
  this.iterator = syncIterator
  this.next = step.bind(this, 'next')
  this.return = step.bind(this, 'return')
  this.throw = step.bind(this, 'throw')
}
const $$asyncIterator = (typeof Symbol === 'function' && Symbol.asyncIterator) || '@@asyncIterator'
AsyncIterator.prototype[$$asyncIterator] = function () {
  return this
}

module.exports = function makeAsyncGenerator (generator) {
  return function () {
    return new AsyncIterator(generator.apply(this, arguments))
  }
}