Coroutines in Typescript

coroutine.ts

type GeneratorFunction<T> = () => Generator<any, T, any>

export default function<T>(g: GeneratorFunction<T>): Promise<T> {
  return new Promise((res, rej) => {
    // create generator
    const r = g()

    function step(green: boolean, fulfilled: any) {
      let result: IteratorResult<any, T> | null

      try {
        // resume with fulfilled value
        result = green ? r.next(fulfilled) : r.throw(fulfilled)
      } catch (err) {
        // reject with unhandled error in coroutine
        return rej(err)
      }

      const { value } = result

      // convert result value to Promise
      const p = value != null && value.constructor === Array
        ? Promise.all(value)
        : Promise.resolve(value)

      // if done, fulfill final value
      if (result.done) return p.then(res, rej)

      // otherwise continue
      p.then(
        x => step(true, x),
        err => step(false, err)
      )
    }

    step(true, null)
  })
}