Coroutines and generators in JavaScript

coroutines-and-generators.js

function* delays() {
  let a = yield delay(800, "Hello, I'm an");
  console.log(a);

  let b = yield delay(400, "async coroutine!");
  console.log(b);
}

const coroutine = nextValue => iterator => {
  const { done, value } = iterator.next(nextValue);

  if (done) {
    return;
  }

  if (value.constructor === Promise) {
    value.then(promiseValue => {
      coroutine(promiseValue)(iterator);
    });
  } else {
    coroutine(value)(iterator);
  }
};

const delay = (ms, result) =>
  new Promise(resolve => setTimeout(() => resolve(result), ms));

coroutine()(delays());