async-await-as-generator.js

import fetch from "isomorphic-fetch";

function getUser(userId) {
  return fetch(`https://jsonplaceholder.typicode.com/users/${userId}`)
    .then((res) => {
      if (!res.ok) {
        throw new Error(res.statusText);
      }
      return res;
    })
    .then((res) => res.json());
}

function throwError() {
  return Promise.reject(new Error("Oops"));
}

// ----- Async/await version

async function main() {
  const user1 = await getUser(1);
  const user2 = await getUser(2);
  // const user2 = await throwError();
  const user3 = await { name: "John Lewis" };
  return user1.name + ", " + user2.name + ", " + user3.name;
}

const resultAsync = main();
resultAsync
  .then(console.log)
  .catch((err) => console.log("Error from async", err));

// ----- Coroutine

function co(genFn, ...args) {
  const it = genFn(...args);

  function handleIteratorResult({ value, done }) {
    if (done) {
      return Promise.resolve(value);
    }

    if (value && value.then) {
      return value
        .then((val) => handleIteratorResult(it.next(val)))
        .catch((err) => handleIteratorResult(it.throw(err)));
    } else {
      return Promise.resolve(it.next(value)).then(handleIteratorResult);
    }
  }

  return handleIteratorResult(it.next());
}

// ----- Generator version

function* mainGen(val) {
  console.log(val);
  const user1 = yield getUser(1);
  const user2 = yield getUser(1);
  // const user2 = yield throwError(2);
  const user3 = yield { name: "John Lewis" };
  return user1.name + ", " + user2.name + ", " + user3.name;
}

const resultGenerator = co(mainGen, 42);
resultGenerator
  .then(console.log)
  .catch((err) => console.log("Error from generator", err));