Benefits from async await vs return promise

Question

I'm just trying to understand the benefits of this:

const populateUsers = done => {
  User.remove({}).then(async () => {
    const userOne = new User(users[0]).save();
    const userTwo = new User(users[1]).save();
    const usersProm = await Promise.all([userOne, userTwo]).then(() => done());
    return usersProm;
  });
};

over this:

const populateUsers = done => {
  User.remove({})
    .then(() => {
      const userOne = new User(users[0]).save();
      const userTwo = new User(users[1]).save();

      return Promise.all([userOne, userTwo]);
    })
    .then(() => done());
};

I came to this problem because eslint suggested my to use async in this function, and I remember the concept, make it work in my app, but I'm not sure why should I use this instead of the original way

Answer 1

Your first version does not go all the way . Do this:

const populateUsers = done => {
  User.remove({}).then(async () => {
    const userOne = new User(users[0]).save();
    const userTwo = new User(users[1]).save();
    await Promise.all([userOne, userTwo]);
    const usersProm = await done();
    return usersProm;
  });
};

There is no difference, it is just that code without these then callbacks is somewhat easier to read.

You might even apply it to the outer function:

const populateUsers = async () => {
  await User.remove({});
  const userOne = new User(users[0]).save();
  const userTwo = new User(users[1]).save();
  await Promise.all([userOne, userTwo]);
  const usersProm = await done();
  return usersProm;
};

Now populateUsers returns the promise instead of undefined .

As concluded in comments: you get an error because populateUsers returns a promise and accepts a done callback argument, while one of these is expected, not both.

Answer 2

Your original code was totally fine.

No, there is no benefit in using the code from your first snippet. You should avoid mixing await and .then(…) syntax ! To use async / await , you'd make the whole function async , not the then callback:

async function populateUsers(done) {
  await User.remove({})
  const userOne = new User(users[0]).save();
  const userTwo = new User(users[1]).save();

  await Promise.all([userOne, userTwo]);
  return done();
}

(Probably you would also remove that done callback - the function already returns a promise)