Node v7.5.0 Released

    return function().then(() => ...).then(() => ...)

    function foo(cb) {
        async.waterfall([
            function(){},
            function(){},
        ])
    }

    function foo2() {
        return function()
            .then(function(){})
            .then(function(){});
    }

Another simple to explain benefits are the fact that you can .then() a promise 10 times and what it does will only happen once, and then the other times you'll just consume the result. You don't need to care if something is (a) already computed, (b) in the process of being computed, (c) scheduled for computation later etc., you can write the exact same code for all cases. Because you're working with values that have dependencies between this, not with processes that need to happen in a certain order.

The bigger benefits show up when you write more and more code and you see how beautifully it all untangles with promises.

And the fact that once you learn promises you can carry further the knwledge to other async patterns like co and async/await (they are all understandable in terms of promises).

Do yourself and everyone you work with a favor and move away from async please, especially in a team promises make everything so much easier to understand! Same advice of you're writing nodejs libraries: have you functions return promises instead of take callbacks! Drop the intellectual lazyness and grow beyond "async by callbacks", it's the worst coding style ever. And everyone will be able to convert almost instantly from promises to async/await since the logic is similar, just syntactic sugar comes over it.

    await asyncFuncOne();
    await asyncFuncTwo();

1. Promises are a value. They can be passed to other functions. This simplifies a lot of control flow.

2. The `.then` method on a promise is similar to `flatMap` in other languages / libs. The next chain will wait for any nested promises to complete. This flattens callbacks so you can see the flow instead of the pyramid of doom.

3. Native promises include a bit of sugar, such as Promise.all. You need a library to do this with callbacks.

4. Promises are the backbone of async/await. You need them to use it.

Example:

    await const [a, b] = Promise.all([getA(), getB()]);

Controller calls the service.

Service returns a call to a repo with any mapping that needs to occur.

Repo calls out to database or external api and returns the promise.

Makes the work very boring and repetitive for the most part which is a good thing I think.

    function someController(req, res) {
        UserService.getUser(req.session.user)
            .then(SomeService.getSomething)
            .then((val) => res.json(val))
            .catch(handleError);
    }

    function getSomething(user) {
        return SomeRepo.getSomething(user)
            .then(MapSomething)
            .catch(alternativeHigherLevelCatch);
    }

    function getSomething(user) {
        return new Promise((resolve, reject) => {
            database.getSomething(
                'defined parameters',
                user,
                resolve,
                reject
            )
        });
    }

The second one is that you can await a promise, which turns your 1-level-deep code to 0 levels deep.

doSomething.then(stepTwo).then(stepThree).catch(errHandler);

  async function Promise_auto(tasks) {
      const keys = Object.keys(tasks);
      const results = { };
      const taskPromises = { };
      function runTask(key) {
          if (!taskPromises[key]) {
              taskPromises[key] = (async () => {
                  let fn = tasks[key];
                  if (fn instanceof Array) {
                      const deps = fn.slice(0, -1);
                      fn = fn.slice(-1)[0];
                      await Promise.all(deps.map(runTask));
                  }
                  results[key] = await fn(results);
              })();
          }
          return taskPromises[key];
      }
      await Promise.all(keys.map(runTask));
      return results;
  }

  (async () => {
      const start = new Date();
      const results = await Promise_auto({
          write_file: ['get_data', 'make_folder', async (results) => {
              console.log('in write_file', results);
              await Promise.delay(1000);
              return 'filename';
          }],
          email_link: ['write_file', async (results) => {
              console.log('in email_link', results);
              await Promise.delay(1000);
              return {file: results.write_file, email: 'user@example.com'};
          }],
          get_data: async () => {
              console.log('in get_data');
              await Promise.delay(1000);
              return [ 'data', 'converted to array' ];
          },
          make_folder: async () => {
              console.log('in make_folder');
              await Promise.delay(900);
              return 'folder';
          },
      });
      console.log('results = ', results);
      console.log(`It took ${(new Date() - start) / 1000} seconds`);
  })();

  function test(name1, name2) {
    let user1 = User.find(name1)
    let user2 = User.find(name2)
    let post1 = user1.then(u => Posts.getLast(u.id))
    let post2 = user2.then(u => Posts.getLast(u.id))
    let comparison = Promise.all([post1, post2])
      .then(([p1, p2]) => DiffService.compare(p1, p2))
    let email1 = Promise.all([user1, comparison])
      .then(([u1, c]) => Email.send(comparison.text(), u1.email))
    let email2 = Promise.all([user2, comparison])
      .then(([u2, c]) => Email.send(comparison.text() ,u2.email))
    return Promise.all([email1, email2])
  }

    a()
    .then(()=>b())
    .then(()=>c())

    a():
    b();
    c();

So promisifying a callback-based API does address it by either letting you use a yield/await abstraction or at the very least promise composition.

Wrapping a callback-based API is such a mechanical process that you can wrap entire modules at a time, so I'm not sure what's hellish about that when it gives you promises, something you can actually work with.