tech.js

fs.writeFile('/tmp/a.txt', 'data', function(err, result) => {
  fs.writeFile('/tmp/b.txt', 'data', function(err, result) => {
    fs.writeFile('/tmp/c.txt', 'data', function(err, result) => {
      fs.writeFile('/tmp/d.txt', 'data', function(err, result) => {
        console.log('exito!');
      });
    });
  });
});

// 1.js
async.map([
  {path: 'tmp/a.txt', data: data}
], (info, cb) => fs.writeFile(info.path, info.data, cb), () => {
  console.log('exito!');
});



function listar() {
  return Promise.resolve(true);
}

function preController(req, res, next) {
  if (req.body.hello) {
    req.hello = true;
    next();
  } else {
    next(new Error('Hello is missing'));
  }
}

function controller(req, res, next) {  
  listar()
    .then((result) => res.json(result))
    .catch((err) => next(err));
}

function controller(req, res, next) {
  listar((err, result) => {
    if (err) {
      next(err);
    } else {
      res.json(result);
    }
  });
}

function errorHandler1(err, req, res, next) {
  if (err.isMongoose) {
    // 
  } else {
    next(err);
  }
  
}

function errorHandler2(err, req, res, next) {  
  res.status(500).send(err);
}

app.use(errorHandler1);
app.use(errorHandler2);

listar()
  .then((result) => console.log('result', result))
  .catch((err) => console.error('result', err));

listar((err, result) => {
  if (err) {
    console.error(err);
  } else {
    console.log('result', result)
  }
});