callback-promise-promise_all.js

db.accounts.findById(id, function(error, user) {
  //....AAAA
  db.accounts.useUsertoDo(user, function(error, something) {
    // use something to do something
  });
});

//------------------------------

var promise1 = db.accounts.findById(id);
var promise2 = user => {
  return db.accounts.useUsertoDo(user);
};

promise1
.then(user => { 
  //....AAAA
  return promise2(user);
})
.then(something => {
  // use something to do something
})
.catch(error => {});

//------------------------------

var promises = [];
promises.push(db.accounts.findById(id));
promises.push(db.accounts.findByName1(name));
promises.push(db.accounts.findByName2(name));

var promiseLast = (id, name1, name2) => {
  return db.accounts.compareNameAndId(id, name1, name2);
};

Promise.all(promises)
.then(values => {
  var id = values[0];
  var name1 = values[1];
  var name2 = values[2];
  return promiseLast(id, name1, name2);
})
.then(something => {})
.catch(error => {});

// ---------------------

function findById(_id, cb) {
  //....
  if (error) {
    cb(error, null);
  } else {
    cb(null, data);
  }
}

// ----------------------

function findById(_id) {
  return new Promise(function(resolve, reject) {
    //....
    if (error) {
        reject(error);
    } else {
        resolve(data);
    }
  });
}