How promise works with nested function calls

Question

I have a piece of code which deals with user's data. There are bunch of nested function calls :

f1(){
   f2(){
     ....
      fn{
        ///
      }
   }
}

fn accesses a database which means it's asynchronous, so I wrote it somehow that it returns a promise and in fn-1 (the function which calls fn) , we use .then() to wait for this promise. But it looks like now I have to return a promise in fn-1 and so on. Is that true ?

var keyValueExists = function(key, value) {
    var query = {};
    query[key] = value;
    return new Promise(function(resolve, reject) {
        User.count(query, function(err, count) {
            if (err) {
                console.log(err);
                console.log('Problem with `.find` function');
                reject('Problem with `.find` function');
            } else {
                resolve(count !== 0);
            }
        });
    });
};

var addUser = function(newUserInfo) {
    var validationResult = Common._validateUserInfo(newUserInfo);
    if (validationResult.isOK) {
        keyValueExists('userName', newUserInfo.userName).then(function(userNameAlreadyExists) {
            if (userNameAlreadyExists) {
                validationResult = {
                    isOK: false,
                    reason: 'Username already exists',
                    infoWithBadInput: 'userName'
                }
            } else {
                var newUserId = generateUserId();
                //TODO: change it somehting more flexible. e.g. a predefined list of attributes to iterate over
                var newUser = {
                    'userName': newUserInfo.userName,
                    'password': newUserInfo.password,
                    'userId': newUserId,
                    'lastModificationTime': Common.getCurrentFormanttedTime(),
                    'createdTime': Common.getCurrentFormanttedTime()
                };
                var user = new User(newUser);
                user.save(function(err) {
                    if (err) {
                        console.log(err);
                        console.log('There is a problem saving the user info');
                    } else {
                        console.log('A new user added: ');
                        console.log(newUser);
                    }
                });
            }
            return validationResult;
        });
    } else {
        return validationResult;
    }
};

addUser returns undefined ! It looks like that the caller of addUser doesn't wait for it !

Answer 1

This is what you are effectively doing in your addUser function

var addUser = function(newUserInfo) {
    var validationResult = Common._validateUserInfo(newUserInfo);
    if (validationResult.isOK) {
        // ... do something asynchronously without returning anything
    } else {
        return validationResult;
    }
}

So, yeah, if validationResult.isOK, adduser WILL return undefined

Here's some code loosely based on your code, but it runs standalone to demonstrate how you possibly should be doing things

var keyValueExists = function(key, value) {
    // pseudo junk, this simulates any username starting with b as existing
    return new Promise(function(resolve, reject) {
        resolve(value.substr(0,1) == 'b'); // barny and betty are dupes, fred and wilma are not
    });
}

var addUser = function (newUserInfo) {
    // var validationResult = Common._validateUserInfo(newUserInfo);
    var validationResult = {isOK: !!~(['fred', 'barny'].indexOf(newUserInfo.userName)), username: newUserInfo.userName}; // dummy code
    if (validationResult.isOK) {
        return keyValueExists('userName', newUserInfo.userName).then(function (userNameAlreadyExists) {
            if (userNameAlreadyExists) {
                validationResult = {
                    isOK: false,
                    reason: 'Username already exists',
                    infoWithBadInput: 'userName',
                    username: newUserInfo.userName
                }
            } else {
                // create new user here
                validationResult.userNumber = (Math.random() * 100000000) | 0;
            }
            return validationResult;
        });
    }
    else {
        // this function always needs to return a promise, even if it is resolved/rejected immediately
        return Promise.reject(validationResult);
    }
}

addUser({userName: 'fred'}).then(function (result) {
    console.log(result);
}).catch(function(err) {
    console.error(err);
});

addUser({userName: 'wilma'}).then(function (result) {
    console.log(result);
}).catch(function(err) {
    console.error(err);
});

addUser({userName: 'barny'}).then(function (result) {
    console.log(result);
}).catch(function(err) {
    console.error(err);
});

addUser({userName: 'betty'}).then(function (result) {
    console.log(result);
}).catch(function(err) {
    console.error(err);
});