諾言如何與嵌套函數調用一起工作

Question

我有一段處理用戶數據的代碼。 有很多嵌套函數調用：

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

fn訪問一個數據庫，這意味着它是異步的，因此我以某種方式編寫了它以返回一個promise並且在fn-1 （調用fn的函數）中，我們使用.then()等待該諾言。 但是看起來現在我必須在fn-1返回一個promise，依此類推。 真的嗎 ？

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返回undefined ！ 看來addUser的調用者沒有等待它！

Answer 1

這是您在addUser函數中有效執行的操作

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

所以，是的，如果validationResult.isOK ，adduser將返回未定義

這是一些基於您的代碼的松散代碼，但是它可以獨立運行以演示您應該如何做

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);
});