.then 的 Promise.all 没有执行

Question

我正在使用 Firestore 检索具有以下 DS 的数据。
我有一个 Company 集合，其中包含一个子集合 Branches
所以我试图检索列出所有 Companies 及其 Branches

代码：

exports.findAll = function (req, res) {
    getcompanies().
    then((companies) => {
        console.log("Main "+ companies) // info: Main TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined
        return res.json(companies);
    })
    .catch((err) => {
        console.log('Error getting documents', err);
    });
}

function getCompanies(){
    var companiesRef = db.collection('companies');

    return companiesRef.get()
    .then((snapshot) => {
        let companies = [];
        return Promise.all(
            snapshot.forEach(doc => {  
                    let company = {};                
                    company.id = doc.id;
                    company.company = doc.data(); 
                    var branchesPromise = getBranchesForCompanyById(company.id);
                    return branchesPromise.then((branches) => {                    
                            company.branches = branches;
                            companies.push(company); 
                            if(snapshot.size === companies.length){
                                console.log("companies - Inside" + JSON.stringify(companies)); //This prints all companies with its branches
                            }
                            return Promise.resolve(companies);
                        })
                        .catch(err => {
                            console.log("Error getting sub-collection documents", err);
                            return Promise.reject(err);
                        }) 
            })
        )
        .then(companies => {
            console.log("Outside " + companies) // This is never executed 
            return companies;
        })
        .catch(err => {
            return err;
        });

    })
    .catch(err => {
        return err;
    });
}

function getBranchesForCompanyById(id){
    var branchesRef = db.collection('companies').doc(id).collection('branches');
    let branches = [];
    return branchesRef.get()
     .then(snapshot => {
        snapshot.forEach(brnch => {
            let branch = {};
            branch.id = brnch.id;
            branch.branch = brnch.data();
            branches.push(branch);
        })
        return branches;
    })
    .catch(err => {
        return err;
    })

 }

我现在已经有了所有需要的数据。

console.log("companies - Inside" + JSON.stringify(companies)); //This prints all companies with its branches

但是 Promise.all 的 then 永远不会被执行。所以得到这个错误 -

info: Main TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined

console.log("Main "+ companies) // info: Main TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined

我觉得我已经遵循了这里指定的所有规则：https://stackoverflow.com/a/31414472/2114024 关于嵌套承诺，不知道我在哪里漏掉了重点。
提前致谢！

Answer 1

根据 Evert 和 Rahul 的意见，感谢你们两位，我已经解决了这里的问题。

1. 我处理了 catch 块中的所有错误
2. Promise.all 没有返回任何内容，因此我将 forEach 转换为 map。

所以这是我更新的代码，它解决了问题：

exports.findAll = function (req, res) {
    getcompanies().
        then((companies) => {
            console.log("Main " + companies) // Prints all companies with its branches
            return res.json(companies);
        })
        .catch((err) => {
            console.log('Error getting documents', err);
            return res.status(500).json({ message: "Error getting the all companies" + err });
        });
}

function getCompanies() {
    var companiesRef = db.collection('companies');

    return companiesRef.get()
        .then((snapshot) => {
            let companies = [];
            return Promise.all(
                snapshot.docs.map(doc => {
                    let company = {};
                    company.id = doc.id;
                    company.company = doc.data();
                    var branchesPromise = getBranchesForCompanyById(company.id);
                    return branchesPromise.then((branches) => {
                        company.branches = branches;
                        companies.push(company);
                        if (snapshot.size === companies.length) {
                            console.log("companies - Inside" + JSON.stringify(companies));
                            return companies;
                        }
                    })
                        .catch(err => {
                            console.log("Error getting sub-collection documents", err);
                            throw new Error(err);
                        })
                })
            )
                .then(companies => {
                    console.log("Outside " + companies); // Executed now
                    return companies[companies.length - 1];
                })
                .catch(err => {
                    throw new Error(err);
                });

        })
        .catch(err => {
            throw new Error(err);
        });
}

function getBranchesForCompanyById(id) {
    var branchesRef = db.collection('companies').doc(id).collection('branches');
    let branches = [];
    return branchesRef.get()
        .then(snapshot => {
            snapshot.forEach(brnch => {
                let branch = {};
                branch.id = brnch.id;
                branch.branch = brnch.data();
                branches.push(branch);
            })
            return branches;
        })
        .catch(err => {
            throw new Error(err);
        })

}

Answer 2

在您的代码中，您可以使用 map 而不是 forEach。 Promise.all 接受一个承诺数组，但 forEach 不返回一个数组

return Promise.all(
    snapshot.map(doc => {
        let company = {};
        company.id = doc.id;
        company.company = doc.data();
        var branchesPromise = getBranchesForCompanyById(company.id);
        return branchesPromise.then((branches) => {
                company.branches = branches;
                companies.push(company);
                if (snapshot.size === companies.length) {
                    console.log("companies - Inside" + JSON.stringify(companies)); //This prints all companies with its branches
                }
                return Promise.resolve(companies);
            })
            .catch(err => {
                console.log("Error getting sub-collection documents", err);
                return Promise.reject(err);
            })
    })
)

Answer 3

我发现至少有 2 个问题：

• forEach 可能不会返回任何内容，您将 forEach 的结果发送到 Promise.all()。
• 如果 Promise.all() 抛出异常，您的一些 catch 处理程序只会抓取错误并返回它。归还它会将其变为非异常。

您实际上也不必为每个 Promise 链添加 catch，只要将 Promise 链的结果反馈到另一个 Promise 链，您可能只需要 1 个 catch 块。

您的then() 函数之一也不应该嵌套得那么深。只需将其提升一个级别，这就是承诺的意义所在。