函数返回空数组，如何解决？ Javascript，猫鼬

Question

I have a javascript, mongoose function which should return an array. So basically what I have is a user schema, and a shopping list schema. User schema has a parameter which is authorizedLists, an array of shopping lists ids. What I want is, that by passing the user id as a parameter, I can show those lists the user is authorized in. This function right now returns an empty array, and what I expected was an array with the shopping list objects. I know it is not working due to Javascript asynchronous way of reading code, but I´m really stucked here. Also I´m relatively new to nodejs and mongoose so I may have made other mistakes but haven´t realized yet.

Code:

function getUserAuthLists(req,res){
    const { userId } = req.params;
    var list = [];
    let i = 0;
    User.findById( userId,async function(err, user){  
        if(err) return res.status(404).send(err);
        console.log(user);
        const array = user.authorizedLists;
        console.log(array);
        array.forEach( function(element){
            console.log(element);
            ShoppingList.findById(element , (error, result)=>{
                if(error) {
                    console.log(error);
                    return res.status(404).send(error);
                }
                list.push(result);
                console.log(list);
            });
            i++;
        });
        if(i === array.length) {
            console.log(list);
            return  res.status(202).send(list);
        }
    });
}

Thanks in advance.

Answer 1

Refactor your function to use promises, collect all authorizedLists and pass it in one-go using the $in operator instead of repeatedly doing findById

async function getUserAuthLists(req, res){
    try {
        const { userId } = req.params;
        let i = 0;
    
        const user = await User.findById(userId).exec();
        const list = await ShoppingList.find({
            _id: {
                $in: user.authorizedLists
            }
        }).exec();
        return  res.status(202).send(list);
    } catch (err) {
        return res.status(404).send(err);
    }
}

Answer 2

Since ShoppingList.findById is an i/o call, control does not wait by default to finish this call and directly jump to list.push(result); with list empty values. You need to use promise to make it wait until it gets results from i/o call and then you should send back response. That way list will be updated for each array item call.

Something like this-

async function getUserAuthLists(req, res) {
    User.findById(userId, async function (err, user) {
        if (err) return res.status(404).send(err);
        console.log(user);
        const array = user.authorizedLists;
        console.log(array);
        await ShoppingListFromDB(array)
            .then((list) => {
                console.log("list--", list);
                return res.status(202).send(list);
            })
            .catch((error) => {
                console.log(error);
                return res.status(404).send(err);
            })
        
    });
}

function ShoppingListFromDB(array) {
    return new Promise((resolve, reject) => {
        let i = 0
        var list = [];
        for (const element of array) {
            console.log("element ", element);
            ShoppingList.findById(element, (error, result) => {
                if (err) {
                    console.log(err)
                    return reject(error);
                }
                i++
                list.push(result)
                if (i == array.length) {
                    return resolve(list)
                }
            })
        }

    })
}

I have created a sample that simulated your requirement.

https://github.com/ajaysikdar/test_server

server

client