res.json() called before the for loop is fully executed-CodePudding

I've got an endpoint in my Application that is supposed to fetch data from a MongoDb Database and return the results to the Client. However, before my for(){} loop finishes execution and an empty array ends up getting sent. How do I change my Code where I can send the Array only when it's completely populated.

Code Below:

app.get("/check/classes/usercart", (req, res) => {

if(req.session.user){

    UserCart.findOne({userId: req.session.user._id}).lean().exec()
    .then((foundUserCart)=>{
        if(foundUserCart){
            console.log("Found user in the UserCart Collection", foundUserCart);

            console.log("Printing User's classCart", foundUserCart.classCart);
            //return res.json({classes: foundUserCart.classCart});

            const classesArray = foundUserCart.classCart;

            let arrayToSend = [];

            for(let i = 0; i < classesArray.length; i  ){
                
                Classes.findById({_id: classesArray[i]}).lean().exec()
                .then((foundClass)=>{
                    console.log("Class Found", foundClass);
                    arrayToSend.push(foundClass);
                    console.log("Printing ArrayToSend", arrayToSend);
                })
                .catch((error)=>{
                    console.log("Error performing findById", error);
                    return res.json({msg: "Server Error"});
                });
            }

            //return arrayToSend;

            //This get's executed before my for loop finishes. I'm new to NodeJs and 
            //the whole asynchronous cycle  
            return res.json({classes: arrayToSend});

        }else{
            return res.json({msg: "No records found for this user"});
        }
        
    })
    .catch((error)=>{
        console.log("Error performing UserCart.find() operation", error);
        return res.json({msg: "Server Error"});
    });

}else{
    return res.redirect("/");
}


});

Please could somebody kindly give me some suggestions? Been stuck on this for a while now. Thanks.

Update: So I got around this by doing the following:

Classes.find({'_id': {$in: classesArray}})
           .then((foundRecords)=>{
               console.log("Found records", foundRecords);
               return res.json({classes: foundRecords}); 
           }).
           catch((error)=>{
            console.log("Error here", error);
 });

This just returns the entire array of records. Probably not the cleanest solution. Could somebody tell me how to do this entire function using Async Await?

CodePudding user response：

Using Promise.all

app.get("/check/classes/usercart", (req, res) => {
    if (req.session.user) {
        return UserCart.findOne({userId: req.session.user._id}).lean().exec()
        .then(foundUserCart => {
            if (foundUserCart) {
                const { classCart } = foundUserCart;
                const arrayToSend = classCart.map(_id => Classes.findById({_id}).lean().exec());
                return Promise.all(arrayToSend)
                    .then(classes => res.json({classes}));
            }
            return res.json({msg: "No records found for this user"});
        })
        .catch((error)=>{
            console.log("Error performing UserCart.find() operation", error);
            return res.json({msg: "Server Error"});
        });
    }
    res.redirect("/");
});

Bonus: Rewrite the above using Async/await

app.get("/check/classes/usercart", async (req, res) => {
    try {
        if (!req.session.user) {
            return res.redirect("/");
        }
        const foundUserCart = await UserCart.findOne({userId: req.session.user._id}).lean().exec();
        
        if (!foundUserCart) {
            return res.json({msg: "No records found for this user"});
        }
        const { classCart } = foundUserCart;
        const arrayToSend = classCart.map(_id => Classes.findById({_id}).lean().exec());
        const classes = await Promise.all(arrayToSend);
        res.json({classes});
    } catch (error) {
        res.json({msg: "Server Error"});
    }
});

CodePudding user response：

You can make your query with multiple IDs in a single operation. That would eliminate the need for the for loop.

const listOfIds = ["46df4667tfs57", "477dfs884v73", "88er58366s"];
Classes.find({ _id: { $in: listOfIds } }).lean().exec()