How to handle this javascript asynchronous function

Question

I have this function in javascript node

exports.getUserEmailByAccess = async (moduleType) => {
 let emailList = []
 const foundUsers = await Access.find({moduleType:moduleType})
 console.log(foundUsers);
 await foundUsers.forEach(access => {
 User.findById(access.userId)
 .then(foundUser => {
 console.log(foundUser.workerEmail);
 emailList.push(foundUser.workerEmail)
 })
 
 });
 console.log(emailList);
 return emailList
}

What I want is to push into emailList array by looping object array, the above approach results in an empty array,so I tried a different following way

exports.getUserEmailByAccess = async (moduleType) => {
 let emailList = []
 const foundUsers = await Access.find({moduleType:moduleType})
 console.log(foundUsers);
 await foundUsers.forEach(access => {
 const foundUser = User.findById(access.userId)
 console.log(foundUser.workerEmail);
 emailList.push(foundUser.workerEmail)
 
 });
 console.log(emailList);
 return emailList
 
}

By doing this , the array list is getting filled but with an [undefined]strong text value, I come from a humble python control structure background, Please can I know why I am not able to push data into array even after using async/await

• Bravo: does foundUsers.forEach return a Promise - if not, you can't await it ... I'm assuming that is NOT Array forEach, in which case, never use Array forEach with async/await, unless you REALLY know what you're doing - and even then, there's always better ways to do it
• Adil Bimzagh: @Bravo is right, forEach doesn't wait for the promise to be resolved, instead you should use a for loop.
• Bravo: the answer depends if you can User.findById in parallel or in series

Answer 1

If the User.findById() returns a promise, I'd recommend using Promise.all() instead of individually running all promises using forEach to fetch documents of all foundUsers:

exports.getUserEmailByAccess = async (moduleType) => {
 const foundUsers = await Access.find({ moduleType: moduleType });
 console.log(foundUsers);
 const userDocs = await Promise.all(
 foundUsers.map((user) => User.findById(user.userId))
 );
 const emailList = userDocs.map((user) => user.workerEmail);
 // or alternatively
 // const emailList = userDocs.map(({workerEmail}) => workerEmail);
 console.log(emailList);
 return emailList;
};

Answer 2

you can give try this

exports.getUserEmailByAccess = async (moduleType) => {
 let emailList = []
 const foundUsers = await Access.find({ moduleType: moduleType })
 console.log(foundUsers);
 await foundUsers.map(async access => {
 let result = await User.findById(access.userId);
 if (result) {
 emailList.push(foundUser.workerEmail)
 }
 });
 console.log(emailList);
 return emailList
}

UPDATED

await Promise.all(
 foundUsers.map(async access => {
 let result = await User.findById(access.userId);
 if (result) {
 emailList.push(foundUser.workerEmail)
 }
 })
])
console.log(emailList);
return emailList

• Adil Bimzagh: map won't wait for the async function to be resolved, the emailList will remain empty.
• Bravo: Array map also doesn't return a Promise - so this is just as bad ... usually it's something like await Promise.all(array.map ..... if you want to do things parallel
• Saurabh: Yes ,it still returns empty array

Answer 3

How are your Models related? I think you might do a populate here if the Access Model has the ObjectId of the User Model. Something like this:

const foundUsers = await Access.find({ moduleType: moduleType }).populate({
 path: 'users', // the name of the field which contains the userId
 select: '-__v' // fields to bring from database. Can add a (-) to NOT bring that field
})

The idea is that you specify all the fields that you need. Then when you receive the data, you can do something like:

foundUsers.users.email

• Dharmaraj: "Can add a (-) to NOT bring that field", you can also specify that in your query using projections