Why am i getting undefined by mapping an array?

I have the following method in my controller:

// @desc   Create a game
// @route  POST /api/games/register
// @access Private
exports.registerGame = asyncHandler(async (req, res, next) => {
const { name, description, employees, hours } = req.body;
// Verify that the hours are valid and dont overlap
const overlappingHours = hours.some((currentHour, currentIndex) => {
return hours.slice(currentIndex + 1).some((nextHour) => {
return (
(currentHour.opening <= nextHour.opening &&
nextHour.opening < currentHour.closing) ||
(currentHour.opening < nextHour.closing &&
nextHour.closing <= currentHour.closing) ||
(nextHour.opening <= currentHour.opening &&
currentHour.closing <= nextHour.closing)
);
});
});
if (overlappingHours) {
return res.status(400).json({
success: false,
error: 'Range of hours are overlapping',
});
}
// Verify that the employees exist
employees.map((employee) => {
const foundEmployee = Employee.findById(employee);
console.log(foundEmployee);
if (!foundEmployee) {
return next(
new ErrorResponse(`Employees not found with id of ${employee}`, 404)
);
}
if (foundEmployee.type !== 'manager') {
return next(new ErrorResponse(`Employees must be managers`, 400));
}
});
console.log(employees);
const game = await Game.create({
name,
description,
employees,
hours,
});
res.status(201).json({
success: true,
data: game,
});
});

A Game contains the employees property which is an array of ids that belong to employees.
I am trying to map the employees array and try to find if the id which is a string in the array employee is the same as a valid id of my Employees collection.

If the employees in the array do not match a valid employee id the api should return an error response with a message that says No employees with id ${employee}

But for some reason i am getting undefined in my console.

Getting the employees from db in a loop is not a good solution, it will have performance problems.

So instead you can get all the employees once using $in operator, and apply the logic in the code.

You can do something like this:

    // Verify that the employees exist
const foundEmploees = await Employee.find({
_id: { $in: employees },
});
const foundEmploeesIds = foundEmploees.map((f) => f._id.toString());
const errors = [];
for (var i = 0; i < employees.length; i++) {
if (!foundEmploeesIds.includes(employees[i])) {
errors.push(`Employees not found with id of ${employees[i]}`);
} else {
const emp = foundEmploees.find(
(e) => e._id.toString() === employees[i]
);
if (emp.type !== 'manager') {
errors.push(`Employees is not manager ${employees[i]}`);
}
}
}
if (errors.length > 0) {
return next(new ErrorResponse(errors.join(','), 404));
}