Synchronous error handling in node.js router - node.js

Question

I am new to MongoDB/backend and have spent a couple of hours on this one. Hoping/expecting there is a silly mistake here!
Essentially, in the process of adding new users to the db, I want to check one of the specified values (the "position code(s)") against a list of acceptable codes. I retrieve these acceptable position codes via an async call and then use an underscore method to compare the specified position code(s) against the acceptable ones. If any are found, I throw an error.
Multiple users can be added at a time and each user can have multiple roles. So both of these are array types.
The most recent thing i've tried is simply using the "throw new Error()" methodology since my understanding is that this should get picked up by the Express middleware. But it's not working. The error message I specify in the error object IS getting printed in the console, but in a "UnhandledPromiseRejectionWarning" error. When running this in Postman, Postman just doesn't return anything (basically appears like it's loading and then says could not get a response").
I also tried wrapping this in a try catch block.
I also tried passing the error to a next() method.
router.route('/add').post(async (req, res) => {
let
newAdults = [],
roleCodes = await Role.find()
.then(roles => _.pluck(roles, "code"))
.catch(err => res.status(400).json('Error: ' + err));
req.body.forEach((el, id) => {
let
firstName = el.firstName,
lastName = el.lastName,
emailAddress = el.emailAddress,
username = el.username,
roles = el.roles;
// one thing we need to lookout for is if an invalid role code is being used.
// here we compare role codes associated with the new adult and all approved
// role codes. this will identify invalid role codes
let invalidCodes = _.difference(_.pluck(roles, "code"), roleCodes)
if (invalidCodes.length > 0)
throw new Error(`Role code(s) ${invalidCodes} for ${firstName} ${lastName} are invalid.`)
Ultimately I want to just return the message I have in my error object.

Answer 1

Since you are throwing the error without handling it in your API, it remains in incomplete state and you get could not get a response error in POSTMAN.
You can try this,
Create new error object array and store the error messages in that object rather then throwing the error.
You can return this error object array in response in case this array has values.
Here is the updated snippet:
router.route('/add').post(async (req, res) => {
let
newAdults = [],
roleCodes = await Role.find()
.then(roles => _.pluck(roles, "code"))
.catch(err => res.status(400).json('Error: ' + err));
let errorArray = [];
req.body.forEach((el, id) => {
let
firstName = el.firstName,
lastName = el.lastName,
emailAddress = el.emailAddress,
username = el.username,
roles = el.roles;
// one thing we need to lookout for is if an invalid role code is being used.
// here we compare role codes associated with the new adult and all approved
// role codes. this will identify invalid role codes
let invalidCodes = _.difference(_.pluck(roles, "code"), roleCodes)
if (invalidCodes.length > 0) {
errorArray.push(`Role code(s) ${invalidCodes} for ${firstName} ${lastName} are invalid.`);
}
if (errorArray && errorArray.length) {
res.status(400).json(errorArray)
}