Why my error is not being handled in express.js? - node.js

Question

I am learning express.js, and I am thinking why I receive UnhandledPromiseRejectionWarning and no error response is being sent after executing http api call..
For error handling I have created helper class and middleware:
/helpers/errorHander.js
------------------------------------
class ErrorHandler extends Error {
constructor(statusCode, message) {
super();
this.statusCode = statusCode;
this.message = message;
}
}
const handleError = (err, res) => {
const { statusCode, message } = err;
res.status(statusCode).json({
status: 'error',
statusCode,
message
});
};
module.exports = {
ErrorHandler,
handleError
};
I have enabled this middleware in app.js file:
/app.js
------------------------------------
const express = require('express');
const app = express();
const bodyParser = require('body-parser');
const config = require('./config/init');
const cors = require('cors');
const { handleError, ErrorHandler } = require('./helpers/errorHandler');
//routes
const apiRoutes = require('./api');
// connect db
config.initializeDB();
// configure bodyParser
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());
// Enable Cors
app.use(cors());
// Set Routes
app.use('/', apiRoutes);
app.get('/error', (req, res) => {
throw new ErrorHandler(500, 'Internal server error');
});
// Enable error handling middleware
app.use((err, req, res, next) => {
handleError(err, res);
});
module.exports = app;
When I perform request to the /error endpoint, it works correctly:
/error endpoint
But when performing request to the endpoint which performs operations with mongoose I am not getting desired result:
/v1/users/ endpoint
user.route.js:
const router = require('express').Router();
const userController = require('./user.controller');
router.get('/', userController.getUsers);
router.post('/', userController.createUser);
module.exports = router;
user.controller.js:
const userService = require('../../../services/user');
module.exports = {
getUsers: (req, res, next) => {
return res.json(userService.getUsers());
},
createUser: (req, res, next) => {
const username = req.body.username;
const password = req.body.password;
res.json(userService.createUser(username, password));
}
};
user.service.js:
const User = require('../models/user');
const { ErrorHandler } = require('../helpers/errorHandler');
module.exports = {
getUsers: async () => {
return User.find({});
},
createUser: async (username, password) => {
const user = new User({ username, password });
try {
await user.save();
return user;
} catch (err) {
if (err.code === 11000) {
throw new ErrorHandler(409, 'Username already exists!');
}
throw new ErrorHandler(500, 'Internal server error');
}
}
};
Console gives such a warning:
Console Output
Why it is not working as I want, and how can I make it work?
=========================================================
Update #1
As user - jfriend00 suggested, I tried to await the promise, and here my code looks like on user.controller.js:
const userService = require('../../../services/user');
module.exports = {
getUsers: (req, res, next) => {
return res.json(userService.getUsers());
},
createUser: async (req, res, next) => {
const username = req.body.username;
const password = req.body.password;
try {
let user = await userService.createUser(username, password);
} catch (err) {
next(err);
}
}
};
And Now I get the desired outcome.