Can't Login To App With Heroku. Works Locally - node.js

Question

Yet Another Heroku Problem....
When working in development mode, users can login to the app no problem.
In Production Mode on Heroku, this is not the case - a POST request to /api/login returns a 500 Internal Server Error.
In Production Mode, my app seems to be connected to the correct database as /api/users returns the full list of registered users.
Heroku just seems to be throwing a 500 error just for the hell of it.
Any ideas why?
The App:
app.js
require('dotenv').config()
const config = require('./utils/config')
const express = require('express')
require('express-async-errors')
const app = express()
const mongoose = require('mongoose')
const cors = require('cors')
const morgan = require('morgan');
const middleware = require('./utils/middleware')
const blogsRouter = require('./controllers/blogs')
const usersRouter = require('./controllers/users')
const loginRouter = require('./controllers/login')
mongoose.connect(config.MONGODB_URI)
.then(() => {
console.log('connected to MongoDB')
})
.catch((error) => {
console.log('error connection to MongoDB:', error.message)
})
app.use(middleware.tokenExtractor)
app.use(cors())
app.use(express.static("build"))
app.use(express.json())
app.use('/api/blogs', blogsRouter)
app.use('/api/users', usersRouter)
app.use('/api/login', loginRouter)
app.use(middleware.errorHandler)
app.use(middleware.unknownEndpoint)
app.use(morgan(':method :url :body'));
module.exports = app
index.js
require('dotenv').config()
const app = require('./app')
const http = require('http')
const logger = require('./utils/logger')
const config = require('./utils/config')
const server = http.createServer(app)
const PORT = process.env.PORT
server.listen(PORT, () => {
logger.info(`Server running on port ${config.PORT}`)
})
loginRouter
const jwt = require('jsonwebtoken')
const bcrypt = require('bcrypt')
const loginRouter = require('express').Router()
const User = require('../models/user')
loginRouter.post('/', async (request, response) => {
const { username, password } = request.body
const user = await User.findOne({ username })
const passwordCorrect = user === null
? false
: await bcrypt.compare(password, user.passwordHash)
if (!(user && passwordCorrect)) {
return response.status(401).json({
error: 'invalid username or password'
})
}
const userForToken = {
username: user.username,
id: user._id,
}
const token = jwt.sign(userForToken, process.env.SECRET)
response
.status(200)
.send({ token, username: user.username, name: user.name })
})
module.exports = loginRouter

Answer 1

The loginRouter contains the following line necessary for generating a token:
const token = jwt.sign(userForToken, process.env.SECRET)
Heroku does not know what process.env.SECRET is unless you tell it.
Go to your app page in Heroku ---> Settings ---> Reveal Config Vars
Click Add
Type SECRET into KEY and your value into VALUE
This then fixes the code.