Page does not export a default function

Question

I'm new to Nextjs and i'm trying to setup a MongoDB connection through a middleware config.

Here is my code:

import type { NextApiRequest, NextApiResponse } from 'next'
import { connectMongoDB } from '../../middlewares/connect-db'

const handler = (req: NextApiRequest, res: NextApiResponse) => {
  if (req.method === 'POST') {
    const { login, pwd } = req.body
    if (login === '[email protected]' && pwd === 'Admin@123') {
      res.status(200).json({ message: 'Successfully logged in!' })
    }
    return res.status(400).json({ error: 'Failed to log in.' })
  }
  return res.status(405).json({ error: 'Unsupported Method' })
}
export default connectMongoDB(handler)

Here is the middleware/connect-db code:

import type { NextApiRequest, NextApiHandler, NextApiResponse } from 'next'
import mongoose from 'mongoose'

export const connectMongoDB = (handler: NextApiHandler) => {
  ;async (req: NextApiRequest, res: NextApiResponse) => {
    //check connection
    if (mongoose.connections[0].readyState) {
      return handler(req, res)
    }

    const { DB_CONNECTION_STRING } = process.env
    if (!DB_CONNECTION_STRING) {
      return res.status(500).json({ error: 'DB Connection Error' })
    }

    mongoose.connection.on('connected', () =>
      console.log('DB Connection stablished')
    )
    mongoose.connection.on('error', () => console.log('DB Connection Error'))
    await mongoose.connect(DB_CONNECTION_STRING)

    return handler(req, res)
  }
}

Here is my project structure:

It worked when I exported de handler function as default. I'm not sure why it is not accepting my export now.

Ty in advance.

I've tried to rename functions, triple checked my DB_CONNECTION_STRING and restarted the app at every change.

Answer 1

The problem stems from the definition of connectMongoDB(). You defined an arrow function in connectMongoDB() but you never output this arrow function. It will output void as a result and the subroutine that will use the export is not expecting void. ^[1]

One solution would be to assign the arrow function to a variable and return it after:

export const connectMongoDB = (handler: NextApiHandler) => {
  let connect = async (req: NextApiRequest, res: NextApiResponse) => {
    •••Rest of code•••
  }
  return connect
}

---

_{^[1] One nit: You have an unexpected token ;.}