TypeError: Router.use() requires a middleware function but got a Object

[decompilexyz]

TypeError: Router.use() requires a middleware function but got a Object

setReqUser.ts

import { Request, Response, NextFunction } from 'express';
import User from "../models/User";
import jwt from "jsonwebtoken";

interface DecodedToken {
id?: string;
}

export default async function setReqUser (req: Request, res: Response, next: NextFunction) {
try {
const cookies = req.cookies;
const token = cookies?.token;

if (!token) return next();

const decodedToken = jwt.verify(token, process.env.JWT_SECRET!) as DecodedToken;

if (decodedToken?.id) {
  const targetUser = await User.findOne({ id: decodedToken.id }).lean();

  if (targetUser) req.user = targetUser;
}

} catch (error) {
console.error(error);
}

next();
};

[IamLizu]

Hi,

Could you please provide a reproduce-able codebase? It would be helpful for the folks answering the question.

[RafaelHDSV]

Router.use() requires a middleware function but got an Object is a runtime error: Express received something that is not a function. The setReqUser source you posted is a function, so the bug is almost always at the call site or in the module interop, not in the JWT logic.

1. You called the middleware instead of passing it

setReqUser is async. Calling it returns a Promise, and a Promise is an object:

// wrong — passes a Promise
app.use(setReqUser())

// right — passes the function
app.use(setReqUser)

This is the most common cause with async middleware.

2. You mounted the module object (CJS / default export)

// CommonJS: this is `{ default: [Function] }`, not the function
const setReqUser = require('./middleware/setReqUser')
app.use(setReqUser)

// use the function
app.use(setReqUser.default)

With ESM, keep a default import matching the default export:

import setReqUser from './middleware/setReqUser.js'
app.use(setReqUser)

import * as setReqUser from '...' then app.use(setReqUser) also passes an object.

3. req.user is a TypeScript problem, not this crash

Assigning req.user needs an Express.Request augmentation. That fails at compile time. It does not produce Router.use() ... got an Object.

You still want cookie-parser before this middleware (app.use(cookieParser())), otherwise req.cookies is undefined and the handler just calls next() — again, not this error.

Quick check: console.log(typeof setReqUser, setReqUser) right before app.use. It must log function. If it logs object, you are in case 1 or 2.