Fix TypeError: Cannot read properties of undefined (reading 'headers') in Express

This error occurs when middleware tries to access req.headers but the request object is undefined, usually because middleware parameters are in the wrong order or a middleware function is invoked incorrectly. Fix it by ensuring your middleware uses the correct (req, res, next) signature and is passed as a reference, not called.

Reading the Stack Trace

Here's what each line means:

• at authMiddleware (/app/src/middleware/auth.js:5:22): The auth middleware at line 5 tries to read headers from the first argument, but the first argument is not the request object.
• at Layer.handle [as handle_request] (/app/node_modules/express/lib/router/layer.js:95:5): Express invoked the middleware function, but the function signature does not match what Express passes.
• at expressInit (/app/node_modules/express/lib/middleware/init.js:40:5): Express initialization middleware ran correctly, so the issue is with the custom middleware definition.

Common Causes

function authMiddleware(req, res, next) {
  const token = req.headers.authorization;
  if (!token) return res.status(401).json({ error: 'Unauthorized' });
  next();
}

// Bug: calling authMiddleware() instead of passing it
app.use(authMiddleware());

function authMiddleware(res, req, next) {
  const token = req.headers.authorization; // res is actually req here
  next();
}

// Missing err as first parameter for error middleware
app.use((req, res, next) => {
  // This is treated as regular middleware
  console.log(req.headers); // Works fine here
});

// But if registered as error handler:
app.use((req, res, next) => {
  // req is actually err, res is actually req
  console.log(req.headers); // Fails if err is a string
});

The Fix

Pass the middleware function as a reference without calling it. Using authMiddleware instead of authMiddleware() lets Express invoke it with the correct (req, res, next) arguments.

function authMiddleware(req, res, next) {
  const token = req.headers.authorization;
  if (!token) return res.status(401).json({ error: 'Unauthorized' });
  next();
}

app.use(authMiddleware());

function authMiddleware(req, res, next) {
  const token = req.headers.authorization;
  if (!token) return res.status(401).json({ error: 'Unauthorized' });
  next();
}

app.use(authMiddleware);

Testing the Fix

const request = require('supertest');
const express = require('express');

function authMiddleware(req, res, next) {
  const token = req.headers.authorization;
  if (!token) return res.status(401).json({ error: 'Unauthorized' });
  next();
}

describe('authMiddleware', () => {
  let app;

  beforeEach(() => {
    app = express();
    app.use(authMiddleware);
    app.get('/protected', (req, res) => {
      res.json({ message: 'success' });
    });
  });

  it('returns 401 when no authorization header is present', async () => {
    const response = await request(app).get('/protected');
    expect(response.status).toBe(401);
    expect(response.body.error).toBe('Unauthorized');
  });

  it('allows request with authorization header', async () => {
    const response = await request(app)
      .get('/protected')
      .set('Authorization', 'Bearer test-token');
    expect(response.status).toBe(200);
    expect(response.body.message).toBe('success');
  });
});

Run your tests:

npm test

Pushing Through CI/CD

git checkout -b fix/middleware-invocation,git add src/app.js src/__tests__/auth.test.js,git commit -m "fix: pass authMiddleware as reference instead of invoking it",git push origin fix/middleware-invocation

Your CI config should look something like this:

name: CI
on:
  pull_request:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: '20'
          cache: 'npm'
      - run: npm ci
      - run: npm test
      - run: npm run lint

The Full Manual Process: 18 Steps

Here's every step you just went through to fix this one bug:

1. Notice the error alert or see it in your monitoring tool
2. Open the error dashboard and read the stack trace
3. Identify the file and line number from the stack trace
4. Open your IDE and navigate to the file
5. Read the surrounding code to understand context
6. Reproduce the error locally
7. Identify the root cause
8. Write the fix
9. Run the test suite locally
10. Fix any failing tests
11. Write new tests covering the edge case
12. Run the full test suite again
13. Create a new git branch
14. Commit and push your changes
15. Open a pull request
16. Wait for code review
17. Merge and deploy to production
18. Monitor production to confirm the error is resolved

Total time: 30-60 minutes. For one bug.

npm install bugstack-sdk

const { initBugStack } = require('bugstack-sdk')

initBugStack({ apiKey: process.env.BUGSTACK_API_KEY })

Deploying the Fix (Manual Path)

1. Run the test suite locally to confirm middleware processes requests correctly.
2. Open a pull request with the middleware registration fix.
3. Wait for CI checks to pass on the PR.
4. Have a teammate review and approve the PR.
5. Merge to main and verify protected routes work in staging.

Frequently Asked Questions