Route.get() requires callback functions but got a object Undefined

# Route.get() requires callback functions but got a object Undefined

The article covers how to solve the following related errors:

Route.get() requires callback functions but got a object Undefined
Route.post() requires callback functions but got a object Undefined

route get requires callback function but got object undefined

The error occurs when you pass a second argument of undefined to the Router.get() or Router.post() methods.

routes/books.js

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import express from 'express';

const router = express.Router();

// ⛔️ Error: Route.get() requires a callback function but got a [object Undefined]
router.get('/', undefined);

// ⛔️ Error: Route.get() requires a callback function but got a [object Undefined]
router.post('/', undefined);


export {router as booksRouter};

This often happens when you misspell the name of the controller function you have to pass as the second argument to Router.get() and Router.post().

You might have also imported the function incorrectly.

# Make sure to pass a function as the second argument to Router.get()

To solve the error, you have to make sure to pass a response handler function as the second argument to the router.get() and router.post() methods.

routes/books.js

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import express from 'express';

const router = express.Router();

// ✅ Works
router.get('/', (req, res) => {
  res.send('Home page of books route');
});

// ✅ Works
router.post('/', (req, res) => {
  res.send('Adding a book to the DB');
});

export {router as booksRouter};

The code for this article is available on GitHub

If you are importing the function you pass to router.get() and router.post() from another file, make sure to import it correctly.

Here are some things to note:

The function has to be exported from the other file, either using the export keyword (ES6) or using module.exports.
Make sure to spell the name of the response handle function correctly.
Make sure the function exists and is defined in the file at the time you are trying to access it.

For example, assume you have the following folder structure.

shell

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index.js
routes/
  └── books.js
controllers/
  └── books-controller.js

# Using ES6 import/export statements

Here is the code for the controllers/books-controller.js module.

controllers/books-controller.js

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export function listBooks(req, res) {
  res.send('Home page of books route');
}

export function createBook(req, res) {
  res.send('Adding a book to the DB');
}

The example uses the ES6 export syntax to export the two functions.

And, here is how you can import the functions in the routes/books.js file.

routes/books.js

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import express from 'express';
import {
  listBooks,
  createBook,
} from '../controllers/books-controller.js';

const router = express.Router();

// Home page for this route
router.get('/', listBooks);

// Create book route
router.post('/', createBook);

export {router as booksRouter};

The code for this article is available on GitHub

We used named imports to import the two functions and registered them on the Router.

And here is how you can start your Express server in the index.js file.

index.js

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import express from 'express';
import {booksRouter} from './routes/books.js';

const app = express();

app.use('/books', booksRouter);


app.listen(5000, function () {
  console.log('Server listening on port 5000');
});

# Using the require() CommonJS syntax

Here is an example that uses the CommonJS require() syntax instead.

controllers/books-controller.js

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function listBooks(req, res) {
  res.send('Home page of books route');
}

function createBook(req, res) {
  res.send('Adding a book to the DB');
}

module.exports = {
  listBooks,
  createBook,
};

The code for this article is available on GitHub

And this is how you can import the functions in the routes/books.js file.

routes/books.js

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const express = require('express');
const {
  listBooks,
  createBook,
} = require('../controllers/books-controller.js');

const router = express.Router();

// Home page for this route
router.get('/', listBooks);

// Create book route
router.post('/', createBook);

module.exports = {
  booksRouter: router,
};

Alternatively, you can import the entire module and access the functions as properties on the module.

routes/books.js

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const express = require('express');

// 👇️ import the entire module
const booksController = require('../controllers/books-controller.js');

const router = express.Router();

// 👇️ access functions as properties on the module
router.get('/', booksController.listBooks);

router.post('/', booksController.createBook);

module.exports = {
  booksRouter: router,
};

If you import the entire module, make sure to access the functions as properties on the module, e.g. booksController.listBooks.

And here is how you can start your Express server in the index.js file.

index.js

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const express = require('express');
const {booksRouter} = require('./routes/books.js');

const app = express();

app.use('/books', booksRouter);

app.listen(5000, function () {
  console.log('Server listening on port 5000');
});

The code for this article is available on GitHub

You can console.log the functions you are importing if the error persists.

Once you correct your import and export statements and pass a function as the second argument to the Router.get() and Router.post() methods, the error should be resolved.

I've written a detailed guide on how to import and export classes and functions using ES6.

# Defining your response handler functions inline

If none of the suggestions helped, you can always define your handler functions in the same file as your Router.

index.js

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const express = require('express');

const router = express.Router();

// 👇️ define Router.get handler
function listBooks(req, res) {
  res.send('Home page of books route');
}

// 👇️ define Router.post handler
function createBook(req, res) {
  res.send('Adding a book to the DB');
}

// 👇️ set Router.get handler function
router.get('/', listBooks);

// 👇️ set Router.post handler function
router.post('/', createBook);

module.exports = {
  booksRouter: router,
};

You can also define the functions inline.

index.js

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const express = require('express');

const router = express.Router();

// 👇️ define handler function inline
router.get('/', (req, res) => {
  res.send('Home page of books route');
});

// 👇️ define handler function inline
router.post('/', (req, res) => {
  res.send('Adding a book to the DB');
});

module.exports = {
  booksRouter: router,
};

The code for this article is available on GitHub

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

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Route.get() requires callback functions but got a object Undefined