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

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Note: if you got an error when setting a view engine, click on the last subheading.

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

The error "TypeError: Router.use() requires a middleware function but got a Object" occurs when you forget to export the Router in one of your .js files.

To solve the error, add the module.exports = router; line to export the Router.

router use required middleware function but got an object

Here is the complete stack trace.

shell

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throw new TypeError('Router.use() requires a middleware function but got a ' + gettype(fn))
^

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

at Function.use (/home/borislav/Desktop/bobbyhadz-rest/bobbyhadz-js/node_modules/express/lib/router/index.js:469:13)
at Function.<anonymous> (/home/borislav/Desktop/bobbyhadz-rest/bobbyhadz-js/node_modules/express/lib/application.js:227:21)

Here is an example of how the error occurs.

blog.js

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const express = require('express');

const router = express.Router();

router.get('/', (req, res) => {
  res.send('Blog home page');
});

router.get('/about', (req, res) => {
  res.send('About Blog');
});

The file makes use of the express.Router function.

Notice that we didn't export anything from the blog.js file.

If I now try to import it in index.js, the error is raised.

index.js

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const express = require('express');
// ⛔️ trying to import from file without export
const blog = require('./blog');

const app = express();

// 👇️ use Router
app.use('/blog', blog);

const port = 5000;

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`);
});

Here is the output in my terminal after starting my server with node index.js.

shell

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

The issue is that we tried to import the Router from blog.js, but the blog.js file doesn't export anything.

To solve the error, make sure to the Router from your files.

index.js

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const express = require('express');

const router = express.Router();

router.get('/', (req, res) => {
  res.send('Blog home page');
});

router.get('/about', (req, res) => {
  res.send('About Blog');
});

// ✅ export the router
module.exports = router;

The code for this article is available on GitHub

The last line exports the Router, so it can be imported into other files.

Make sure that you haven't mistyped module.exports. Forgetting the s in exports often causes the error.

If you use ES6 Modules, use the following syntax instead.

blog.js

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// ✅ using ES6 Modules syntax
import express from 'express';

const router = express.Router();

router.get('/', (req, res) => {
  res.send('Blog home page');
});

router.get('/about', (req, res) => {
  res.send('About Blog');
});

// ✅ export your router
export default router;

The code for this article is available on GitHub

We used export default router instead of module.exports = router;.

And, here is how we can import the blog.js file and use the Router.

index.js

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import express from 'express';

// ✅ importing the Router
import blog from './blog.js';

const app = express();

app.use('/blog', blog);

const port = 5000;

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`);
});

Notice that the extension of the file has to be specified when importing local files with ES6 modules.

You have to make sure to add an export statement in all of the modules from which you are importing.

Trying to import from a module that doesn't export anything causes the error.

If the module you are importing from is empty or you've forgotten to export a value, the error is raised.

# Export the Router with a Named export instead

Alternatively, you can export the Router as a named export.

This is useful when you need to export multiple functions, classes or variables from the same file.

index.js

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const express = require('express');

const router = express.Router();

router.get('/', (req, res) => {
  res.send('Blog home page');
});

router.get('/about', (req, res) => {
  res.send('About Blog');
});

// 👇️ export an object instead of the Router directly
module.exports = {
  blog: router,
};

The code for this article is available on GitHub

We export an object instead of exporting the Router directly.

You can add multiple properties to the object if you need to export multiple values from the file.

And here is how we'd import the Router in a different file.

index.js

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const express = require('express');

// 👇️ import the Router
const {blog} = require('./blog');

const app = express();

// 👇️ use Router
app.use('/blog', blog);

const port = 5000;

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`);
});

Notice that we wrapped the import in curly braces to destructure the blog Router from the module's exports.

Here is the equivalent example using ES6 Modules import/export syntax.

blog.js

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// ✅ using ES6 Modules
import express from 'express';

const router = express.Router();

router.get('/', (req, res) => {
  res.send('Blog home page');
});

router.get('/about', (req, res) => {
  res.send('About Blog');
});

// 👇️ export the router aliasing it to `blog`
export {router as blog};

The code for this article is available on GitHub

I aliased the router export to blog to make the variable name more clear in the other file.

And here is how we'd import the Router in the other file.

index.js

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import express from 'express';

// 👇️ import the Router
import {blog} from './blog.js';

const app = express();

// 👇️ use Router
app.use('/blog', blog);

const port = 5000;

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`);
});

Notice that we use curly braces {} with named imports.

This is not the case with default imports and exports.

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

The error "TypeError: Router.use() requires a middleware function but got a string" occurs when you pass a string as the second argument to the app.use() method.

If you meant to set a template engine, use the app.set() function instead.

router use required middleware function but got a string

Here is an example of how the error occurs.

index.js

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// ⛔️ TypeError: Router.use() requires a middleware function but got a string
app.use('view engine', 'ejs');

Instead, you should use the app.set() function.

index.js

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app.set('view engine', 'ejs');

The code for this article is available on GitHub

Make sure you have the view engine installed.

For example:

npm install ejs (if you use ejs)
npm install pug (if you use pug)

A template engine enables you to use static template files in your application and replaces variables with actual values.

Here is a complete example that uses the pug template engine.

Make sure you have pug installed if you want to run the code snippet.

shell

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npm install pug

Here is the index.js file that sets pug as the template engine.

index.js

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const express = require('express');

const app = express();

app.set('view engine', 'pug');

app.get('/', (req, res) => {
  res.render('index', {
    title: 'bobbyhadz.com',
    message: 'Hello world',
  });
});

const port = 5000;

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`);
});

Now create a views directory at the root of your project (where your package.json file is) and add the following index.pug file.

Your folder structure should look as follows.

shell

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my-express-app
  └── views
      └── index.pug
  └── index.js

folder structure of express application

index.pug

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html
  head
    title= title
  body
    h1= message

If I refresh my browser I can see the pug engine has replaced the placeholders with the given values.

page successfully loaded

The code for this article is available on GitHub

# Restart your development server

If the error persists, try to restart your development server.

Open your terminal and press Ctrl + C to stop your dev server and restart it.

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

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