Creating an Express Router

Introduction

An Express Router helps organize routes in your web app. It keeps your code clean and easy to manage.

You want to group related routes, like all user-related paths, in one place.

You need to split a large app into smaller parts for easier maintenance.

You want to reuse routes in different parts of your app.

You want to apply middleware only to a specific group of routes.

Syntax

Express

const express = require('express');
const router = express.Router();

router.get('/path', (req, res) => {
  res.send('Response text');
});

module.exports = router;

Use express.Router() to create a new router instance.

Define routes on the router just like on the main app.

Examples

This example creates a router with a GET route at '/hello' that sends 'Hello World'.

Express

const express = require('express');
const router = express.Router();

router.get('/hello', (req, res) => {
  res.send('Hello World');
});

module.exports = router;

This router handles a POST request to '/submit' and sends a confirmation message.

Express

const express = require('express');
const router = express.Router();

router.post('/submit', (req, res) => {
  res.send('Form submitted');
});

module.exports = router;

This router logs every request before handling the '/info' route.

Express

const express = require('express');
const router = express.Router();

router.use((req, res, next) => {
  console.log('Request received');
  next();
});

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

module.exports = router;

Sample Program

This program creates an Express app and a router. The router has a GET route at '/greet'. The app uses the router with the prefix '/api', so the full path is '/api/greet'. When you visit that URL, you see the greeting message.

Express

const express = require('express');
const app = express();
const router = express.Router();

// Define a route on the router
router.get('/greet', (req, res) => {
  res.send('Hello from the router!');
});

// Use the router with a prefix
app.use('/api', router);

// Start the server
app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Remember to export your router with module.exports so you can use it in your main app.

Mount routers on paths using app.use('/path', router) to organize routes under that path.

Routers can have their own middleware to handle requests before routes.

Summary

Express Router helps organize routes into smaller, manageable parts.

Create a router with express.Router() and define routes on it.

Use app.use() to mount routers under specific paths.

Practice

1. What is the main purpose of using express.Router() in an Express app?

easy

A. To handle database connections

B. To start the Express server

C. To create a modular set of routes that can be mounted separately

D. To serve static files

Solution

1. Step 1: Understand what express.Router() does

   This function creates a new router object that can hold routes separately from the main app.

2. Step 2: Identify the purpose of modular routes

   Modular routes help organize code by grouping related routes, making the app easier to maintain.

3. Final Answer:

   To create a modular set of routes that can be mounted separately -> Option C

4. Quick Check:

   Router = modular routes [OK]

Hint: Router organizes routes separately for cleaner code [OK]

Common Mistakes:

• Thinking router starts the server
• Confusing router with database handling
• Assuming router serves static files

2. Which of the following is the correct way to create a new Express router?

easy

A. const router = express.Router();

B. const router = new express.Router();

C. const router = express.createRouter();

D. const router = Router();

Solution

1. Step 1: Recall the Express Router creation syntax

   The correct method is calling Router() as a function on the express object.

2. Step 2: Check each option for syntax correctness

   const router = express.Router(); uses express.Router() correctly. const router = new express.Router(); wrongly uses new. const router = express.createRouter(); uses a non-existent method. const router = Router(); calls Router() without express..

3. Final Answer:

   const router = express.Router(); -> Option A

4. Quick Check:

   Correct syntax = express.Router() [OK]

Hint: Use express.Router() without new keyword [OK]

Common Mistakes:

• Using new keyword with Router
• Calling a non-existent createRouter method
• Calling Router without express prefix

3. Given this code snippet, what will be the output when a GET request is made to /api/users?

const express = require('express');
const app = express();
const router = express.Router();

router.get('/users', (req, res) => {
  res.send('User list');
});

app.use('/api', router);

app.listen(3000);

medium

A. Cannot GET /api/users

B. Error: router not defined

C. 404 Not Found

D. User list

Solution

1. Step 1: Understand route mounting with app.use('/api', router)

   This mounts the router under the path /api, so routes inside router are prefixed with /api.

2. Step 2: Check the router route /users

   The router handles GET requests to /users, so combined with the mount path, the full path is /api/users.

3. Final Answer:

   User list -> Option D

4. Quick Check:

   Mounted path + router path = /api/users [OK]

Hint: Router path combines with app.use prefix [OK]

Common Mistakes:

• Forgetting the mount path prefix
• Expecting route at root instead of /api
• Assuming router not mounted

4. Identify the error in this Express router code:

const express = require('express');
const router = express.Router;

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

module.exports = router;

medium

A. Missing parentheses when creating router instance

B. Route path should be '/info/' with trailing slash

C. res.send should be res.json

D. module.exports should export an object, not router

Solution

1. Step 1: Check router creation syntax

   The code uses express.Router without parentheses, so it assigns the function itself, not an instance.

2. Step 2: Confirm correct router instantiation

   It should be express.Router() to create a router instance that supports route methods.

3. Final Answer:

   Missing parentheses when creating router instance -> Option A

4. Quick Check:

   Router needs () to create instance [OK]

Hint: Router must be called as a function with () [OK]

Common Mistakes:

• Forgetting () after express.Router
• Thinking trailing slash is required
• Confusing res.send with res.json

5. You want to create two routers: one for /users and one for /products. How do you correctly mount them in your Express app so that requests to /users/list and /products/list work as expected?

hard

A. app.use(usersRouter); app.use(productsRouter);

B. app.use('/users', usersRouter); app.use('/products', productsRouter);

C. app.use('/list', usersRouter); app.use('/list', productsRouter);

D. app.get('/users', usersRouter); app.get('/products', productsRouter);

Solution

1. Step 1: Understand router mounting with path prefixes

   To handle routes under /users and /products, mount each router with those paths using app.use().

2. Step 2: Check each option for correct usage

   app.use('/users', usersRouter); app.use('/products', productsRouter); mounts routers correctly with prefixes. app.use(usersRouter); app.use(productsRouter); mounts without prefixes, so routes won't be under desired paths. app.use('/list', usersRouter); app.use('/list', productsRouter); mounts both under /list, causing conflicts. app.get('/users', usersRouter); app.get('/products', productsRouter); uses app.get incorrectly with routers.

3. Final Answer:

   app.use('/users', usersRouter); app.use('/products', productsRouter); -> Option B

4. Quick Check:

   Mount routers with correct path prefixes [OK]

Hint: Use app.use with path prefix for each router [OK]

Common Mistakes:

• Mounting routers without path prefix
• Using app.get instead of app.use for routers
• Mounting multiple routers on same path