req.params for route parameters in Express

Introduction

req.params lets you get values from the URL path in Express. It helps your app know what the user wants based on the URL.

When you want to get an ID from the URL like /user/123 to find a user.

When your route needs to handle different items by name or ID from the URL.

When you want to build URLs that change based on user input or selection.

Syntax

Express

app.get('/route/:paramName', (req, res) => {
  const value = req.params.paramName;
  // use value
});

Use a colon (:) before the parameter name in the route path.

Access the parameter value with req.params.paramName.

Examples

This route gets the user ID from the URL like /user/5 and sends it back.

Express

app.get('/user/:id', (req, res) => {
  res.send(`User ID is ${req.params.id}`);
});

This route gets two parameters from the URL, showing how to handle multiple route params.

Express

app.get('/post/:postId/comment/:commentId', (req, res) => {
  res.send(`Post ${req.params.postId}, Comment ${req.params.commentId}`);
});

Sample Program

This Express app listens on port 3000. It has a route /product/:productId that reads the productId from the URL and sends it back in the response.

Express

import express from 'express';
const app = express();
const port = 3000;

app.get('/product/:productId', (req, res) => {
  const productId = req.params.productId;
  res.send(`Product ID requested: ${productId}`);
});

app.listen(port, () => {
  console.log(`Server running on http://localhost:${port}`);
});

Important Notes

Route parameters are always strings. Convert them if you need numbers.

If the URL does not match the route pattern, req.params will be empty.

You can have multiple parameters in one route, each with its own name.

Summary

req.params gets values from the URL path in Express routes.

Use a colon before the parameter name in the route path.

Access parameters with req.params.paramName inside the route handler.

Practice

1. What does req.params represent in an Express route handler?

easy

A. An object containing query string parameters from the URL

B. An array of middleware functions

C. A function to send responses to the client

D. An object containing route parameters extracted from the URL path

Solution

1. Step 1: Understand the role of req.params

   req.params holds key-value pairs for parameters defined in the route path, extracted from the URL.

2. Step 2: Differentiate from other request properties

   Query parameters are accessed via req.query, not req.params. Response functions and middleware arrays are unrelated.

3. Final Answer:

   An object containing route parameters extracted from the URL path -> Option D

4. Quick Check:

   req.params = route parameters [OK]

Hint: Remember: req.params gets values from URL path, not query [OK]

Common Mistakes:

• Confusing req.params with req.query
• Thinking req.params is a function
• Assuming req.params contains request body data

2. Which of the following is the correct way to define a route with a parameter named id in Express?

easy

A. app.get('/user/*id', (req, res) => { ... })

B. app.get('/user/id', (req, res) => { ... })

C. app.get('/user/:id', (req, res) => { ... })

D. app.get('/user/?id', (req, res) => { ... })

Solution

1. Step 1: Recognize route parameter syntax

   Express uses a colon : before the parameter name in the route path to define route parameters.

2. Step 2: Evaluate each option

   app.get('/user/:id', (req, res) => { ... }) correctly uses /:id. Options A, B, and C use incorrect syntax for route parameters.

3. Final Answer:

   app.get('/user/:id', (req, res) => { ... }) -> Option C

4. Quick Check:

   Route params start with ':' [OK]

Hint: Use ':' before param name in route path [OK]

Common Mistakes:

• Omitting the colon before parameter name
• Using query syntax instead of route param syntax
• Using wildcard '*' incorrectly

3. Given the route definition app.get('/product/:productId', (req, res) => { res.send(req.params.productId); });, what will be the response when a client requests /product/12345?

medium

A. The string '12345'

B. An error because productId is undefined

C. The string '/product/12345'

D. An empty response

Solution

1. Step 1: Understand route parameter extraction

   The route defines a parameter productId which captures the part of the URL after /product/.

2. Step 2: Check what is sent in the response

   The handler sends req.params.productId, which will be the string '12345' from the URL.

3. Final Answer:

   The string '12345' -> Option A

4. Quick Check:

   req.params.productId = '12345' [OK]

Hint: req.params.paramName matches URL part after ':' [OK]

Common Mistakes:

• Expecting full URL or path in req.params
• Confusing req.params with req.query
• Assuming undefined if param not accessed correctly

4. Consider this Express route: app.get('/order/:orderId', (req, res) => { const id = req.params.id; res.send(id); }); What is the problem with this code?

medium

A. req.params cannot be used in GET routes

B. The route parameter is named orderId but accessed as id, causing undefined

C. The route path is missing a parameter

D. res.send cannot send strings

Solution

1. Step 1: Compare route parameter name and access key

   The route defines parameter orderId, but the code accesses req.params.id, which does not exist.

2. Step 2: Understand the effect of this mismatch

   Accessing req.params.id returns undefined, so res.send(id) sends 'undefined'.

3. Final Answer:

   The route parameter is named orderId but accessed as id, causing undefined -> Option B

4. Quick Check:

   Param name mismatch causes undefined [OK]

Hint: Access req.params using exact route param name [OK]

Common Mistakes:

• Using wrong parameter name in req.params
• Assuming req.params works like req.body
• Ignoring case sensitivity in parameter names

5. You want to create an Express route that accepts two parameters: category and itemId. Which code correctly defines the route and sends back a JSON object with both parameters?

hard

A. app.get('/shop/:category/:itemId', (req, res) => { res.json({ category: req.params.category, itemId: req.params.itemId }); });

B. app.get('/shop/:category/:itemId', (req, res) => { res.json({ category: req.query.category, itemId: req.query.itemId }); });

C. app.get('/shop/:category/:itemId', (req, res) => { res.json({ category: req.params.category, itemId: req.query.itemId }); });

D. app.get('/shop/:category/:itemId', (req, res) => { res.json({ category: req.body.category, itemId: req.body.itemId }); });

Solution

1. Step 1: Define route with two parameters

   The route path /shop/:category/:itemId correctly defines two route parameters.

2. Step 2: Access parameters from req.params

   Both category and itemId must be accessed from req.params to get values from the URL path.

3. Step 3: Verify response sends JSON with both parameters

   app.get('/shop/:category/:itemId', (req, res) => { res.json({ category: req.params.category, itemId: req.params.itemId }); }); sends a JSON object with both parameters from req.params. Other options incorrectly use req.query or req.body.

4. Final Answer:

   app.get('/shop/:category/:itemId', (req, res) => { res.json({ category: req.params.category, itemId: req.params.itemId }); }); -> Option A

5. Quick Check:

   Use req.params for route params, send JSON [OK]

Hint: Use req.params for all route parameters in URL path [OK]

Common Mistakes:

• Mixing req.params with req.query or req.body
• Not defining all parameters in route path
• Sending plain text instead of JSON when needed