Route parameters in Express

Introduction

Route parameters let you capture parts of a URL as variables. This helps you handle dynamic content like user profiles or product pages.

When you want to get a user ID from the URL to show their profile.

When you need to fetch a product by its ID from the URL.

When building URLs that change based on the item, like blog posts or articles.

When you want to create routes that accept different values without making many separate routes.

Syntax

Express

app.get('/path/:parameterName', (req, res) => {
  const param = req.params.parameterName;
  res.send(`Value is ${param}`);
});

Route parameters start with a colon : in the route path.

You access the parameter value inside the handler with req.params.parameterName.

Examples

This route captures the id from the URL like /user/123.

Express

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

This route captures two parameters: category and id.

Express

app.get('/product/:category/:id', (req, res) => {
  res.send(`Category: ${req.params.category}, Product ID: ${req.params.id}`);
});

Multiple parameters can be used to get nested data like comments on posts.

Express

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

Sample Program

This simple Express app listens on port 3000. It has one route that takes a userId from the URL and sends it back in the response.

Try visiting http://localhost:3000/user/42 in your browser to see the output.

Express

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

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

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

Important Notes

Route parameters only capture the part of the URL where you put the : placeholder.

If you want to accept optional parameters, you can add a question mark like :param?.

Always validate or sanitize route parameters before using them to avoid security issues.

Summary

Route parameters let you get dynamic values from the URL.

Use req.params to access these values inside your route handler.

This helps build flexible routes for things like user profiles, products, or posts.

Practice

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

easy

A. An object containing route parameter values from the URL

B. A function to send responses to the client

C. A middleware to parse JSON bodies

D. A method to define static file paths

Solution

1. Step 1: Understand route parameters

   Route parameters are parts of the URL defined with a colon, like /user/:id.

2. Step 2: Accessing parameters in Express

   Express stores these dynamic values in req.params as an object with keys matching parameter names.

3. Final Answer:

   An object containing route parameter values from the URL -> Option A

4. Quick Check:

   req.params = route parameters object [OK]

Hint: Remember: req.params holds URL dynamic parts [OK]

Common Mistakes:

• Confusing req.params with req.body
• Thinking req.params is a function
• Using req.query instead of req.params

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

easy

A. app.get('/post/:postId', (req, res) => { ... })

B. app.get('/post/postId', (req, res) => { ... })

C. app.get('/post/?postId', (req, res) => { ... })

D. app.get('/post/*postId', (req, res) => { ... })

Solution

1. Step 1: Syntax for route parameters

   Express uses a colon : before the parameter name to mark it as dynamic.

2. Step 2: Check each option

   app.get('/post/:postId', (req, res) => { ... }) uses /:postId correctly. Options A, B, and D use incorrect syntax for parameters.

3. Final Answer:

   app.get('/post/:postId', (req, res) => { ... }) -> Option A

4. Quick Check:

   Colon before param name = correct route param syntax [OK]

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

Common Mistakes:

• Omitting the colon before parameter name
• Using query syntax instead of route params
• Using wildcard * incorrectly for parameters

3. What will be the output of the following Express route handler when a GET request is made to /user/42?

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

medium

A. User ID is undefined

B. User ID is 42

C. User ID is :id

D. Error: Cannot read property 'id' of undefined

Solution

1. Step 1: Identify the route parameter

   The route defines a parameter :id, so req.params.id will hold the value from the URL.

2. Step 2: Check the request URL and response

   The URL /user/42 sets id to '42', so the response sends 'User ID is 42'.

3. Final Answer:

   User ID is 42 -> Option B

4. Quick Check:

   req.params.id = '42' from URL [OK]

Hint: Route param value replaces :id in req.params [OK]

Common Mistakes:

• Expecting req.params to be undefined
• Using req.query instead of req.params
• Forgetting to use backticks for template literals

4. Consider this Express route:

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

What is the problem with this code?

medium

A. req.params is not accessible in GET routes

B. The route path is missing a colon before productId

C. res.send cannot send strings

D. The route parameter name and req.params key do not match

Solution

1. Step 1: Check route parameter name

   The route defines parameter productId, so req.params will have a key named productId.

2. Step 2: Check usage of req.params

   The code uses req.params.product, which does not exist, so id will be undefined.

3. Final Answer:

   The route parameter name and req.params key do not match -> Option D

4. Quick Check:

   Param name mismatch causes undefined value [OK]

Hint: Match param name exactly in req.params key [OK]

Common Mistakes:

• Using wrong key name from req.params
• Forgetting colon in route path
• Assuming req.params is always empty

5. You want to create an Express route that handles URLs like /order/123/item/456 to get both orderId and itemId. Which route definition and parameter access is correct?

hard

A. app.get('/order/:order/item/:item', (req, res) => { res.send(`Order: ${req.params.orderId}, Item: ${req.params.itemId}`); })

B. app.get('/order/:orderId/item/:itemId', (req, res) => { res.send(`Order: ${req.params.item}, Item: ${req.params.order}`); })

C. app.get('/order/:orderId/item/:itemId', (req, res) => { res.send(`Order: ${req.params.orderId}, Item: ${req.params.itemId}`); })

D. app.get('/order/:orderId/item/:itemId', (req, res) => { res.send(`Order: ${req.query.orderId}, Item: ${req.query.itemId}`); })

Solution

1. Step 1: Define route with two parameters

   The route must have :orderId and :itemId to capture both values from the URL.

2. Step 2: Access parameters correctly

   Use req.params.orderId and req.params.itemId to get the values inside the handler.

3. Final Answer:

   app.get('/order/:orderId/item/:itemId', (req, res) => { res.send(`Order: ${req.params.orderId}, Item: ${req.params.itemId}`); }) -> Option C

4. Quick Check:

   Route params and req.params keys must match [OK]

Hint: Use matching param names in route and req.params [OK]

Common Mistakes:

• Mixing req.params with req.query
• Using wrong parameter names in handler
• Defining route without all needed parameters