Route parameters in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing /user/42?

Consider this Express route:

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

What will the server respond with when a client requests /user/42?

Express

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

A User ID is 42

B Error: Cannot read property 'id' of undefined

C User ID is undefined

D User ID is :id

💡 Hint

Look at how route parameters are accessed via req.params.

The route parameter ':id' captures the part of the URL after '/user/'. It is accessible as req.params.id. So, requesting '/user/42' sets req.params.id to '42'.

📝 Syntax

intermediate

Which route definition correctly captures a product ID?

You want to define a route in Express that captures a product ID from the URL like /product/123. Which of these route definitions is correct?

A app.get('/product/:productId?', (req, res) => { ... });

B app.get('/product/productId', (req, res) => { ... });

C app.get('/product/:productId', (req, res) => { ... });

D app.get('/product/:productId*', (req, res) => { ... });

💡 Hint

Route parameters start with a colon and match one segment.

Option C correctly uses ':productId' to capture the product ID as a route parameter. Option C treats 'productId' as a literal path segment. Option C makes the parameter optional, which is not required here. Option C uses a wildcard which captures multiple segments, not just one.

🔧 Debug

advanced

Why does this route not capture the ID parameter?

Given this Express route:

app.get('/order/:id', (req, res) => { res.send(`Order ID: ${req.params.orderId}`); });

What will happen when a client requests /order/99?

Express

app.get('/order/:id', (req, res) => { res.send(`Order ID: ${req.params.orderId}`); });

A Order ID: undefined

B Order ID: 99

C Error: Cannot read property 'orderId' of undefined

D 404 Not Found

💡 Hint

Check the name used in the route parameter and the name used in req.params.

The route defines the parameter as ':id', so req.params.id contains the value. But the code tries to access req.params.orderId, which is undefined. So the output shows 'Order ID: undefined'.

❓ state_output

advanced

What is the value of req.params after requesting /blog/2023/07?

Consider this Express route:

app.get('/blog/:year/:month', (req, res) => { res.json(req.params); });

What JSON will the server send when a client requests /blog/2023/07?

Express

app.get('/blog/:year/:month', (req, res) => { res.json(req.params); });

A {}

B {"year":"2023","month":"07"}

C {"month":"07"}

D {"year":"2023/07"}

💡 Hint

Each colon-prefixed segment captures one part of the URL path.

The route has two parameters: ':year' and ':month'. The URL '/blog/2023/07' matches both, so req.params contains both keys with their values.

🧠 Conceptual

expert

Which option correctly explains route parameter behavior with optional segments?

Given this route:

app.get('/archive/:year/:month?', (req, res) => { res.send(req.params); });

Which statement is true about accessing /archive/2022 and /archive/2022/12?

A Accessing /archive/2022 sets req.params to { year: '2022', month: null } and /archive/2022/12 sets req.params to { year: '2022', month: '12' }

B Accessing /archive/2022 causes a 404 error because :month is missing

C Accessing /archive/2022 sets req.params to { year: '2022', month: undefined } and /archive/2022/12 sets req.params to { year: '2022', month: '12' }

D Accessing /archive/2022 sets req.params to { year: '2022' } and /archive/2022/12 sets req.params to { year: '2022', month: '12' }

💡 Hint

The question mark after :month makes it optional.

The question mark means the :month parameter is optional. When omitted, req.params does not include the month key at all. When present, it is included with its value.

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