req.params for route parameters in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does req.params contain in this route?

Consider this Express route:

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

What will be the output if a client requests /user/42?

Express

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

A {"user":"42"}

B {"params":"42"}

C {}

D {"id":"42"}

💡 Hint

Look at how the route defines the parameter name after the colon.

The req.params object contains key-value pairs where keys are the names defined in the route path. Here, :id means req.params will have a key id with the value from the URL.

📝 Syntax

intermediate

Which route correctly captures two parameters in req.params?

You want to capture both category and itemId from the URL. Which route definition is correct?

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

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

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

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

💡 Hint

Parameters are separated by slashes, not commas or ampersands.

Express route parameters are defined by prefixing names with a colon and separating them by slashes. Option A correctly uses /:category/:itemId.

🔧 Debug

advanced

Why does req.params not contain expected values?

Given this code:

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

What happens when a client requests /post/123?

Express

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

A The response is undefined because req.params.post does not exist.

B The response is '123' as expected.

C The server throws a SyntaxError.

D The response is an empty object {}.

💡 Hint

Check the exact parameter name used in the route and in the code.

The route defines the parameter as postId, but the code tries to access req.params.post. Since post is not defined, it returns undefined.

❓ state_output

advanced

What is the value of req.params for this route and URL?

Route:

app.get('/files/:fileName.:ext', (req, res) => { res.send(req.params); });

URL requested: /files/report.pdf
What does req.params contain?

Express

app.get('/files/:fileName.:ext', (req, res) => { res.send(req.params); });

A {"fileName":"report","ext":".pdf"}

B {"fileName":"report.pdf"}

C {"fileName":"report","ext":"pdf"}

D {}

💡 Hint

Express supports dot-separated parameters in routes.

The route uses :fileName.:ext to capture the file name and extension separately. For /files/report.pdf, fileName is 'report' and ext is 'pdf'.

🧠 Conceptual

expert

Which statement about req.params is true?

Choose the correct statement about req.params in Express routes.

A <code>req.params</code> contains key-value pairs for all URL query parameters.

B <code>req.params</code> only contains parameters defined in the route path with colons.

C <code>req.params</code> is always an empty object unless middleware is used.

D <code>req.params</code> merges route parameters and body parameters automatically.

💡 Hint

Think about how Express separates route parameters from query and body data.

req.params holds only the parameters defined in the route path using colon syntax. Query parameters are in req.query, and body data is in req.body.

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