Route parameters in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are route parameters in Express?

Route parameters are parts of the URL that act as placeholders for values. They let you capture values from the URL and use them in your code.

beginner

How do you define a route parameter in an Express route?

You define a route parameter by adding a colon (:) before a name in the route path, like '/user/:id'. This means 'id' is a parameter you can access.

beginner

How do you access route parameters inside an Express route handler?

You access route parameters using 'req.params'. For example, if your route is '/user/:id', you get the id with 'req.params.id'.

intermediate

What happens if you define multiple route parameters in Express?

You can define multiple parameters like '/user/:userId/book/:bookId'. Each parameter is available in 'req.params' by its name, e.g., 'req.params.userId' and 'req.params.bookId'.

intermediate

Can route parameters be optional in Express? How?

Yes, by adding a question mark (?) after the parameter name, like '/user/:id?'. This means the parameter is optional and the route matches with or without it.

How do you define a route parameter named 'postId' in Express?

A /post/?postId

B /post/postId

C /post/:postId

D /post/*postId

Route parameters are defined with a colon before the name, like '/post/:postId'.

Where do you find the value of a route parameter inside an Express route handler?

A req.body

B req.params

C req.query

D req.headers

Route parameters are stored in 'req.params' as key-value pairs.

What does the route '/user/:id?' mean in Express?

A The 'id' parameter is required

B The route matches only if 'id' is a string

C The route matches only if 'id' is a number

D The 'id' parameter is optional

Adding '?' after a parameter name makes it optional.

If you have a route '/product/:category/:id', how do you access the 'category' parameter?

A req.params.category

B req.query.category

C req.body.category

D req.headers.category

Route parameters are accessed via 'req.params' by their names.

Which of these is NOT a valid route parameter definition in Express?

A /item/itemId

B /item/:itemId/details

C /item/:itemId

D /item/:itemId?

Without a colon, 'itemId' is treated as a fixed path segment, not a parameter.

Explain how to create and use route parameters in Express routes.

Think about how URLs can carry data like user IDs.

Describe how to make a route parameter optional and why you might want to do that.

Optional means the URL can have that part or skip it.

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