req.query for query strings in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does req.query contain?

In an Express app, when a client sends a URL like /search?term=book&sort=asc, what will req.query contain?

A {"term": "book", "sort": "asc"}

B ["term", "book", "sort", "asc"]

C "term=book&sort=asc"

D undefined

💡 Hint

Think about how query strings are parsed into key-value pairs.

The req.query object contains the parsed query string parameters as key-value pairs. For the URL /search?term=book&sort=asc, req.query will be { term: 'book', sort: 'asc' }.

📝 Syntax

intermediate

Accessing a query parameter safely

Given the URL /items?category=tools, which code correctly accesses the category query parameter in Express?

Express

app.get('/items', (req, res) => {
  // Access category here
});

A const category = req.params.category;

B const category = req.query.category;

C const category = req.body.category;

D const category = req.query[0];

💡 Hint

Query parameters come from the URL after the question mark.

Query parameters are accessed via req.query. Route parameters use req.params, and POST data uses req.body.

🔧 Debug

advanced

Why is req.query.page undefined?

An Express route expects a query parameter page. The URL is /list?page=2. The code is:

app.get('/list', (req, res) => {
  const page = req.query.page;
  res.send(`Page number is ${page}`);
});

But the response shows Page number is undefined. What is the likely cause?

A The client did not send the query string properly; the URL is missing the question mark.

B The Express app is missing middleware to parse query strings.

C The query parameter name is case-sensitive and should be uppercase.

D The route path is incorrect and does not match the URL.

💡 Hint

Check the URL format carefully.

If the URL does not contain a question mark before query parameters, req.query will be empty. The URL must be /list?page=2, not /listpage=2.

❓ state_output

advanced

What is the output of this Express route?

Consider this Express route:

app.get('/filter', (req, res) => {
  const { type = 'all', limit = 10 } = req.query;
  res.json({ type, limit });
});

What will be the JSON response for the URL /filter?limit=5?

A {"type": "all", "limit": 5}

B {"type": undefined, "limit": 5}

C {"type": "all", "limit": "5"}

D {"type": "all", "limit": 10}

💡 Hint

Remember that query parameters are strings by default.

Query parameters are strings, so limit is the string "5". The default type is used because it was not provided.

🧠 Conceptual

expert

Handling multiple values for the same query key

If a client sends the URL /search?tag=js&tag=node&tag=express, what will req.query.tag contain in Express by default?

A undefined

B ["js", "node", "express"]

C "js"

D "express"

💡 Hint

Express uses a simple parser for query strings by default.

By default, Express's query parser returns the last value for repeated keys. So req.query.tag will be the string "express".

Practice

1. What does req.query represent in an Express app?

easy

A. An object containing URL query parameters sent after the question mark

B. The body data sent in a POST request

C. The headers of the HTTP request

D. The cookies sent by the client

Solution

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

   req.query holds key-value pairs from the URL query string, which comes after the question mark in a URL.

2. Step 2: Differentiate from other request parts

   Body data is accessed via req.body, headers via req.headers, and cookies via req.cookies. So, req.query is specifically for query parameters.

3. Final Answer:

   An object containing URL query parameters sent after the question mark -> Option A

4. Quick Check:

   req.query = URL query parameters [OK]

Hint: Remember: query = URL after '?' [OK]

Common Mistakes:

• Confusing query parameters with body data
• Thinking req.query holds headers or cookies
• Assuming req.query is a string instead of an object

2. Which of the following is the correct way to access a query parameter named id in Express?

easy

A. req.params.id

B. req.query.id

C. req.body.id

D. req.headers.id

Solution

1. Step 1: Identify where query parameters are stored

   Query parameters are stored in req.query, so to get the value of id, use req.query.id.

2. Step 2: Differentiate from other request properties

   req.params is for route parameters, req.body is for POST data, and req.headers is for HTTP headers.

3. Final Answer:

   req.query.id -> Option B

4. Quick Check:

   Query param 'id' = req.query.id [OK]

Hint: Query params always use req.query.[name] [OK]

Common Mistakes:

• Using req.params for query strings
• Trying to get query data from req.body
• Accessing query params from headers

3. Given this Express route handler, what will be logged if the URL is /search?term=books&page=2?

app.get('/search', (req, res) => {
  console.log(req.query.term);
  console.log(req.query.page);
  res.send('Done');
});

medium

A. term\npage

B. undefined\nundefined

C. books\n2

D. books\nundefined

Solution

1. Step 1: Understand the URL query string

   The URL has query parameters: term=books and page=2. These are accessible via req.query.term and req.query.page.

2. Step 2: Check what is logged

   The code logs req.query.term which is "books" and req.query.page which is "2" (strings).

3. Final Answer:

   books 2 -> Option C

4. Quick Check:

   req.query.term = 'books', req.query.page = '2' [OK]

Hint: Query values come as strings from req.query [OK]

Common Mistakes:

• Expecting numbers instead of strings
• Confusing query params with route params
• Assuming undefined if query exists

4. What is the problem with this Express route handler?

app.get('/items', (req, res) => {
  const page = req.query.page;
  if (page > 0) {
    res.send(`Page number is ${page}`);
  } else {
    res.send('Invalid page');
  }
});

medium

A. res.send cannot be called inside an if statement

B. req.query.page is undefined, causing a crash

C. The route should use req.params instead of req.query

D. req.query.page is a string, so numeric comparison may fail

Solution

1. Step 1: Identify the type of req.query.page

   Query parameters are strings by default, so req.query.page is a string, e.g., "1".

2. Step 2: Understand comparison behavior

   Comparing a string with a number using > can lead to unexpected results because string comparison is lexicographic. The code should convert page to a number first.

3. Final Answer:

   req.query.page is a string, so numeric comparison may fail -> Option D

4. Quick Check:

   Convert query strings to numbers before numeric checks [OK]

Hint: Convert query strings to numbers before numeric comparisons [OK]

Common Mistakes:

• Not converting query params to numbers
• Assuming req.query.page is a number
• Using req.params instead of req.query incorrectly

5. You want to create an Express route that reads optional query parameters sort and limit. If limit is missing or not a positive number, it should default to 10. Which code snippet correctly implements this behavior?

hard

A. app.get('/data', (req, res) => {
const sort = req.query.sort || 'asc';
const limit = Number(req.query.limit) > 0 ? Number(req.query.limit) : 10;
res.send(`Sort: ${sort}, Limit: ${limit}`);
});

B. app.get('/data', (req, res) => {
const sort = req.query.sort;
const limit = req.query.limit || 10;
res.send(`Sort: ${sort}, Limit: ${limit}`);
});

C. app.get('/data', (req, res) => {
const sort = req.query.sort ?? 'desc';
const limit = parseInt(req.query.limit) || 10;
res.send(`Sort: ${sort}, Limit: ${limit}`);
});

D. app.get('/data', (req, res) => {
const sort = req.query.sort || 'desc';
const limit = req.query.limit > '0' ? req.query.limit : 10;
res.send(`Sort: ${sort}, Limit: ${limit}`);
});

Solution

1. Step 1: Default sort parameter

   Use req.query.sort || 'asc' to default falsy/missing values to 'asc'.

2. Step 2: Validate limit as positive number

   Number(req.query.limit) > 0 ? Number(req.query.limit) : 10 properly converts, checks positivity (>0), and defaults for missing/undefined/NaN/zero/negative.

3. Step 3: Why others fail

   B: No sort default; limit || 10 accepts invalid like '-1'. C: parseInt(req.query.limit) || 10 accepts negatives ('-1' is truthy). D: req.query.limit > '0' does string comparison, accepts invalid like 'a' ("a" > "0").

4. Final Answer:

   Number(req.query.limit) > 0 ? Number(req.query.limit) : 10 -> Option A

5. Quick Check:

   Convert and validate query params before use [OK]

Hint: Always convert and validate query params before use [OK]

Common Mistakes:

• Not converting query params to numbers
• Using || default with parseInt causing wrong defaults
• Comparing strings directly to numbers