req.query for query strings in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is req.query in Express?

req.query is an object in Express that holds the query string parameters sent in the URL of a GET request.

beginner

How do you access a query parameter named name in Express?

You use req.query.name to get the value of the name parameter from the URL query string.

beginner

Example URL: /search?term=books&sort=asc. How does req.query look?

req.query will be an object: { term: 'books', sort: 'asc' }.

beginner

What type of data does req.query contain?

req.query contains all query parameters as strings inside an object. Even numbers or booleans come as strings.

intermediate

Can req.query handle multiple values for the same key?

Yes, if the URL has repeated keys like ?tag=js&tag=node, req.query.tag will be an array: ['js', 'node'].

What does req.query represent in Express?

A An object containing URL query parameters

B The body of a POST request

C The headers of the HTTP request

D The cookies sent by the client

req.query holds the query string parameters from the URL, like ?key=value.

How do you access the value of ?page=3 in Express?

A <code>req.headers.page</code>

B <code>req.query.page</code>

C <code>req.body.page</code>

D <code>req.params.page</code>

Query parameters are accessed via req.query.

If the URL is /items?color=red&color=blue, what is req.query.color?

A 'red'

B undefined

C 'blue'

D ['red', 'blue']

Repeated query keys become arrays in req.query.

What data type are values inside req.query?

A Strings

B Numbers

C Booleans

D Objects

All query values come as strings by default.

Which HTTP method usually sends data accessible via req.query?

A PUT

B POST

C GET

D DELETE

Query strings are mostly used with GET requests.

Explain how req.query works in Express and how you use it to get query string values.

Think about how URLs have ?key=value and how Express reads that.

Describe a real-life example where you might use req.query in a web app.

Imagine a shopping site where you filter products by category or price.

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