req.query for query strings in Express - Mini Project: Build & Apply

Using req.query to Read Query Strings in Express

📖 Scenario: You are building a simple Express server that responds to requests with personalized messages based on query strings in the URL.

🎯 Goal: Learn how to use req.query in Express to read query string parameters and use them in your response.

📋 What You'll Learn

Create an Express app with a GET route at /greet

Use req.query to read the name parameter from the URL

Send a greeting message that includes the name value

Add a default greeting if name is not provided

💡 Why This Matters

🌍 Real World

Web servers often use query strings to get information from users without needing forms. For example, search pages or filters use query strings.

💼 Career

Understanding how to read query strings with req.query is essential for backend developers working with Express to build APIs and web applications.

1

Set up Express app and route

Create an Express app by requiring express and calling express(). Then create a GET route at /greet with a callback function that takes req and res as parameters.

Express

const express = require('express');
const app = express();

// Your code here

Hint

Remember to require Express and create an app instance before defining routes.

2

Read the name query parameter

Inside the /greet route callback, create a variable called name and set it to req.query.name to read the name query string parameter.

Express

const express = require('express');
const app = express();

app.get('/greet', (req, res) => {
  // Create a variable called name and assign req.query.name
  // Your code here
});

Hint

Use req.query to access query string parameters by name.

3

Send a greeting message using the name parameter

Still inside the /greet route, use res.send() to send a message that says Hello, {name}! if name exists. If name is missing, send Hello, guest! instead.

Express

const express = require('express');
const app = express();

app.get('/greet', (req, res) => {
  const name = req.query.name;
  // Send greeting message using res.send()
  // Your code here
});

Hint

Use an if statement to check if name exists and send the appropriate message.

4

Start the server on port 3000

Add app.listen(3000) to start the Express server on port 3000.

Express

const express = require('express');
const app = express();

app.get('/greet', (req, res) => {
  const name = req.query.name;
  if (name) {
    res.send(`Hello, ${name}!`);
  } else {
    res.send('Hello, guest!');
  }
});

// Start the server on port 3000
// Your code here

Hint

Use app.listen(3000) to start the server on port 3000.

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