Query string parsing in Express

Introduction

Query string parsing helps you get information from the URL after the question mark. This lets your app understand what the user wants.

When you want to read filters or search terms from a URL like /products?category=books

When you need to get page numbers or limits for showing lists, like /items?page=2&limit=10

When you want to customize responses based on user choices sent in the URL

When handling form submissions sent via GET method

When building APIs that accept parameters in the URL query

Syntax

Express

app.get('/path', (req, res) => {
  const value = req.query.key;
  // use value
  res.send(`Value is ${value}`);
});

req.query is an object containing all query parameters as strings.

If a parameter is missing, its value will be undefined.

Examples

Reads the term parameter from the query string like /search?term=books

Express

app.get('/search', (req, res) => {
  const term = req.query.term;
  res.send(`Search term: ${term}`);
});

Uses a default value if page is not provided in the URL.

Express

app.get('/items', (req, res) => {
  const page = req.query.page || 1;
  res.send(`Page number: ${page}`);
});

Extracts multiple query parameters at once using object destructuring.

Express

app.get('/filter', (req, res) => {
  const { category, sort } = req.query;
  res.send(`Category: ${category}, Sort: ${sort}`);
});

Sample Program

This Express app listens on port 3000. When you visit /products?category=books&page=2, it reads the query parameters and shows them in the response.

Express

import express from 'express';

const app = express();
const port = 3000;

app.get('/products', (req, res) => {
  const category = req.query.category || 'all';
  const page = Number(req.query.page) || 1;
  res.send(`Showing products in category: ${category}, page: ${page}`);
});

app.listen(port, () => {
  console.log(`Server running on http://localhost:${port}`);
});

Important Notes

Query parameters are always strings. Convert them to numbers or booleans if needed.

Use default values to avoid errors when parameters are missing.

Express automatically parses the query string into req.query.

Summary

Query string parsing lets you read extra info from URLs.

Use req.query in Express to get query parameters as an object.

Always handle missing or wrong types by setting defaults or converting values.

Practice

1. In Express, which object lets you access the query parameters from a URL like /search?term=books?

easy

A. req.query

B. req.params

C. req.body

D. req.url

Solution

1. Step 1: Understand Express request objects

   Express provides different objects to access parts of the request: req.params for route params, req.body for POST data, and req.query for query strings.

2. Step 2: Identify query string access

   The query string is the part after ? in a URL. Express parses it into req.query as an object.

3. Final Answer:

   req.query -> Option A

4. Quick Check:

   Query string = req.query [OK]

Hint: Query strings are in req.query object in Express [OK]

Common Mistakes:

• Confusing req.params with req.query
• Trying to read query from req.body
• Using req.url directly without parsing

2. Which of the following is the correct way to read a query parameter named page in an Express route handler?

easy

A. const page = req.query.page;

B. const page = req.body.page;

C. const page = req.params.page;

D. const page = req.url.page;

Solution

1. Step 1: Recall query parameter access syntax

   Query parameters are accessed via req.query as key-value pairs.

2. Step 2: Match correct syntax

   To get the page parameter, use req.query.page. Other options access wrong objects.

3. Final Answer:

   const page = req.query.page; -> Option A

4. Quick Check:

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

Hint: Use req.query.paramName to get query parameters [OK]

Common Mistakes:

• Using req.params for query parameters
• Trying to read query from req.body
• Accessing req.url.page which is undefined

3. Given this Express route handler:

app.get('/items', (req, res) => {
  const limit = Number(req.query.limit) || 10;
  res.send(`Limit is ${limit}`);
});

What will be the response if the URL is /items?limit=5?

medium

A. "Limit is 10"

B. "Limit is undefined"

C. "Limit is 5"

D. "Limit is NaN"

Solution

1. Step 1: Convert query string value to number

   The code uses Number(req.query.limit) to convert the string '5' to number 5.

2. Step 2: Use default if conversion fails

   If conversion results in 0 or NaN, it defaults to 10. Here, 5 is valid, so limit = 5.

3. Final Answer:

   "Limit is 5" -> Option C

4. Quick Check:

   Number('5') = 5, so output = "Limit is 5" [OK]

Hint: Convert query strings to number before use [OK]

Common Mistakes:

• Not converting string to number
• Assuming req.query.limit is a number
• Ignoring default fallback logic

4. Consider this Express route:

app.get('/search', (req, res) => {
  const term = req.query.term.toLowerCase();
  res.send(term);
});

What error will occur if the URL is /search (no query parameter)?

medium

A. SyntaxError: Unexpected token

B. TypeError: Cannot read property 'toLowerCase' of undefined

C. ReferenceError: term is not defined

D. No error, response is empty string

Solution

1. Step 1: Check for missing query parameter

   If term is missing, req.query.term is undefined.

2. Step 2: Calling method on undefined causes error

   Calling toLowerCase() on undefined throws a TypeError.

3. Final Answer:

   TypeError: Cannot read property 'toLowerCase' of undefined -> Option B

4. Quick Check:

   Undefined.toLowerCase() throws TypeError [OK]

Hint: Check if query param exists before using methods [OK]

Common Mistakes:

• Assuming query param always exists
• Not handling undefined values
• Expecting empty string instead of error

5. You want to handle a query parameter sort that can be either asc or desc. If missing or invalid, default to asc. Which code snippet correctly implements this in Express?

hard

A. const sort = req.query.sort === 'asc' ? 'asc' : req.query.sort === 'desc' ? 'desc' : 'desc';

B. const sort = req.query.sort || 'asc';

C. const sort = req.query.sort === 'asc' ? 'asc' : 'desc';

D. const sort = ['asc', 'desc'].includes(req.query.sort) ? req.query.sort : 'asc';

Solution

1. Step 1: Validate query parameter value

   We must check if req.query.sort is either 'asc' or 'desc'. Using includes() on an array is a clean way.

2. Step 2: Provide default if invalid or missing

   If the value is not in the allowed list, default to 'asc'. This avoids invalid values.

3. Final Answer:

   const sort = ['asc', 'desc'].includes(req.query.sort) ? req.query.sort : 'asc'; -> Option D

4. Quick Check:

   Check allowed values with includes() and default [OK]

Hint: Use includes() to validate query param values before use [OK]

Common Mistakes:

• Not validating allowed values
• Using simple || which accepts invalid strings
• Overcomplicating with nested ternaries