Validating route params and query in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing /user/42 with valid params?

Consider this Express route that validates a numeric user ID from the route params and a 'role' query parameter. What will the server respond with when the URL is /user/42?role=admin?

Express

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

app.get('/user/:id', (req, res) => {
  const id = Number(req.params.id);
  const role = req.query.role;
  if (isNaN(id) || id <= 0) {
    return res.status(400).send('Invalid user ID');
  }
  if (!role || (role !== 'admin' && role !== 'user')) {
    return res.status(400).send('Invalid role');
  }
  res.send(`User ID: ${id}, Role: ${role}`);
});

A Invalid user ID

B User ID: 42, Role: admin

C Invalid role

D Internal Server Error

💡 Hint

Check how the route extracts and validates params and query values.

The route converts the 'id' param to a number and checks if it's positive. It also checks if the 'role' query is either 'admin' or 'user'. Since 42 is valid and 'admin' is allowed, the response is the formatted string.

📝 Syntax

intermediate

Which option causes a syntax error in route param validation?

Which of the following Express route handlers contains a syntax error when trying to validate a numeric route param 'id'?

Express

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

A app.get('/item/:id', (req, res) => { const id = Number(req.params.id); if (id < 0) { res.send('Invalid'); } else { res.send('Valid'); } });

B app.get('/item/:id', (req, res) => { const id = Number(req.params.id); if (isNaN(id)) res.send('Invalid'); else res.send('Valid'); });

C app.get('/item/:id', (req, res) => { const id = Number(req.params.id); if isNaN(id) { res.send('Invalid'); } else { res.send('Valid'); } });

D app.get('/item/:id', (req, res) => { const id = Number(req.params.id); if (!id) res.send('Invalid'); else res.send('Valid'); });

💡 Hint

Look closely at the syntax of the if statement.

Option C misses parentheses around the condition in the if statement, causing a syntax error.

❓ state_output

advanced

What is the response when query param is missing?

Given this Express route that expects a 'page' query parameter as a positive integer, what will the server respond with when the URL is /search (no query params)?

Express

app.get('/search', (req, res) => {
  const page = Number(req.query.page);
  if (!page || page < 1) {
    return res.status(400).send('Page query param required and must be positive');
  }
  res.send(`Page number: ${page}`);
});

A Page query param required and must be positive

B Page number: 0

C Page number: NaN

D Internal Server Error

💡 Hint

Consider what happens when req.query.page is undefined.

When no 'page' query param is provided, Number(undefined) is NaN, which is falsy, so the condition triggers the 400 response.

🔧 Debug

advanced

Why does this route always return 'Invalid ID' even for valid numeric IDs?

Examine this Express route. It should accept numeric IDs but always returns 'Invalid ID'. What is the cause?

Express

app.get('/product/:id', (req, res) => {
  const id = req.params.id;
  if (typeof id !== 'number') {
    return res.send('Invalid ID');
  }
  res.send(`Product ID: ${id}`);
});

A The id variable is not declared properly

B The route path is incorrect and does not match requests

C The res.send call is missing parentheses

D req.params.id is always a string, so typeof id !== 'number' is always true

💡 Hint

Check the type of route params in Express.

Route params in Express are always strings. The check typeof id !== 'number' fails because id is never a number.

🧠 Conceptual

expert

Which validation approach correctly handles optional query params with defaults?

You want to validate an optional 'limit' query parameter that defaults to 10 if missing or invalid. Which code snippet correctly implements this in Express?

A

const limit = parseInt(req.query.limit) || 10;
if (limit <= 0) return res.status(400).send('Limit must be positive');

B

const limit = req.query.limit ?? 10;
if (typeof limit !== 'number') return res.status(400).send('Limit must be a number');

C

const limit = Number(req.query.limit);
if (!limit) limit = 10;
if (limit < 1) return res.status(400).send('Limit must be positive');

D

const limit = Number(req.query.limit) || 10;
if (limit < 1) return res.status(400).send('Limit must be positive');

💡 Hint

Consider how to parse and default numeric query params safely.

Option A uses parseInt to convert the string to a number, defaults to 10 if falsy, and checks if limit is positive. This handles missing or invalid inputs correctly.