Why understanding req matters in Express

Introduction

Understanding req helps you get information from the user's request. This lets your app respond correctly.

When you want to read data sent by a user in a form.

When you need to check what URL or path the user asked for.

When you want to get details like cookies or headers from the user's browser.

When you want to handle different types of requests like GET or POST.

When you want to read query parameters from the URL.

Syntax

Express

app.get('/path', (req, res) => {
  // use req to get info
  const data = req.body;
  const query = req.query;
  const params = req.params;
  // respond
  res.send('Got your data');
});

req stands for request and holds all info sent by the user.

You use req.body for form data, req.query for URL queries, and req.params for URL parts.

Examples

Get a user ID from the URL part and send it back.

Express

app.get('/user/:id', (req, res) => {
  const userId = req.params.id;
  res.send(`User ID is ${userId}`);
});

Read a search term from the URL query and respond with it.

Express

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

Read form data sent by POST and greet the user.

Express

app.post('/submit', (req, res) => {
  const name = req.body.name;
  res.send(`Hello, ${name}`);
});

Sample Program

This program starts a server that listens for POST requests at /greet. It reads the name from the request body and replies with a greeting. If no name is sent, it says Hello, Guest!

Express

import express from 'express';
const app = express();
app.use(express.json());

app.post('/greet', (req, res) => {
  const name = req.body.name || 'Guest';
  res.send(`Hello, ${name}!`);
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Always use middleware like express.json() to read JSON data from req.body.

Check if the data exists in req before using it to avoid errors.

Understanding req helps you make your app respond to what users send.

Summary

req holds all info sent by the user to your server.

You use req.params, req.query, and req.body to get different parts of the request.

Knowing how to use req helps your app understand and respond to users correctly.

Practice

1. In Express, what does the req object represent?

easy

A. Information sent by the user to the server

B. The server's response to the user

C. A database connection

D. A middleware function

Solution

1. Step 1: Understand the role of req in Express

   The req object contains all the information sent by the user to the server, such as URL parameters, query strings, and request body.

2. Step 2: Differentiate req from other objects

   The server's response is handled by res, not req. Database connections and middleware are separate concepts.

3. Final Answer:

   Information sent by the user to the server -> Option A

4. Quick Check:

   req = user info [OK]

Hint: Remember: req = request from user [OK]

Common Mistakes:

• Confusing req with res (response)
• Thinking req is a database connection
• Mixing req with middleware functions

2. Which of the following is the correct way to access URL parameters in Express?

easy

A. req.body.params

B. req.params

C. req.query.params

D. req.url.params

Solution

1. Step 1: Recall how Express stores URL parameters

   Express stores URL parameters in req.params, which holds key-value pairs from the route.

2. Step 2: Check other options for correctness

   req.body holds POST data, req.query holds query strings, and req.url is a string, not an object with params.

3. Final Answer:

   req.params -> Option B

4. Quick Check:

   URL params = req.params [OK]

Hint: URL params are always in req.params [OK]

Common Mistakes:

• Using req.body to get URL params
• Confusing query strings with URL params
• Trying to access params from req.url string

3. What will be logged by this Express route handler?

app.get('/user/:id', (req, res) => {
  console.log(req.query.name);
  res.send('Done');
});
// Request URL: /user/123?name=Alice

medium

A. undefined

B. 123

C. Alice

D. Error

Solution

1. Step 1: Identify what req.query.name accesses

   req.query holds query string parameters from the URL, so req.query.name gets the value of the 'name' query parameter.

2. Step 2: Match the request URL query

   The URL has ?name=Alice, so req.query.name is 'Alice'.

3. Final Answer:

   Alice -> Option C

4. Quick Check:

   Query param 'name' = Alice [OK]

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

Common Mistakes:

• Confusing req.params with req.query
• Expecting URL param instead of query param
• Assuming req.query is undefined if not in route

4. Identify the error in this Express route handler:

app.post('/submit', (req, res) => {
  const data = req.query.body;
  res.send(data);
});

medium

A. Using req.query instead of req.body to access POST data

B. Missing res.end() after res.send()

C. Route should be app.get, not app.post

D. req.body is not available without middleware

Solution

1. Step 1: Check how POST data is accessed

   POST data is sent in the request body and accessed via req.body, not req.query.

2. Step 2: Identify the mistake in the code

   The code incorrectly tries to get POST data from req.query.body, which is wrong; it should be req.body.

3. Final Answer:

   Using req.query instead of req.body to access POST data -> Option A

4. Quick Check:

   POST data = req.body, not req.query [OK]

Hint: POST data is in req.body, not req.query [OK]

Common Mistakes:

• Accessing POST data from req.query
• Forgetting to use body-parser middleware
• Confusing HTTP methods and data locations

5. You want to create an Express route that responds differently based on a URL parameter type and a query parameter sort. Which code correctly accesses both parameters?

hard

A. app.get('/items/:type', (req, res) => { const t = req.body.type; const s = req.body.sort; res.send(`${t} sorted by ${s}`); });

B. app.get('/items/:type', (req, res) => { const t = req.query.type; const s = req.params.sort; res.send(`${t} sorted by ${s}`); });

C. app.get('/items', (req, res) => { const t = req.params.type; const s = req.query.sort; res.send(`${t} sorted by ${s}`); });

D. app.get('/items/:type', (req, res) => { const t = req.params.type; const s = req.query.sort; res.send(`${t} sorted by ${s}`); });

Solution

1. Step 1: Identify where URL and query parameters are stored

   URL parameters like type are in req.params, and query parameters like sort are in req.query.

2. Step 2: Check each option for correct usage

   app.get('/items/:type', (req, res) => { const t = req.params.type; const s = req.query.sort; res.send(`${t} sorted by ${s}`); }); correctly uses req.params.type and req.query.sort. app.get('/items/:type', (req, res) => { const t = req.query.type; const s = req.params.sort; res.send(`${t} sorted by ${s}`); }); swaps them incorrectly. app.get('/items', (req, res) => { const t = req.params.type; const s = req.query.sort; res.send(`${t} sorted by ${s}`); }); misses the URL param in route. app.get('/items/:type', (req, res) => { const t = req.body.type; const s = req.body.sort; res.send(`${t} sorted by ${s}`); }); wrongly uses req.body which is for POST data.

3. Final Answer:

   req.params.type and req.query.sort -> Option D

4. Quick Check:

   URL param = req.params, query param = req.query [OK]

Hint: URL params in req.params, query params in req.query [OK]

Common Mistakes:

• Swapping req.params and req.query
• Missing URL param in route path
• Using req.body for GET parameters