res.set for response headers in Express - Mini Project: Build & Apply

Setting Response Headers with res.set in Express

📖 Scenario: You are building a simple Express server that sends back a greeting message. To make your response more informative and secure, you want to add custom HTTP headers.

🎯 Goal: Learn how to use res.set in Express to add custom response headers before sending the response.

📋 What You'll Learn

Create an Express app with a single GET route at '/'

Use res.set to add a custom header 'X-Custom-Header' with value 'HelloHeader'

Add another header 'Content-Type' with value 'text/plain'

Send a plain text response 'Welcome to the header demo!'

💡 Why This Matters

🌍 Real World

Web servers often need to send custom headers for security, caching, or metadata. Using <code>res.set</code> in Express helps control these headers easily.

💼 Career

Backend developers use Express to build APIs and web servers. Knowing how to set response headers is essential for controlling client-server communication.

1

Create the Express app and basic route

Create a variable called express that requires the 'express' module. Then create a variable called app by calling express(). Finally, add a GET route at path '/' with a callback function that takes req and res as parameters.

Express

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

// Your code here

Hint

Use require('express') to import Express and express() to create the app. Use app.get to define the route.

2

Add custom headers using res.set

Inside the GET route callback, use res.set to add a header 'X-Custom-Header' with value 'HelloHeader'. Also add a header 'Content-Type' with value 'text/plain'.

Express

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

app.get('/', (req, res) => {
  // Use res.set to add headers here
});

Hint

Call res.set twice with the exact header names and values.

3

Send the plain text response

After setting the headers, use res.send to send the text 'Welcome to the header demo!' as the response body.

Express

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

app.get('/', (req, res) => {
  res.set('X-Custom-Header', 'HelloHeader');
  res.set('Content-Type', 'text/plain');
  // Send response here
});

Hint

Use res.send with the exact string to send the response.

4

Start the Express server

Add a line to start the server by calling app.listen on port 3000 with a callback function that logs 'Server running on port 3000'.

Express

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

app.get('/', (req, res) => {
  res.set('X-Custom-Header', 'HelloHeader');
  res.set('Content-Type', 'text/plain');
  res.send('Welcome to the header demo!');
});

// Start the server here

Hint

Use app.listen with port 3000 and a callback that logs the message.

Practice

1. What does res.set do in an Express response?

easy

A. It sets or updates HTTP headers before sending the response.

B. It sends the response body to the client.

C. It closes the server connection immediately.

D. It logs request details to the console.

Solution

1. Step 1: Understand the purpose of res.set

   res.set is used to add or modify HTTP headers in the response before sending it.

2. Step 2: Differentiate from other response methods

   Sending the body is done by res.send, not res.set. Logging or closing connections are unrelated.

3. Final Answer:

   It sets or updates HTTP headers before sending the response. -> Option A

4. Quick Check:

   res.set sets headers [OK]

Hint: Remember: res.set changes headers, not body or connection [OK]

Common Mistakes:

• Confusing res.set with res.send
• Thinking res.set sends the response
• Assuming res.set closes the connection

2. Which of the following is the correct syntax to set a single header using res.set?

easy

A. res.set('Content-Type', application/json);

B. res.set('Content-Type', 'application/json');

C. res.set({Content-Type: 'application/json'});

D. res.set('Content-Type' => 'application/json');

Solution

1. Step 1: Recall correct method signature

   The correct syntax for setting one header is res.set(name, value) where both are strings.

2. Step 2: Check each option

   res.set('Content-Type', 'application/json'); uses correct syntax with two string arguments. res.set('Content-Type' => 'application/json'); uses invalid arrow syntax. res.set({Content-Type: 'application/json'}); uses invalid object syntax - unquoted key Content-Type (hyphen requires quotes). res.set('Content-Type', application/json); misses quotes around value.

3. Final Answer:

   res.set('Content-Type', 'application/json'); -> Option B

4. Quick Check:

   Two string args for single header [OK]

Hint: Use two strings for single header: name and value [OK]

Common Mistakes:

• Using arrow => instead of comma
• Passing unquoted values
• Confusing object syntax for single header

3. What will be the value of the Content-Type header after this code runs?

res.set({
  'Content-Type': 'text/html',
  'Cache-Control': 'no-cache'
});
res.set('Content-Type', 'application/json');

medium

A. Undefined (header not set)

B. 'text/html'

C. 'no-cache'

D. 'application/json'

Solution

1. Step 1: Understand header setting order

   First, Content-Type is set to 'text/html' via object. Then it is overwritten by res.set('Content-Type', 'application/json').

2. Step 2: Determine final header value

   The last set value for Content-Type is 'application/json', so that is the final header value.

3. Final Answer:

   'application/json' -> Option D

4. Quick Check:

   Last header set wins [OK]

Hint: Later res.set calls overwrite previous headers [OK]

Common Mistakes:

• Assuming first set value stays
• Confusing other headers with Content-Type
• Thinking headers merge values

4. Identify the error in this Express code snippet:

app.get('/data', (req, res) => {
  res.send('Hello');
  res.set('X-Custom-Header', '123');
});

medium

A. Headers must be set before sending the response body.

B. res.set cannot set custom headers.

C. res.send should be called twice to set headers.

D. No error; code works fine.

Solution

1. Step 1: Check order of res.set and res.send

   Headers must be set before calling res.send because sending the body finalizes the response.

2. Step 2: Identify the mistake

   Here, res.send is called first, then res.set, which is too late to add headers.

3. Final Answer:

   Headers must be set before sending the response body. -> Option A

4. Quick Check:

   Set headers before send [OK]

Hint: Always set headers before sending response body [OK]

Common Mistakes:

• Setting headers after res.send
• Thinking res.set can't set custom headers
• Calling res.send multiple times

5. You want to set multiple headers including Content-Type and Cache-Control in one call, but only if a condition is true. Which code correctly uses res.set with condition?

hard

A. res.set({'Content-Type': isJson ? 'application/json' : 'text/html'}); res.set('Cache-Control', 'no-store');

B. res.set('Content-Type', isJson ? 'application/json' : 'text/html', 'Cache-Control', 'no-store');

C. if (isJson) res.set({'Content-Type': 'application/json', 'Cache-Control': 'no-store'});

D. res.set('Content-Type', 'application/json' && 'Cache-Control', 'no-store');

Solution

1. Step 1: Understand how to set multiple headers conditionally

   Using an object with res.set allows setting many headers at once. Wrapping in an if statement applies condition.

2. Step 2: Evaluate each option

   if (isJson) res.set({'Content-Type': 'application/json', 'Cache-Control': 'no-store'}); correctly uses an if condition and sets multiple headers in one object. res.set('Content-Type', isJson ? 'application/json' : 'text/html', 'Cache-Control', 'no-store'); has invalid syntax with multiple pairs in one call. res.set({'Content-Type': isJson ? 'application/json' : 'text/html'}); res.set('Cache-Control', 'no-store'); splits headers into two calls but is valid; however, question asks for one call. res.set('Content-Type', 'application/json' && 'Cache-Control', 'no-store'); uses invalid logical operator in arguments.

3. Final Answer:

   if (isJson) res.set({'Content-Type': 'application/json', 'Cache-Control': 'no-store'}); -> Option C

4. Quick Check:

   if + res.set(object) for conditional multiple [OK]

Hint: Use object syntax inside if to set multiple headers [OK]

Common Mistakes:

• Passing multiple header pairs as separate arguments
• Using logical operators inside res.set arguments
• Setting headers outside condition