req.headers for HTTP headers in Express - Step-by-Step Execution

Concept Flow - req.headers for HTTP headers

Client sends HTTP request

↓

Express receives request

↓

req object created

↓

req.headers populated with HTTP headers

↓

Access req.headers in route handler

↓

Use header values for logic or response

When Express gets a request, it creates req.headers with all HTTP headers sent by the client. You can read these headers inside your route handler.

Execution Sample

Express

app.get('/', (req, res) => {
  console.log(req.headers);
  res.send('Headers logged');
});

This code logs all HTTP headers from the incoming request to the console.

Execution Table

Step	Action	req.headers content	Result
1	Client sends GET / with headers: Host, User-Agent, Accept	{host: 'localhost:3000', 'user-agent': 'Mozilla/5.0', accept: 'text/html'}	Express receives request with headers
2	Express creates req object and fills req.headers	{host: 'localhost:3000', 'user-agent': 'Mozilla/5.0', accept: 'text/html'}	req.headers ready for use
3	Route handler runs and logs req.headers	{host: 'localhost:3000', 'user-agent': 'Mozilla/5.0', accept: 'text/html'}	Headers printed to console
4	Response sent with message 'Headers logged'	N/A	Client receives response
5	No more steps	N/A	Request handling complete

💡 Request handled fully; req.headers accessed and response sent

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
req.headers	undefined	{host: 'localhost:3000', 'user-agent': 'Mozilla/5.0', accept: 'text/html'}	{host: 'localhost:3000', 'user-agent': 'Mozilla/5.0', accept: 'text/html'}	Same as after Step 3

Key Moments - 2 Insights

Why does req.headers contain all headers in lowercase keys?

Express converts all header names to lowercase in req.headers for consistency, as shown in the execution_table rows where keys like 'User-Agent' appear as 'user-agent'.

Can I modify req.headers to change client headers?

No, req.headers is read-only for the incoming request. Modifying it won't affect the client or the request itself, as the execution_table shows it only being read.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2. What does req.headers contain?

A Only the Host header

B An empty object

C An object with all HTTP headers sent by the client

D A string of headers

💡 Hint

Check the 'req.headers content' column at Step 2 in the execution_table

At which step does the route handler log the headers?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for the action mentioning logging req.headers in the execution_table

If the client sends a new header 'X-Test: 123', how would req.headers change?

A It would replace all headers with only 'x-test'

B It would include 'x-test': '123' in req.headers

C It would ignore the new header

D It would cause an error

💡 Hint

Refer to how req.headers contains all client headers in the execution_table rows

Concept Snapshot

req.headers holds all HTTP headers from the client request.
Keys are lowercase strings.
Access it inside route handlers via req.headers.
It is read-only and reflects client-sent headers.
Use it to read info like user-agent, cookies, or custom headers.

Full Transcript

When a client sends an HTTP request to an Express server, Express creates a request object named req. This object includes a property called headers, which contains all the HTTP headers sent by the client. These headers are stored as key-value pairs with lowercase keys for consistency. Inside your route handler, you can access req.headers to read these headers. For example, you might log them or use them to customize your response. The req.headers object is read-only and reflects exactly what the client sent. This process happens automatically when Express receives the request and before your route handler runs.