Status codes and headers in NestJS - Performance & Optimization

Performance: Status codes and headers

MEDIUM IMPACT

This affects how quickly the browser understands the server response and how efficiently it processes caching and content rendering.

Sending HTTP responses with correct status codes and caching headers

NestJS

res.status(200).set({ 'Cache-Control': 'public, max-age=3600' }).send(data);

Correct status codes and caching headers reduce redundant network requests and speed up content loading.

📈 Performance GainReduces network round-trips and improves LCP by enabling browser caching

Sending HTTP responses with correct status codes and caching headers

NestJS

res.status(200).send(data); // No caching headers or incorrect status code for errors

Missing or incorrect status codes and headers cause browsers to re-fetch resources unnecessarily or misinterpret response meaning.

📉 Performance CostIncreases network requests and delays LCP due to lack of caching and error handling

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No caching headers, incorrect status codes	N/A	0	0	[X] Bad
Correct status codes with caching headers	N/A	0	0	[OK] Good

Rendering Pipeline

Status codes and headers are processed early by the browser to decide how to handle the response, affecting caching, rendering, and error handling.

→Network

→Cache Lookup

→Rendering

⚠️ BottleneckNetwork round-trips caused by missing or incorrect caching headers

Core Web Vital Affected

LCP

This affects how quickly the browser understands the server response and how efficiently it processes caching and content rendering.

Optimization Tips

1Always use correct HTTP status codes to communicate response meaning.

2Set Cache-Control headers to enable efficient browser caching.

3Avoid sending unnecessary headers that increase response size.

Performance Quiz - 3 Questions

How do correct HTTP status codes affect web performance?

A They increase the size of the response, slowing down loading.

B They help browsers understand response meaning and avoid unnecessary retries.

C They only affect server logs and have no impact on performance.

D They force browsers to ignore caching headers.

DevTools: Network

How to check: Open DevTools > Network tab, reload the page, click a resource, and inspect the Status and Response Headers sections.

What to look for: Look for correct HTTP status codes (200, 304, 404, etc.) and caching headers like Cache-Control or Expires to confirm efficient response handling.

Practice

1. In NestJS, what does the HTTP status code 201 usually indicate in a response?

easy

A. A new resource was successfully created

B. The request was malformed or invalid

C. The requested resource was not found

D. The server encountered an internal error

Solution

1. Step 1: Understand HTTP status codes

   HTTP status codes communicate the result of a client's request. Code 201 means "Created".

2. Step 2: Match 201 with its meaning

   201 specifically means a new resource was successfully created on the server.

3. Final Answer:

   A new resource was successfully created -> Option A

4. Quick Check:

   201 = Created [OK]

Hint: 201 means resource created successfully [OK]

Common Mistakes:

• Confusing 201 with 404 (not found)
• Thinking 201 means server error
• Mixing 201 with 400 (bad request)

2. Which of the following is the correct way to set a custom header X-Custom-Header with value 123 in a NestJS controller using Express response?

easy

A. res.setHeader('X-Custom-Header', 789);

B. res.set('X-Custom-Header', '123');

C. res.header('X-Custom-Header', '789');

D. res.addHeader('X-Custom-Header', '123');

Solution

1. Step 1: Recall Express response methods

   Express response object uses res.set() to set headers.

2. Step 2: Check method and value types

   res.set() accepts header name and string value. res.set('X-Custom-Header', '123'); uses correct method and string value.

3. Final Answer:

   res.set('X-Custom-Header', '123'); -> Option B

4. Quick Check:

   Use res.set() to set headers [OK]

Hint: Use res.set() to set headers in NestJS [OK]

Common Mistakes:

• Using res.setHeader (not Express method)
• Passing number instead of string as header value
• Using non-existent res.addHeader method

3. What will be the HTTP status code and header in the response when this NestJS controller method runs?

async getData(@Res() res) {
  res.status(204).set('X-Info', 'No Content').send();
}

medium

A. Status 204 with no headers

B. Status 200 with header X-Info: No Content

C. Status 204 with header X-Info: No Content

D. Status 404 with header X-Info: No Content

Solution

1. Step 1: Analyze status and header setting

   The code sets status to 204 and header 'X-Info' to 'No Content'.

2. Step 2: Understand 204 status behavior

   Status 204 means "No Content" and allows custom headers. The header is set correctly.

3. Final Answer:

   Status 204 with header X-Info: No Content -> Option C

4. Quick Check:

   Status 204 + header set = Status 204 with header X-Info: No Content [OK]

Hint: res.status(204).set() sets status and headers [OK]

Common Mistakes:

• Assuming 204 means no headers allowed
• Confusing default status 200 with explicit 204
• Ignoring header set after status

4. Identify the error in this NestJS controller method that tries to set status and headers:

async update(@Res() res) {
  res.status(200);
  res.set('Content-Type', 'application/json');
  res.send({ success: true });
  res.status(201);
}

medium

A. Calling res.status(201) after res.send() has no effect

B. res.set() must be called before res.status()

C. res.send() cannot send JSON objects

D. res.status() cannot be called twice

Solution

1. Step 1: Review response flow

   res.send() sends the response and ends it. Any code after it won't affect response.

2. Step 2: Check status calls order

   Calling res.status(201) after res.send() does nothing because response is already sent.

3. Final Answer:

   Calling res.status(201) after res.send() has no effect -> Option A

4. Quick Check:

   res.send() ends response, later status calls ignored [OK]

Hint: Set status before res.send(), not after [OK]

Common Mistakes:

• Trying to set status after sending response
• Thinking res.send() can't send JSON
• Believing res.set() must come before res.status()

5. You want to send a 202 Accepted status with a custom header X-Request-ID and a JSON body in NestJS. Which code snippet correctly achieves this?

hard

A. res.send({ message: 'Accepted' }).status(202).set('X-Request-ID', requestId);

B. res.set('X-Request-ID', requestId).status(202).send('Accepted');

C. res.status(202).send({ message: 'Accepted' }).set('X-Request-ID', requestId);

D. res.status(202).set('X-Request-ID', requestId).json({ message: 'Accepted' });

Solution

1. Step 1: Order of method calls matters

   In Express, chaining status, set, and json works if status is set before sending.

2. Step 2: Check each option's correctness

   Options A and C call send before completing status/header setup, invalid. res.set('X-Request-ID', requestId).status(202).send('Accepted'); sets header and status but uses res.send('Accepted') sending plain text instead of JSON object. res.status(202).set('X-Request-ID', requestId).json({ message: 'Accepted' }); correctly sets status, header, sends JSON.

3. Step 3: Confirm best practice

   res.status(202).set('X-Request-ID', requestId).json({ message: 'Accepted' }); is the clearest and most common pattern: status, set header, then send JSON.

4. Final Answer:

   res.status(202).set('X-Request-ID', requestId).json({ message: 'Accepted' }); -> Option D

5. Quick Check:

   Status then headers then send JSON [OK]

Hint: Chain status, set, then send/json in that order [OK]

Common Mistakes:

• Calling send before setting status or headers
• Using send instead of json for JSON body
• Setting headers after sending response