Status code conventions in Express - Step-by-Step Execution

Concept Flow - Status code conventions

Client sends HTTP request

↓

Server processes request

↓

Determine response status code

↓

Send response with status code

↓

Client receives response and acts accordingly

This flow shows how a server using Express decides and sends HTTP status codes back to the client after processing a request.

Execution Sample

Express

app.get('/user', (req, res) => {
  if (!req.query.id) {
    res.status(400).send('Bad Request');
  } else {
    res.status(200).send('User found');
  }
});

This Express route sends a 400 status if no user ID is given, otherwise sends 200.

Execution Table

Step	Condition Checked	Status Code Set	Response Sent	Explanation
1	Is req.query.id present?	No	No	Check if user ID is missing
2	req.query.id missing	400	'Bad Request'	Send 400 Bad Request because ID is missing
3	Is req.query.id present?	Yes	No	User ID is present, proceed
4	req.query.id present	200	'User found'	Send 200 OK with user found message

💡 Response sent with appropriate status code based on presence of user ID

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
req.query.id	undefined or value	undefined or value	value if present	value if present
res.statusCode	unset	400 if missing	200 if present	400 or 200 depending on condition

Key Moments - 2 Insights

Why do we use different status codes like 400 and 200?

Because 400 means the client sent a bad request (missing ID), while 200 means the request was successful. See execution_table rows 2 and 4.

What happens if we forget to set a status code?

Express defaults to 200 OK if no status is set. So the client might think the request succeeded even if it didn't.

Visual Quiz - 3 Questions

Look at the execution table, what status code is sent when req.query.id is missing?

A 404

B 400

C 200

D 500

💡 Hint

Check row 2 in the execution_table where req.query.id is missing

At which step does the server send a 200 status code?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look at the execution_table row where status code 200 is set

If we remove res.status(400), what status code will Express send when ID is missing?

A 200

B 400

C 404

D 500

💡 Hint

Recall Express defaults to 200 if no status is set (see key_moments answer 2)

Concept Snapshot

Status codes tell the client how the request went.
400 means bad request (client error).
200 means success.
Use res.status(code) before sending response.
Always send correct status for clarity.

Full Transcript

In Express, when a client sends a request, the server checks conditions like if required data is present. Based on that, it sets a status code using res.status(code). For example, if a user ID is missing, the server sends 400 Bad Request. If the ID is present, it sends 200 OK. This helps the client understand if the request succeeded or failed. If no status is set, Express defaults to 200. So always set the right status code before sending a response.

Practice

1. In Express, which status code is conventionally used to indicate a successful GET request?

easy

A. 301 Moved Permanently

B. 404 Not Found

C. 500 Internal Server Error

D. 200 OK

Solution

1. Step 1: Understand HTTP status codes for success

   The status code 200 means the request was successful and the server returned the requested data.

2. Step 2: Match the code to the GET request success

   For a successful GET request, 200 OK is the standard code to indicate success.

3. Final Answer:

   200 OK -> Option D

4. Quick Check:

   Success code for GET = 200 OK [OK]

Hint: 200 means success, use it for successful GET requests [OK]

Common Mistakes:

• Using 404 for success
• Using 500 for client errors
• Confusing 301 with success

2. Which of the following is the correct way to set a 404 status code in Express?

easy

A. res.statusCode = 404; res.send('Not Found')

B. res.sendStatus(200)

C. res.status(404).send('Not Found')

D. res.code(404).send('Not Found')

Solution

1. Step 1: Recall Express method to set status code

   Express uses res.status(code) to set the HTTP status code before sending a response.

2. Step 2: Verify correct syntax for 404

   res.status(404).send('Not Found') correctly sets status 404 and sends the message.

3. Final Answer:

   res.status(404).send('Not Found') -> Option C

4. Quick Check:

   Use res.status(code) to set status [OK]

Hint: Use res.status(code) before send() to set status [OK]

Common Mistakes:

• Using res.code() which doesn't exist
• Setting res.statusCode directly without chaining
• Using sendStatus(200) for 404

3. What status code will the following Express code send to the client?

app.get('/data', (req, res) => {
  res.status(201).send('Created');
});

medium

A. 201 Created

B. 404 Not Found

C. 200 OK

D. 500 Internal Server Error

Solution

1. Step 1: Identify the status code set in the code

   The code uses res.status(201) which sets the HTTP status code to 201.

2. Step 2: Understand the meaning of 201

   Status 201 means the request was successful and a new resource was created.

3. Final Answer:

   201 Created -> Option A

4. Quick Check:

   res.status(201) sends 201 Created [OK]

Hint: res.status(201) means resource created successfully [OK]

Common Mistakes:

• Assuming default 200 status
• Confusing 201 with 404
• Ignoring the status() call

4. You wrote this Express code but clients always get status 200 instead of 400:

app.post('/submit', (req, res) => {
  if (!req.body.name) {
    res.status(400);
    res.send('Name is required');
  }
});

What is the main problem?

medium

A. res.send() must come before res.status()

B. res.status(400) must be followed by return to stop execution

C. 400 is not a valid status code

D. res.status() does not set the status code

Solution

1. Step 1: Analyze the code flow after setting status 400

   res.status(400) sets the status but code continues to run after sending response.

2. Step 2: Understand Express response behavior

   Without return, Express may continue and send default 200 later or cause errors.

3. Final Answer:

   res.status(400) must be followed by return to stop execution -> Option B

4. Quick Check:

   Use return after res.status().send() to stop further processing [OK]

Hint: Add return after res.status().send() to prevent default 200 [OK]

Common Mistakes:

• Not returning after sending response
• Calling res.send() before res.status()
• Thinking 400 is invalid

5. You want to send a 204 No Content status after deleting a resource in Express. Which code snippet correctly does this?

hard

A. res.status(204).send()

B. res.sendStatus(204).send('Deleted')

C. res.status(204).send('Deleted')

D. res.send(204)

Solution

1. Step 1: Understand 204 No Content meaning

   204 means success but no response body should be sent.

2. Step 2: Choose code that sends 204 without content

   res.status(204).send() sends status 204 with empty body, which is correct.

3. Step 3: Identify incorrect options

   res.sendStatus(204).send('Deleted') tries to chain send() after sendStatus(), which is invalid. res.status(204).send('Deleted') sends a body with 204, which breaks the rule. res.send(204) sends 204 as body, not status.

4. Final Answer:

   res.status(204).send() -> Option A

5. Quick Check:

   204 means no content, so send empty response [OK]

Hint: Use res.status(204).send() to send no content response [OK]

Common Mistakes:

• Sending body with 204 status
• Using sendStatus(204).send()
• Sending status code as body