Conditional requests handling in Express - Step-by-Step Execution

Concept Flow - Conditional requests handling

Client sends request with If-None-Match or If-Modified-Since header

↓

Server checks resource's ETag or Last-Modified date

↓

Resource not changed

↓

Respond 304 Not Modified

↓

Client receives response

The server checks if the client's cached version matches the current resource using headers, then decides to send the resource or a 304 status.

Execution Sample

Express

app.get('/data', (req, res) => {
  const etag = '12345';
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.set('ETag', etag).send('Fresh data');
  }
});

This Express route sends a 304 status if the client's ETag matches, otherwise sends fresh data with an ETag.

Execution Table

Step	Request Header	ETag Check	Action	Response Status	Response Body
1	No If-None-Match header	No match	Send data with ETag	200	"Fresh data"
2	If-None-Match: 12345	Match	Send 304 Not Modified	304	No body
3	If-None-Match: 67890	No match	Send data with ETag	200	"Fresh data"
4	If-None-Match: 12345	Match	Send 304 Not Modified	304	No body

💡 Requests end after sending either 304 or 200 response based on ETag match.

Variable Tracker

Variable	Start	Request 1	Request 2	Request 3	Request 4
req.headers['if-none-match']	undefined	undefined	"12345"	"67890"	"12345"
etag	"12345"	"12345"	"12345"	"12345"	"12345"
ETag match?	N/A	No	Yes	No	Yes
Response status	N/A	200	304	200	304

Key Moments - 2 Insights

Why does the server send a 304 status instead of the data sometimes?

When the client's If-None-Match header matches the server's ETag, the server knows the client has the latest data and sends 304 to avoid resending.

What happens if the client sends no If-None-Match header?

The server treats it as no match and sends the full data with a fresh ETag, as shown in step 1 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the response status at step 2 when If-None-Match is '12345'?

A 200

B 304

C 404

D 500

💡 Hint

Check row 2 under Response Status in execution_table.

At which request does the server send fresh data because the ETag does not match?

A Request 2

B Request 4

C Request 3

D Request 1

💡 Hint

Look at ETag match? and Response status columns in variable_tracker for requests.

If the server changes the ETag to '67890', what would happen when the client sends If-None-Match: '12345'?

A Server sends 200 with fresh data

B Server sends 304 Not Modified

C Server sends 404 Not Found

D Server ignores the header

💡 Hint

Compare the ETag with the If-None-Match header to decide response status.

Concept Snapshot

Conditional Requests Handling in Express:
- Client sends If-None-Match or If-Modified-Since headers.
- Server compares with current ETag or Last-Modified.
- If match, respond 304 Not Modified (no body).
- If no match, respond 200 with fresh data and ETag.
- Saves bandwidth by avoiding resending unchanged data.

Full Transcript

In Express, conditional requests help save bandwidth by letting the server check if the client's cached data is still fresh. The client sends headers like If-None-Match with an ETag value. The server compares this with its current ETag. If they match, the server sends a 304 Not Modified status without the data. If they don't match or the header is missing, the server sends the full data with a fresh ETag. This process repeats for each request, ensuring clients only download data when it changes.