304 Not Modified
The client sent a conditional GET request (with If-Modified-Since or If-None-Match), and the server determined that the resource hasn't changed. The client can use its cached copy. This is a browser caching optimization — no body is sent, saving bandwidth.
What can I do?
- Not an error — it means your browser is loading a cached version (faster and efficient).
How to debug & fix
- Return ETag and/or Last-Modified headers on all cacheable GET responses
- Check If-None-Match and If-Modified-Since request headers before generating full responses
- Use strong ETags (content hash) for precise cache validation
- Confirm your cache validation logic is consistent across load-balanced instances; stale ETags cause cache misses.
Code Example
app.get('/data', (req, res) => {
const etag = getEtag(data);
if (req.headers['if-none-match'] === etag) {
return res.status(304).end();
}
res.setHeader('ETag', etag).json(data);
});Related Status Codes
How HTTP Status Codes Work
Every HTTP response carries a three-digit status code that tells the client — browser, API consumer, or search-engine crawler — exactly what happened. The first digit defines the class: 1xx informational (request in progress), 2xx success, 3xx redirection, 4xx client error (bad request, missing auth, not found), and 5xx server failure.
Status codes are standardised in RFC 9110 (HTTP Semantics, 2022). Extensions like WebDAV (RFC 4918) and rate-limit headers (RFC 6585) added codes beyond the core set. When a client receives an unrecognised code, the rule is to treat it as the generic x00 of its class.
Why the Right Code Matters
Semantically correct codes help search engines index accurately (301 passes link equity; 410 removes pages faster than 404), allow API clients to implement correct retry logic (429 + Retry-After, 503 + Retry-After), and let monitoring systems distinguish bugs (500) from load issues (503) from auth failures (401/403).
Looking up a different status code? The full reference covers all HTTP codes with causes, fix guides, and copyable code examples for Node.js and Python.
Browse the full HTTP Status Code reference →