HTTP 204 No Content

HTTP 204 is No Content: the request succeeded and there is deliberately no body. RFC 9110 section 15.3.5 says a 204 ends with its headers and “cannot contain content or trailers”. It also tells a browser to stay where it is: a link or form that gets a 204 back leaves the current page in place.

What it’s for

DELETE /orders/4182 HTTP/1.1

HTTP/1.1 204 No Content

• A DELETE that worked.
• A PUT or PATCH that saved, when the client already holds the new state. Send the new ETag with it: headers on a 204 still describe the resource.
• Beacons and analytics endpoints whose answer nobody reads.
• CORS preflights: the cors middleware for Express answers OPTIONS with 204 by default.

When the client needs the result back, send 200 with it. When the work will happen later, 202.

Gotchas

• response.json() throws on a 204. There’s nothing to parse. Check response.status === 204 before reading the body.
• No Content-Length. RFC 9110 section 8.6 forbids it on a 204. A body sent anyway breaks framing: on a reused HTTP/1.1 connection the client can read those bytes as the start of the next response.
• Express hides the mistake. res.status(204).json(data) strips the body and Content-Type, so the client gets nothing and no error is logged.
• An empty search result isn’t a 204. GET /orders?status=lost with no matches is a 200 with []; clients then handle one shape.
• Not for pages. Google treats a 204 as no content and may report the URL as a soft 404 in Search Console.

Caching

204 is one of the codes RFC 9110 section 15.1 makes heuristically cacheable, so a cache may store a GET that answered 204 without any Cache-Control. If an empty answer can change soon, say so with Cache-Control: no-cache or no-store.

In your stack

res.status(204).end();                      // Express; res.sendStatus(204) works too

location = /beacon { return 204; }

return "", 204                              # Flask
return Response(status_code=204)            # FastAPI
return HttpResponse(status=204)             # Django

w.WriteHeader(http.StatusNoContent) // a Write after this returns http.ErrBodyNotAllowed