How to programmatically send a 404 response with Express/Node?

Updated Answer for Express 4.x

Rather than using res.send(404) as in old versions of Express, the new method is:

res.sendStatus(404);

Express will send a very basic 404 response with "Not Found" text:

HTTP/1.1 404 Not Found
X-Powered-By: Express
Vary: Origin
Content-Type: text/plain; charset=utf-8
Content-Length: 9
ETag: W/"9-nR6tc+Z4+i9RpwqTOwvwFw"
Date: Fri, 23 Oct 2015 20:08:19 GMT
Connection: keep-alive

Not Found

Since Express 4.0, there's a dedicated sendStatus function:

res.sendStatus(404);

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If you're using an earlier version of Express, use the status function instead.

res.status(404).send('Not found');