node-express error : express deprecated res.send(status): Use res.sendStatus(status) instead

You could try this:

res.status(200).send((results[0].id).toString());

Guys are right - it doesn't allow numbers. Prooflink: http://expressjs.com/4x/api.html#res.send

This is because you are sending numeric value in the res.send.

You could send a json object or convert it to string.

(as mentioned in the comments already)

The manual states:

The body parameter can be a Buffer object, a String, an object, or an Array.

So integers aren't directly supported and need to be converted to one of those types first. For instance:

response.send(String(idTest));