Bluebird - Promise.resolve doesn't work but return works

Question

I have this function which returns a Promise

save(data) => {
    return mymongo.insert(data).then(
        (result) => { 
           console.log('hello mongo');
           console.log(result);
           return Promise.resolve(result); 
        }
    )
}

in my router I call that function

app.post('/printers', (req, res, next) => {
    save(req.body).then(
        (result) => {
            console.log('hello') // is not called, nothing is printed
            console.log(result) // is not called
            res.send(result)
        }
    ).catch(res.send);
});

then with curl I do the request

curl -v -d '{ ... }' -X POST http://localhost:8080/printers

It is pretty fast and looks fine

*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 8080 (#0)
> POST /printers HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.52.1
> Accept: */*
> Content-Length: 35
> Content-Type: application/x-www-form-urlencoded
> 
* upload completely sent off: 35 out of 35 bytes
< HTTP/1.1 200 OK
< X-Powered-By: Express
< Content-Type: application/json; charset=utf-8
< Content-Length: 2
< ETag: W/"2-vyGp6PvFo4RvsFtPoIWeCReyIC8"
< Date: Fri, 21 Apr 2017 01:01:30 GMT
< Connection: keep-alive
< 
* Curl_http_done: called premature == 0
* Connection #0 to host localhost left intact

so no timeout but no response and in console I see only the logs from save()

hello mongo
{ .... }

when I replace return Promise.resolve(result) with return result then I receive the response from the server and console.log(result) gets executed and in my console I see

hello mongo
{ .... }
hello
{ .... }

Answer 1

This may be a problem with the mongodb implementation of promises, since the return value in the insert(...).then callback is treated by that implementation. I don't find any reference that it is Promise/A+ compliant, and your code seems to prove it is not.

If your concern is to use the Promise API of your liking (probably bluebird in your case), then do the conversion at an earlier stage, so you are not dependent on mongodb's then implementation:

save(data) => {
    return Promise.resolve(mymongo.insert(data)).then(
        (result) => { 
           console.log('hello mongo');
           console.log(result);
           return result;
        }
    )
}