var doc = db.collection('cities').doc('SF');

var observer = doc.onSnapshot(docSnapshot => {
  console.log(`Received doc snapshot: ${docSnapshot}`);
  // ...
}, err => {
  console.log(`Encountered error: ${err}`);
});

function getTest(req, res, done){

    let docs = [];
    var docu = fBase.collection('test');

    var observer = docu.onSnapshot(docSnapshot => {
        docSnapshot.forEach(doc => {
            docs.push(doc.data());
            console.log(docs);
        });

        res.json({response: docs});

    }, err => {
        console.log(`Encountered error: ${err}`);
    });

}

_http_outgoing.js:470
    throw new ERR_HTTP_HEADERS_SENT('set');
    ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:470:11)
........