MongoDB refuses Connection on Windows 10 machine - node.js

Question

i'm running a Node.js app on my Windows 10 machine. I have Mongoose installed and setup like this:
// Init Database
var mongoose = require("mongoose");
var mongoDB = "mongodb://127.0.0.1/sampledata"
mongoose.connect(mongoDB);
var db = mongoose.connection;
// Check DB connection
db.once("open", function(){
console.log("Connected to MongoDB");
});
// Check for DB errors
db.on("error", function(err){
console.log(err);
});
Now, when I start the server with nodemon, the following error appears in the console:
{ MongoNetworkError: failed to connect to server [127.0.0.1:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]
at Pool.<anonymous> (C:\Users\...\node_modules\mongodb-core\lib\topologies\server.js:564:11)
at emitOne (events.js:96:13)
at Pool.emit (events.js:188:7)
at Connection.<anonymous> (C:\Users\...\node_modules\mongodb-core\lib\connection\pool.js:317:12)
at Connection.g (events.js:291:16)
at emitTwo (events.js:106:13)
at Connection.emit (events.js:191:7)
at Socket.<anonymous> (C:\...\node_modules\mongodb-core\lib\connection\connection.js:246:50)
at Socket.g (events.js:291:16)
at emitOne (events.js:96:13)
at Socket.emit (events.js:188:7)
at emitErrorNT (net.js:1278:8)
at _combinedTickCallback (internal/process/next_tick.js:74:11)
at process._tickCallback (internal/process/next_tick.js:98:9)
name: 'MongoNetworkError',
errorLabels: [ 'TransientTransactionError' ]
}
When I try to start mongo over the console, I get the following error:
C:\Program Files\MongoDB\Server\4.0\bin>mongo
MongoDB shell version v4.0.2
connecting to: mongodb://127.0.0.1:27017
2018-10-03T19:05:24.194+0200 E QUERY [js] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed: SocketException: Error connecting to 127.0.0.1:27017 :: caused by :: Es konnte keine Verbindung hergestellt werden, da der Zielcomputer die Verbindung verweigerte (in english: A connection couldn't be established, because the target computer has refused the connection). :
connect#src/mongo/shell/mongo.js:257:13
#(connect):1:6
exception: connect failed
I've never encountered this error before and as stupid as it sounds, yesterday everything worked fine. Haven't changed the code or anything, so it must be an issue with my computer I guess.
Do you guys know what could cause this error?

Answer 1

TL;DR
Start mongod server
mongod
Open another terminal window
Start mongo shell
mongo
Longer Version:
Open Command prompt as an administrator and run netstat -anb, check which port mongodb is listening to (Usually 27017). If you dont see a listing for mongo then it means that mongod is not running, so go ahead and start mongod.exe from the mongodb binaries(or if environment variables are set, run mongod from anywhere on your cmd).
if this doesnt work, your firewall is probably blocking access on that port, so check that out

Answer 2

I was getting this error as well. In my case, I was trying to upgrade to MongoDB 4.0 without first upgrading to 3.6.
Please see https://docs.mongodb.com/manual/release-notes/4.0/#upgrade-procedures if you recently updated MongoDB.