Error when starting the application

[adjoawadee]

I'm able to install all dependencies. After I run the application, the GraphQL playground starts but it's accompanied with an error in the log.
UnhandledPromiseRejectionWarning: MongoNetworkError: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017] at Pool.<anonymous> (C:\Users\ernestina.wadee\Documents\GitHub\advanced-graphql\node_modules\mongodb-core\lib\topologies\server.js:505:11) at emitOne (events.js:116:13) at Pool.emit (events.js:211:7) at Connection.<anonymous> (C:\Users\ernestina.wadee\Documents\GitHub\advanced-graphql\node_modules\mongodb-core\lib\connection\pool.js:329:12) at Object.onceWrapper (events.js:317:30) at emitTwo (events.js:126:13) at Connection.emit (events.js:214:7) at Socket.<anonymous> (C:\Users\ernestina.wadee\Documents\GitHub\advanced-graphql\node_modules\mongodb-core\lib\connection\connection.js:245:50) at Object.onceWrapper (events.js:315:30) at emitOne (events.js:116:13) at Socket.emit (events.js:211:7) at emitErrorNT (internal/streams/destroy.js:64:8) at _combinedTickCallback (internal/process/next_tick.js:138:11) at process._tickCallback (internal/process/next_tick.js:180:9) (node:12940) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1) (node:12940) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

[antrancs]

Make sure you have started MongoDB by running "mongod" from the terminal.