MongoError: failed to connect to server [localhost:27017] on first connect [Mong oError: connect ECONNREFUSED 127.0.0.1:27017] at Pool. (D:\2020\vue后台管理\node-elm\node_modules_mongodb-core @2.1.20@mongodb-core\lib\topologies\server.js:336:35) at Pool.emit (events.js:198:13) at Connection. (D:\2020\vue后台管理\node-elm\node_modules_mongod b-core@2.1.20@mongodb-core\lib\connection\pool.js:280:12) at Object.onceWrapper (events.js:286:20) at Connection.emit (events.js:198:13) at Socket. (D:\2020\vue后台管理\node-elm\node_modules_mongodb-co re@2.1.20@mongodb-core\lib\connection\connection.js:189:49) at Object.onceWrapper (events.js:286:20) at Socket.emit (events.js:198:13) at emitErrorNT (internal/streams/destroy.js:91:8) at emitErrorAndCloseNT (internal/streams/destroy.js:59:3) at process._tickCallback (internal/process/next_tick.js:63:19) [nodemon] app crashed - waiting for file changes before starting...