How can I resolve this mongoose error

DAN\node_modules\mongoose\lib\connection.js:695
    throw new MongooseError('The `uri` parameter to `openUri()` must be a ' +
          ^

MongooseError: The `uri` parameter to `openUri()` must be a string, got "undefined". Make sure the first parameter to `mongoose.connect()` or `mongoose.createConnection()` is a string.
    at NativeConnection.Connection.openUri (C:\Users\bhide\Desktop\APPOINTMENT BOOKING FOR SEVASADAN\node_modules\mongoose\lib\connection.js:695:11)
    at C:\Users\bhide\Desktop\APPOINTMENT BOOKING FOR SEVASADAN\node_modules\mongoose\lib\index.js:414:10
    at C:\Users\bhide\Desktop\APPOINTMENT BOOKING FOR SEVASADAN\node_modules\mongoose\lib\helpers\promiseOrCallback.js:41:5
    at new Promise (<anonymous>)
    at promiseOrCallback (C:\Users\bhide\Desktop\APPOINTMENT BOOKING FOR SEVASADAN\node_modules\mongoose\lib\helpers\promiseOrCallback.js:40:10)
    at Mongoose._promiseOrCallback (C:\Users\bhide\Desktop\APPOINTMENT BOOKING FOR SEVASADAN\node_modules\mongoose\lib\index.js:1288:10)
    at Mongoose.connect (C:\Users\bhide\Desktop\APPOINTMENT BOOKING FOR SEVASADAN\node_modules\mongoose\lib\index.js:413:20)
    at Object.<anonymous> (C:\Users\bhide\Desktop\APPOINTMENT BOOKING FOR SEVASADAN\config\dbconfig.js:3:10)
    at Module._compile (node:internal/modules/cjs/loader:1126:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1180:10)
[nodemon] app crashed - waiting for file changes before starting...