on { useUnifiedTopology: true } to the MongoClient constructor. code example

Example: DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.

mongoose.connect(mongoConnectionString, {useNewUrlParser: true, useUnifiedTopology: true});