Check Mongoose Connection Status

Mongoose connections have a readyState property that contains a number representing the current state of the connection, 0-4. These states are as follows:

• 0 = disconnected
• 1 = connected
• 2 = connecting
• 3 = disconnecting
• 4 = invalid credentials

These event names will emit when the state changes.

// Demonstrate the readyState and on event emitters
console.log(mongoose.connection.readyState); //logs 0
mongoose.connection.on('connecting', () => { 
  console.log('connecting')
  console.log(mongoose.connection.readyState); //logs 2
});
mongoose.connection.on('connected', () => {
  console.log('connected');
  console.log(mongoose.connection.readyState); //logs 1
});
mongoose.connection.on('disconnecting', () => {
  console.log('disconnecting');
  console.log(mongoose.connection.readyState); // logs 3
});
mongoose.connection.on('disconnected', () => {
  console.log('disconnected');
  console.log(mongoose.connection.readyState); //logs 0
});
// Connect to a MongoDB server running on 'localhost:27017' and use the
// 'test' database.
await mongoose.connect('mongodb://localhost:27017/test', {
  useNewUrlParser: true // Boilerplate for Mongoose 5.x
});

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