Example code from the article https://www.joshwulf.com/blog/2020/02/shun-the-mutant/

testable-state-machine.js

const GRPCConnection = gRPC => url => {
  let connected = false
  const _connection = gRPC(url)
  _connection.on('connect', () => (connected = true))
  _connection.on('disconnect', () => (connected = false))
  return {
    getIsConnected(): connected,
    connect: () => connected || _connection.connect()
    send: command => _connection.send(command)
  }
}