Testing

• manual testing is a terrible use of resource, invest in automated testing whenever possible
• unit tests are supposed to be fast so developers can run them frequently without getting bored
• your 'test doubles' are dysfunctional if they involve writing things twice, use lightweight mocks
  • in a language without static types, even lightweight mocks are redundant as the test harness can be passed in where collaborators normally would be
• unit tests are for verifying behaviour and enforcing it
• an integration test can be used to check the parts unit tests can't reach, in particular that the service can communicate with its immediately adjacent services

Analogy

A dishwasher gets dishes clean by splashing them gently. If you tried that yourself you'd get nowhere, but the machine is relentless and thorough - that's the difference between manual and automated testing.