There are two critical aspects that affect the testability of a good application:

• Writing code that can be tested easily
• Having self-contained, easily reproducible tests

The first part is about structuring code so that the code business logic is isolated from dependencies such as external services, and so on. This allows mocking the dependencies about these boundaries to allow the test cases to exercise the execution flow along various interesting paths. For example, consider you are writing the flight search feature on a travel marketplace. There are two aspects to this:

• Obtaining prices for a sector from various providers
• Running some business logic to filter and sort the results

Now, it may not be possible to get various error scenarios reliably reproduced by making direct calls to the provider. Also, it might be expensive (and insecure) to give API keys to...