When is mocking useful?

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• TDD - Test Driven Development

• Write application agains API that is not ready yet or not controlled by you.

• Replace a complex object with a simpler one.

• Isolate parts of the system to test them on their own.

• Speed up tests (e.g. eliminate remote calls, eliminate database calls).

• Simulate cases that are hard to replicate. (What if the other system fails?)

• Unit tests.