Welcome to Writing Quality, Tested Code, the third module of the Software Engineering Fundamentals course. Good architecture from Module 2 only pays off if the code inside it stays readable and correct as it changes. You’ll start with clean code: naming, function size, and error handling, turning a tangled invoice-total function from Ledgerly into five small, well-named pieces.

From there, you’ll build a real automated safety net. You’ll learn the testing pyramid, write unit tests with test doubles instead of hitting a real payment gateway, and see what test coverage does and doesn’t guarantee. The module then adds behaviour-driven development: writing scenarios in a shared language business and engineering both understand, and the Gherkin syntax — Feature, Scenario, Background, Scenario Outline — that makes those scenarios executable. A guided project closes the module by cleaning, testing, and specifying a real subscription-pause feature end to end.

Every test in this module runs for real and passes for real. Start with Lesson 1, where you’ll clean up Ledgerly’s messiest function.