Testing

A test for place_order needs a payment gateway, an SMS client, and a tax calculator. Wire in the real ones and every test run charges a card and texts a phone number. A unit test is supposed to run in milliseconds with no network, so all three have to be swapped for something else. That “something else” comes in five kinds, each solving a different problem, and treating them as interchangeable is why “just mock it” arguments in code review go in circles. ...

A test written after the code tends to check what the code does, not what it was supposed to do. Test-driven development fixes that by flipping the order: write the test, watch it fail, then write the smallest piece of code that makes it pass. Because the test exists before the code it checks, it can’t quietly confirm whatever the code happens to return. Kent Beck formalized the loop as red, green, refactor in his 2002 book Test-Driven Development: By Example. The name stuck. ...

A function can have 100% unit test coverage and the page that calls it can still return a 500 in production. That is not a contradiction. Unit tests, integration tests, and end-to-end tests check different things, and a suite that’s heavy on one and light on another has a specific, predictable hole in it. What unit, integration, and end-to-end tests each check Take a sign-up feature: a function validates the input, writes a row to a database, and sends a welcome email. Three tests can cover it, and each one answers a different question. ...