What makes a good unit test?

A good unit test is fast, isolated, repeatable, self-checking and readable. These are often remembered as the FIRST properties.

• Fast: it runs in milliseconds, so people run it all the time.
• Isolated: it does not depend on other tests, on test order, or on shared state.
• Repeatable: the same code gives the same result on any machine, with no dependence on the clock, the network or random values.
• Self-checking: it asserts the result itself, and nobody reads logs to decide if it passed.
• Readable: the name states the scenario and the expected outcome, for example rejects_order_with_negative_quantity, and the body shows arrange, act and assert.

It also tests one behavior, so a failure points to one cause. And it fails for the right reason: breaking the code makes it red, while refactoring that keeps behavior does not.

A good test is a short specification that fails when the behavior breaks.