Writing Tests

Core Principle

Tests prove behavior works. A test that can't fail is worthless. A test that tests mocks instead of real code is theater.

Writing Good Tests

One behavior per test

Each test should verify exactly one thing. If the test name needs "and" in it, split it into two tests.

Good:  "creates user with valid email"
Good:  "rejects user with duplicate email"
Bad:   "creates user and sends welcome email and updates counter"

Derive test cases from three sources