Separate setup, the single action under test, and assertions into clear phases for readable tests.
Tangled setup and assertions make failures hard to diagnose and intent unclear.
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A test that mixes setup, action, and verification is hard to read and debug. The Arrange-Act-Assert (AAA) pattern makes each test's intent obvious and isolates the one action being exercised.
// Bad — interleaved, unclear what's under test
expect(svc.charge(makeUser(), 100).status).toBe("ok");
// Good — AAA
const user = makeUser(); // Arrange
const result = svc.charge(user, 100); // Act
expect(result.status).toBe("ok"); // Assert