principle-test-behavior-not-implementation

测行为，不测实现

A test calls the code the way its users do and asserts the result they observe against a literal expected value. A test that asserts which calls the code made, or restates a constant the code contains, does neither.

测试按用户方式调用代码，并用字面期望值断言他们观察到的结果。断言代码做了哪些调用，或复述代码里某个常量的测试，两样都没做。

The check: before you keep a test, ask whether it would still pass if every function it imports returned undefined. If yes, it observes no behavior and cannot fail for a defect. Rewrite the assertion or delete the test.

自检：保留测试前，问若它 import 的每个函数都返回 undefined，测试是否仍过。是，就观察不到行为，也不会因缺陷失败。改断言或删测试。

Why: A test that cannot fail for a defect costs CI time and review attention and catches nothing. A constant pin also fails when someone edits the constant or the prompt it restates, so it prevents that edit.

为什么： 不能因缺陷失败的测试白耗 CI 时间和审阅注意力，什么也抓不到。钉死常量也会在有人改常量或它复述的 prompt 时失败，从而阻止那次编辑。

Five shapes that still pass when every imported function returns undefined:

在每个 import 函数返回 undefined 时仍会过的五种形状：

• Weak or no assertion. No expect, or only toBeDefined, toBeTruthy, not.toThrow, toBeInstanceOf, toBeGreaterThan(0).

• Mock or absence only. Only toHaveBeenCalled, not.toHaveBeenCalled, toBeUndefined, toEqual([]), toHaveLength(0), not.toBe(wrongValue).

• Self-referential. The expected value comes from the code under test: expect(f(a)).toBe(f(a)), expect(parsed.url).toBe(buildUrl(...)).

• Constant pin. The assertion restates a hand-maintained constant, config default, table row, or prompt string: expect(LIMITS.maxTools).toBe(8), expect(PROMPT).toContain("You are").

• Fixture asserts fixture. The assertion reads data the test built or a value computed in beforeEach, and the subject never runs inside the body.

• 弱断言或无断言。 没有 expect，或只有 toBeDefined、toBeTruthy、not.toThrow、toBeInstanceOf、toBeGreaterThan(0)。

• 只测 mock 或缺失。 只有 toHaveBeenCalled、not.toHaveBeenCalled、toBeUndefined、toEqual([])、toHaveLength(0)、not.toBe(wrongValue)。

• 自指。 期望值来自被测代码：expect(f(a)).toBe(f(a))、expect(parsed.url).toBe(buildUrl(...))。

• 钉死常量。 断言复述手维护常量、配置默认值、表行或 prompt 字符串：expect(LIMITS.maxTools).toBe(8)、expect(PROMPT).toContain("You are")。

• fixture 断言 fixture。 断言读的是测试自己建的数据或 beforeEach 算出的值，被测主体在 body 里根本没跑。

The fix: call the subject inside the test body with one concrete input and assert the literal output or the observable effect, expect(slugify("Hello, World!")).toBe("hello-world"). For an absence, assert the presence on the other input in the same test. For a constant, test the mechanism that reads it with one input instead of restating the value. For a mock, assert the payload it received or the state after the call, not that it was called. When no such assertion exists, delete the test.

修法： 在测试 body 里用一个具体输入调用主体，断言字面输出或可观察效果，如 expect(slugify("Hello, World!")).toBe("hello-world")。测缺失时，同测里断言另一输入上的存在。测常量时，测读取它的机制加一个输入，别复述值。测 mock 时，断言它收到的载荷或调用后状态，别断言「被调用过」。没有这种断言就删测试。

Keep a test of a relation across a table’s rows (a key present in two tables, a parent that exists), and a compile-time check in a *.test-d.ts file.

保留跨表行关系的测试（两表都有的 key、存在的 parent），以及 *.test-d.ts 里的编译期检查。