test-driven-development

Jesse Vincent ｜ 測試與驗證 ｜ 原文 ｜ MIT

先寫會失敗的測試、再寫最少程式碼，紅綠重構一路走到底

---
name: test-driven-development
description: 實作任何功能或修 bug 時，在寫實作程式碼之前使用
---

# Test-Driven Development (TDD)

## 概述

先寫測試。看著它失敗。寫最少的程式碼讓它通過。

**核心原則：** 如果你沒看過測試失敗，你就不知道它測的是不是對的東西。

**違反規則的字面規定，就是違反規則的精神。**

## 何時使用

**一律要用：**
- 新功能
- 修 bug
- 重構
- 行為變更

**例外（要問你的搭檔）：**
- 用完即丟的原型
- 產生出來的程式碼
- 設定檔

在想「就這一次跳過 TDD」嗎？停。那是合理化。

## 鐵律

```
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
```

測試之前就寫了程式碼？刪掉。重來。

**沒有例外：**
- 不要把它留著當「參考」
- 寫測試的時候不要「改編」它
- 不要去看它
- 刪掉就是刪掉

從測試重新實作。就這樣。

## Red-Green-Refactor

```dot
digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}
```

### RED - 寫會失敗的測試

寫一個最小的測試，顯示應該發生什麼。

<Good>
```typescript
test('retries failed operations 3 times', async () => {
  let attempts = 0;
  const operation = () => {
    attempts++;
    if (attempts < 3) throw new Error('fail');
    return 'success';
  };

  const result = await retryOperation(operation);

  expect(result).toBe('success');
  expect(attempts).toBe(3);
});
```
名稱清楚、測真實的行為、只測一件事
</Good>

<Bad>
```typescript
test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(3);
});
```
名稱含糊、測的是 mock，不是程式碼
</Bad>

**需求：**
- 一個行為
- 名稱清楚
- 真實的程式碼（除非不可避免，否則不要用 mock）

### 驗證 RED - 看著它失敗

**必做。絕對不能跳過。**

```bash
npm test path/to/test.test.ts
```

確認：
- 測試是失敗（不是出錯）
- 失敗訊息是預期的
- 失敗是因為功能還不存在（不是打錯字）

**測試通過了？** 你測的是既有的行為。修測試。

**測試出錯？** 修好錯誤，重跑，直到它正確地失敗。

### GREEN - 最少的程式碼

寫出能讓測試通過的最簡單程式碼。

<Good>
```typescript
async function retryOperation<T>(fn: () => Promise<T>): Promise<T> {
  for (let i = 0; i < 3; i++) {
    try {
      return await fn();
    } catch (e) {
      if (i === 2) throw e;
    }
  }
  throw new Error('unreachable');
}
```
剛好夠通過
</Good>

<Bad>
```typescript
async function retryOperation<T>(
  fn: () => Promise<T>,
  options?: {
    maxRetries?: number;
    backoff?: 'linear' | 'exponential';
    onRetry?: (attempt: number) => void;
  }
): Promise<T> {
  // YAGNI
}
```
過度設計
</Bad>

不要加功能、不要重構其他程式碼，也不要在測試之外「改進」。

### 驗證 GREEN - 看著它通過

**必做。**

```bash
npm test path/to/test.test.ts
```

確認：
- 測試通過
- 其他測試仍然通過
- 輸出乾淨（沒有錯誤、警告）

**測試失敗？** 修程式碼，不要修測試。

**其他測試失敗？** 現在就修。

**「其他測試」指的是專案的整套測試，不只是你的那個檔案。** 你寫的測試全綠，不等於套件全綠。在你說這個改動做完之前，要跑專案的測試指令（單純的 `pytest`、`npm test`、`cargo test`，repo 用什麼就用什麼），即使你的任務只指名了一個測試檔。任務裡的範圍說明，限定的是交付物，不是你的驗證。那次執行顯示的任何失敗，包括不是你造成的，都要在你的報告裡點名；你看著它捲過去、卻沒提到的紅燈測試，就是因為遺漏而被偽造的報告。

### REFACTOR - 清理

只有在綠燈之後：
- 移除重複
- 改善命名
- 抽出 helper

讓測試保持綠燈。不要加行為。

### 重複

為下一個功能寫下一個會失敗的測試。

## 好的測試

| 品質 | 好 | 壞 |
|---------|------|-----|
| **最小** | 只測一件事。名稱裡有「and」？拆開。 | `test('validates email and domain and whitespace')` |
| **清楚** | 名稱描述行為 | `test('test1')` |
| **顯示意圖** | 展示想要的 API | 模糊了程式碼應該做什麼 |

寫或改任何測試時，讀 [writing-good-tests.md](writing-good-tests.md)，裡面有讓測試保持誠實的規則：
- 寫測試之前，先說出哪個 production 變更會讓這個測試失敗
- 斷言真實的行為，絕對不要斷言 mock 的行為
- 只給測試用的程式碼放在測試工具裡，不要放進 production 的 class
- mock 一個依賴之前，先理解它的副作用

## 常見的合理化藉口

| 藉口 | 現實 |
|--------|---------|
| 「太簡單了，不用測」 | 簡單的程式碼也會壞。寫測試只要 30 秒。 |
| 「我之後再測」 | 後來才寫的測試一寫出來就通過，這什麼都證明不了。它們可能測錯東西、測的是實作而不是行為，或漏掉你忘記的邊界情況。你從沒看過它失敗，所以你從沒證明它抓得到 bug。測試先行會逼出那次失敗。 |
| 「事後測試也達到同樣的目標（重精神、不重儀式）」 | 事後測試回答的是「這段程式碼做了什麼？」；先寫測試回答的是「這段程式碼應該做什麼？」事後寫的測試，會被你已經寫好的程式碼影響：你驗證的是你記得的情況，不是你本來會發現的情況。有覆蓋率，卻沒有測試有效的證明。 |
| 「已經手動測過了」 | 手動測試是臨時的：沒有你測過什麼的紀錄、程式碼變更時沒辦法重跑、壓力下很容易漏掉情況。「我試的時候有用」≠ 全面。自動化測試每次都用同樣的方式執行。 |
| 「刪掉做了 X 小時的東西太浪費」 | 這是沉沒成本謬誤，不管怎樣那些時間都已經花掉了。真正的選擇是：用 TDD 重寫（高信心）vs. 留著它再事後補測試（低信心、很可能有 bug）。留著你無法信任的程式碼才是浪費。 |
| 「留著當參考，先寫測試」 | 你會去改編它。那就是事後測試。刪掉就是刪掉。 |
| 「需要先探索一下」 | 可以。把探索的成果丟掉，從 TDD 開始。 |
| 「很難測 = 設計不清楚」 | 聽測試的話。難測 = 難用。 |
| 「TDD 會拖慢我」 | TDD「就是」務實的路：它在 commit 之前抓到 bug、預防回歸、讓你可以無所畏懼地重構。所謂「務實」的捷徑，意味著在正式環境除錯：更慢，不是更快。 |
| 「手動測試比較快」 | 手動測試無法證明邊界情況。每次變更你都要重測。 |
| 「既有的程式碼沒有測試」 | 你正在改進它。幫既有的程式碼補上測試。 |

## 警訊：停下來，重來

- 先寫程式碼，再寫測試
- 實作之後才寫測試
- 測試一寫出來就通過
- 說不出測試為什麼失敗
- 「之後」才加測試
- 合理化「就這一次」
- 「我已經手動測過了」
- 「事後測試也達到同樣的目的」
- 「重點是精神，不是儀式」
- 「留著當參考」或「改編既有的程式碼」
- 「已經花了 X 小時，刪掉太浪費」
- 「TDD 太教條，我是在務實」
- 「這個不一樣，因為……」

**這些全部都表示：刪掉程式碼。用 TDD 重來。**

## 範例：修 Bug

**Bug：** 空的 email 被接受了

**RED**
```typescript
test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});
```

**驗證 RED**
```bash
$ npm test
FAIL: expected 'Email required', got undefined
```

**GREEN**
```typescript
function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}
```

**驗證 GREEN**
```bash
$ npm test
PASS
```

**REFACTOR**
需要的話，把驗證抽出來給多個欄位用。

## 驗證檢查表

標記工作完成之前：

- [ ] 每個新的函式／方法都有測試
- [ ] 實作之前，看過每個測試失敗
- [ ] 每個測試都是因為預期的原因失敗（功能不存在，不是打錯字）
- [ ] 為每個測試寫了最少的程式碼讓它通過
- [ ] 所有測試通過
- [ ] 輸出乾淨（沒有錯誤、警告）
- [ ] 測試使用真實的程式碼（除非不可避免，否則不要用 mock）
- [ ] 邊界情況和錯誤都有涵蓋

勾不滿所有的框？你跳過了 TDD。重來。

## 卡住的時候

| 問題 | 解法 |
|---------|----------|
| 不知道怎麼測 | 寫出你希望有的 API。先寫斷言。問你的搭檔。 |
| 測試太複雜 | 設計太複雜。簡化 interface。 |
| 什麼都得 mock | 程式碼耦合太緊。用 dependency injection。 |
| 測試的 setup 很龐大 | 抽出 helper。還是很複雜？簡化設計。 |

## 與除錯的整合

發現 bug 了？寫出會失敗、能重現它的測試。照著 TDD 循環走。測試既證明修正，也預防回歸。

沒有測試，絕對不要修 bug。

## 最終規則

```
Production code → test exists and failed first
Otherwise → not TDD
```

沒有你的搭檔的許可，不得有例外。

---
name: test-driven-development
description: Use when implementing any feature or bugfix, before writing implementation code
---

# Test-Driven Development (TDD)

## Overview

Write the test first. Watch it fail. Write minimal code to pass.

**Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing.

**Violating the letter of the rules is violating the spirit of the rules.**

## When to Use

**Always:**
- New features
- Bug fixes
- Refactoring
- Behavior changes

**Exceptions (ask your human partner):**
- Throwaway prototypes
- Generated code
- Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

## The Iron Law

```
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
```

Write code before the test? Delete it. Start over.

**No exceptions:**
- Don't keep it as "reference"
- Don't "adapt" it while writing tests
- Don't look at it
- Delete means delete

Implement fresh from tests. Period.

## Red-Green-Refactor

```dot
digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}
```

### RED - Write Failing Test

Write one minimal test showing what should happen.

<Good>
```typescript
test('retries failed operations 3 times', async () => {
  let attempts = 0;
  const operation = () => {
    attempts++;
    if (attempts < 3) throw new Error('fail');
    return 'success';
  };

  const result = await retryOperation(operation);

  expect(result).toBe('success');
  expect(attempts).toBe(3);
});
```
Clear name, tests real behavior, one thing
</Good>

<Bad>
```typescript
test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(3);
});
```
Vague name, tests mock not code
</Bad>

**Requirements:**
- One behavior
- Clear name
- Real code (no mocks unless unavoidable)

### Verify RED - Watch It Fail

**MANDATORY. Never skip.**

```bash
npm test path/to/test.test.ts
```

Confirm:
- Test fails (not errors)
- Failure message is expected
- Fails because feature missing (not typos)

**Test passes?** You're testing existing behavior. Fix test.

**Test errors?** Fix error, re-run until it fails correctly.

### GREEN - Minimal Code

Write simplest code to pass the test.

<Good>
```typescript
async function retryOperation<T>(fn: () => Promise<T>): Promise<T> {
  for (let i = 0; i < 3; i++) {
    try {
      return await fn();
    } catch (e) {
      if (i === 2) throw e;
    }
  }
  throw new Error('unreachable');
}
```
Just enough to pass
</Good>

<Bad>
```typescript
async function retryOperation<T>(
  fn: () => Promise<T>,
  options?: {
    maxRetries?: number;
    backoff?: 'linear' | 'exponential';
    onRetry?: (attempt: number) => void;
  }
): Promise<T> {
  // YAGNI
}
```
Over-engineered
</Bad>

Don't add features, refactor other code, or "improve" beyond the test.

### Verify GREEN - Watch It Pass

**MANDATORY.**

```bash
npm test path/to/test.test.ts
```

Confirm:
- Test passes
- Other tests still pass
- Output pristine (no errors, warnings)

**Test fails?** Fix code, not test.

**Other tests fail?** Fix now.

**"Other tests" means the project's suite, not just your file.** A
green run of the test you wrote is not a green suite. Before you call
the change done, run the project's test command (bare `pytest`,
`npm test`, `cargo test` — whatever the repo uses) even when your task
named only one test file. A scope statement in your task bounds the
deliverable, not your verification. Any failure that run shows —
including one you didn't cause — goes in your report by name; a red
test you watched scroll past and didn't mention is a report falsified
by omission.

### REFACTOR - Clean Up

After green only:
- Remove duplication
- Improve names
- Extract helpers

Keep tests green. Don't add behavior.

### Repeat

Next failing test for next feature.

## Good Tests

| Quality | Good | Bad |
|---------|------|-----|
| **Minimal** | One thing. "and" in name? Split it. | `test('validates email and domain and whitespace')` |
| **Clear** | Name describes behavior | `test('test1')` |
| **Shows intent** | Demonstrates desired API | Obscures what code should do |

When writing or changing any test, read [writing-good-tests.md](writing-good-tests.md) for the rules that keep tests honest:
- Name the production change that would make the test fail — before writing it
- Assert on real behavior, never on mock behavior
- Keep test-only code in test utilities, out of production classes
- Understand a dependency's side effects before mocking it

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests written after pass immediately — which proves nothing. They may test the wrong thing, test the implementation instead of the behavior, or miss the edge case you forgot. You never watched it fail, so you never proved it can catch the bug. Test-first forces that failure. |
| "Tests after achieve same goals (spirit not ritual)" | Tests-after answer "what does this do?"; tests-first answer "what should this do?" Tests written after are biased by the code you already wrote — you verify the cases you remembered, not the ones you'd have discovered. Coverage without proof the tests work. |
| "Already manually tested" | Manual testing is ad-hoc: no record of what you covered, no way to re-run it when the code changes, easy to forget cases under pressure. "Worked when I tried it" ≠ comprehensive. Automated tests run the same way every time. |
| "Deleting X hours is wasteful" | Sunk cost fallacy — that time is already spent either way. The real choice: rewrite with TDD (high confidence) vs. keep it and bolt tests on after (low confidence, likely bugs). Keeping code you can't trust is the waste. |
| "Keep as reference, write tests first" | You'll adapt it. That's testing after. Delete means delete. |
| "Need to explore first" | Fine. Throw away exploration, start with TDD. |
| "Test hard = design unclear" | Listen to test. Hard to test = hard to use. |
| "TDD will slow me down" | TDD IS the pragmatic path: catches bugs before commit, prevents regressions, lets you refactor without fear. "Pragmatic" shortcuts mean debugging in production — slower, not faster. |
| "Manual test faster" | Manual doesn't prove edge cases. You'll re-test every change. |
| "Existing code has no tests" | You're improving it. Add tests for existing code. |

## Red Flags - STOP and Start Over

- Code before test
- Test after implementation
- Test passes immediately
- Can't explain why test failed
- Tests added "later"
- Rationalizing "just this once"
- "I already manually tested it"
- "Tests after achieve the same purpose"
- "It's about spirit not ritual"
- "Keep as reference" or "adapt existing code"
- "Already spent X hours, deleting is wasteful"
- "TDD is dogmatic, I'm being pragmatic"
- "This is different because..."

**All of these mean: Delete code. Start over with TDD.**

## Example: Bug Fix

**Bug:** Empty email accepted

**RED**
```typescript
test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});
```

**Verify RED**
```bash
$ npm test
FAIL: expected 'Email required', got undefined
```

**GREEN**
```typescript
function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}
```

**Verify GREEN**
```bash
$ npm test
PASS
```

**REFACTOR**
Extract validation for multiple fields if needed.

## Verification Checklist

Before marking work complete:

- [ ] Every new function/method has a test
- [ ] Watched each test fail before implementing
- [ ] Each test failed for expected reason (feature missing, not typo)
- [ ] Wrote minimal code to pass each test
- [ ] All tests pass
- [ ] Output pristine (no errors, warnings)
- [ ] Tests use real code (mocks only if unavoidable)
- [ ] Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

## When Stuck

| Problem | Solution |
|---------|----------|
| Don't know how to test | Write wished-for API. Write assertion first. Ask your human partner. |
| Test too complicated | Design too complicated. Simplify interface. |
| Must mock everything | Code too coupled. Use dependency injection. |
| Test setup huge | Extract helpers. Still complex? Simplify design. |

## Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

## Final Rule

```
Production code → test exists and failed first
Otherwise → not TDD
```

No exceptions without your human partner's permission.