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Unit Test 實踐守則 (四) — 如何寫出一個好的 Unit Test

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Summary

Jack Yu’s Part 4 of a unit test best practices series (in Traditional Chinese). Covers the 3A Pattern (Arrange-Act-Assert) and good unit test naming conventions, with examples in both C# and JavaScript (describe-it). Translated from/informed by “Unit Testing: Principles, Practices, and Patterns” (Vladimir Khorikov).

Jack Yu 單元測試最佳實踐系列第 4 部分（繁體中文）。涵蓋 3A 模式（Arrange-Act-Assert）和良好的單元測試命名規範，有 C# 和 JavaScript（describe-it）示例。譯自/基於《單元測試的藝術》（Vladimir Khorikov）。

Key Points

  • 3A Pattern: Arrange (準備 — setup data, mocks, expected results), Act (行動 — typically ONE line calling the behavior under test), Assert (驗證 — verify outcome)
  • Arrange: don’t expose implementation logic here (no 1+1 for expected values, no if-else in test code)
  • Act: should be exactly one line; multiple lines suggest either: hard to diagnose failures, or poor encapsulation in production code
  • Assert: may have multiple assertions for Unit of Behavior tests (vs. single result for Unit of Code tests); assert outcomes, not implementation details (e.g., “login succeeded” not “hash was called”)
  • Test naming guidelines (from Khorikov):
    1. No rigid naming rules
    2. Name as if describing a scenario to a domain expert (not a programmer)
    3. Separate words with underscores
  • Naming evolution example: IsDeliveryValid_InvalidDate_ReturnsFalse → Delivery_with_past_date_is_invalid
  • describe-it (JS): use nested describe-it with human-readable descriptions following the same guidelines; hierarchical grouping improves readability

Insights

The instruction “don’t expose implementation logic in Arrange” is subtle but important: if expectedResult = hashPassword(input) appears in Arrange, the test will break when you refactor the implementation even if the behavior is still correct. Tests should be behavioral contracts, not implementation mirrors. The “Act = one line” rule is a useful smell detector for overly complex production code. The naming evolution from IsDeliveryValid_InvalidDate_ReturnsFalse to Delivery_with_past_date_is_invalid nicely demonstrates how progressively more business-language names make tests self-documenting.

Connections

Raw Excerpt

如何寫出一個好的 Unit Test：在此範例中看到了 arrange, act 以及 assert, 這是著名的 3A pattern。不能暴露程式的實作邏輯在這裡。

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