Test Master

Unit test writing guidelines and patterns for creating testable, maintainable, and effective tests.

Quick Reference

Principle	Core Idea
Testable Code	Make functions testable before writing tests
Clear Use Cases	Tests should document function behavior
Efficient Coverage	Boundary values > redundant middle values
Maintainability	Tests shouldn't break on unrelated changes

Workflow Routing

Intent	Workflow
Create new test file	workflows/create-test.md
Improve existing test	workflows/improve-test.md

Core Principles

1. Testable Code First

Before writing tests, ensure the function is testable:

•Separate time dependencies → Pass time as parameter
•Separate API calls → Extract business logic to pure functions
•Separate runtime dependencies → Isolate browser APIs

typescript

// ❌ Hard to test
const formatTime = (data) => {
  return { ...data, createdAt: new Date().toISOString() };
};

// ✅ Testable
const formatTime = (data, currentTime: Date) => {
  return { ...data, createdAt: currentTime.toISOString() };
};

2. Tests as Documentation

Test names should clearly document use cases:

typescript

// ❌ Vague
it('calculates rate', () => { ... });

// ✅ Clear use case
it('applies 20% discount for VIP customers', () => { ... });

3. Efficient Coverage

Focus on boundary values, not redundant cases:

typescript

// ❌ Redundant (25, 30, 35, 40 all test same logic)
it('25세는 성인이다', ...);
it('30세는 성인이다', ...);

// ✅ Efficient (boundary values: 19, 20)
it('경계값 20세는 성인이다', ...);
it('경계값 19세는 미성년자이다', ...);

4. List Testing Pattern

For arrays: test 0, 1, 2 items only:

•0: Empty array handling
•1: Basic logic works
•2: Loop/iteration works

Key Conventions

File Structure

code

src/utils/format-date.ts → src/utils/format-date.test.ts

Use .test.ts not .spec.ts for consistency.

Test Structure (Given-When-Then)

typescript

describe(calculateTax.name, () => {
  test("applies 10% tax rate for regular income", () => {
    // given
    const income = 50000;
    const taxRate = 0.1;

    // when
    const result = calculateTax(income, taxRate);

    // then
    expect(result.taxAmount).toBe(5000);
  });
});

Factory Pattern for Mock Data

typescript

// Use factory for test data
const vipUser = userFactory.build({
  membershipLevel: "VIP", // Explicit test point
  discountRate: 0.2,
});

References

Topic	Reference
Testable code design	references/unit-test-standard.md
Test conventions	references/unit-test-convention.md
Time-dependent tests	references/time-test.md
Parameterized tests	references/parameterized-test.md

Anti-Patterns

Anti-Pattern	Problem	Solution
`as any`, `@ts-ignore`	Hides type errors	Fix types properly
Empty catch `catch(e) {}`	Silences errors	Handle or rethrow
Inline large snapshots	Poor readability	Use helper functions
DOM tests in unit tests	Slow, fragile	Test pure logic only
Testing implementation	Breaks on refactor	Test behavior