coding-standards

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Detects code smells, anti-patterns, and readability issues. Use when implementing features, reviewing code, or refactoring.
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SKILL.md

---
name: coding-standards
description: Detects code smells, anti-patterns, and readability issues. Use when implementing features, reviewing code, or refactoring.
---

# Universal Coding Standards

## Technical Anti-patterns (Red Flag Patterns)

Immediately stop and reconsider design when detecting the following patterns:

### Code Quality Anti-patterns
1. **Writing similar code 3 or more times** - Violates Rule of Three
2. **Multiple responsibilities mixed in a single file** - Violates Single Responsibility Principle (SRP)
3. **Defining same content in multiple files** - Violates DRY principle
4. **Making changes without checking dependencies** - Potential for unexpected impacts
5. **Disabling code with comments** - Should use version control
6. **Error suppression** - Hiding problems creates technical debt
7. **Excessive use of type assertions (as)** - Abandoning type safety

### Design Anti-patterns
- **"Make it work for now" thinking** - Accumulation of technical debt
- **Patchwork implementation** - Unplanned additions to existing code
- **Optimistic implementation of uncertain technology** - Designing unknown elements assuming "it'll probably work"
- **Symptomatic fixes** - Surface-level fixes that don't solve root causes
- **Unplanned large-scale changes** - Lack of incremental approach

## Basic Principles

- **Aggressive Refactoring** - Prevent technical debt and maintain health
- **No Unused "Just in Case" Code** - Violates YAGNI principle (Kent Beck)

## Comment Writing Rules

- **Function Description Focus**: Describe what the code "does"
- **No Historical Information**: Do not record development history
- **Timeless**: Write only content that remains valid whenever read
- **Conciseness**: Keep explanations to necessary minimum

## Error Handling Fundamentals

### Fail-Fast Principle
Fail quickly on errors to prevent processing continuation in invalid states. Error suppression is prohibited.

For detailed implementation methods (Result type, custom error classes, layered error handling, etc.), refer to language and framework-specific rules.

## Rule of Three - Criteria for Code Duplication

How to handle duplicate code based on Martin Fowler's "Refactoring":

| Duplication Count | Action | Reason |
|-------------------|--------|--------|
| 1st time | Inline implementation | Cannot predict future changes |
| 2nd time | Consider future consolidation | Pattern beginning to emerge |
| 3rd time | Implement commonalization | Pattern established |

### Criteria for Commonalization

**Cases for Commonalization**
- Business logic duplication
- Complex processing algorithms
- Areas likely requiring bulk changes
- Validation rules

**Cases to Avoid Commonalization**
- Accidental matches (coincidentally same code)
- Possibility of evolving in different directions
- Significant readability decrease from commonalization
- Simple helpers in test code

## Common Failure Patterns and Avoidance Methods

### Pattern 1: Error Fix Chain
**Symptom**: Fixing one error causes new errors
**Cause**: Surface-level fixes without understanding root cause
**Avoidance**: Identify root cause with 5 Whys before fixing

### Pattern 2: Abandoning Type Safety
**Symptom**: Excessive use of any type or as
**Cause**: Impulse to avoid type errors
**Avoidance**: Handle safely with unknown type and type guards

### Pattern 3: Implementation Without Sufficient Testing
**Symptom**: Many bugs after implementation
**Cause**: Ignoring Red-Green-Refactor process
**Avoidance**: Always start with failing tests

### Pattern 4: Ignoring Technical Uncertainty
**Symptom**: Frequent unexpected errors when introducing new technology
**Cause**: Assuming "it should work according to official documentation" without prior investigation
**Avoidance**:
- Record certainty evaluation at the beginning of task files
- For low certainty cases, create minimal verification code first

### Pattern 5: Insufficient Existing Code Investigation
**Symptom**: Duplicate implementations, architecture inconsistency, integration failures
**Cause**: Insufficient understanding of existing code before implementation
**Avoidance Methods**:
- Before implementation, always search for similar functionality (using domain, responsibility, configuration patterns as keywords)
- Similar functionality found -> Use that implementation (do not create new implementation)
- Similar functionality is technical debt -> Create ADR improvement proposal before implementation
- No similar functionality exists -> Implement new functionality following existing design philosophy
- Record all decisions and rationale in "Existing Codebase Analysis" section of Design Doc

## Debugging Techniques

### 1. Error Analysis Procedure
1. Read error message (first line) accurately
2. Focus on first and last of stack trace
3. Identify first line where your code appears

### 2. 5 Whys - Root Cause Analysis
```
Symptom: Build error
Why1: Type definitions don't match -> Why2: Interface was updated
Why3: Dependency change -> Why4: Package update impact
Why5: Major version upgrade with breaking changes
Root cause: Inappropriate version specification
```

### 3. Minimal Reproduction Code
To isolate problems, attempt reproduction with minimal code:
- Remove unrelated parts
- Replace external dependencies with mocks
- Create minimal configuration that reproduces problem

## Type Safety Fundamentals

**Type Safety Principle**: Use `unknown` type with type guards. `any` type disables type checking and causes runtime errors.

**any Type Alternatives (Priority Order)**
1. **unknown Type + Type Guards**: Use for validating external input
2. **Generics**: When type flexibility is needed
3. **Union Types/Intersection Types**: Combinations of multiple types
4. **Type Assertions (Last Resort)**: Only when type is certain

**Type Guard Implementation Pattern**
```typescript
function isUser(value: unknown): value is User {
  return typeof value === 'object' && value !== null && 'id' in value && 'name' in value
}
```

**Type Complexity Management**
- Field Count: Up to 20 (split by responsibility if exceeded, external API types are exceptions)
- Optional Ratio: Up to 30% (separate required/optional if exceeded)
- Nesting Depth: Up to 3 levels (flatten if exceeded)
- Type Assertions: Review design if used 3+ times
- **External API Types**: Relax constraints and define according to reality (convert appropriately internally)

## Refactoring Techniques

**Basic Policy**
- Small Steps: Maintain always-working state through gradual improvements
- Safe Changes: Minimize the scope of changes at once
- Behavior Guarantee: Ensure existing behavior remains unchanged while proceeding

**Implementation Procedure**: Understand Current State -> Gradual Changes -> Behavior Verification -> Final Validation

**Priority**: Duplicate Code Removal > Large Function Division > Complex Conditional Branch Simplification > Type Safety Improvement

## Implementation Completeness Assurance

### Required Procedure for Impact Analysis

**Completion Criteria**: Complete all 3 stages

#### 1. Discovery
```bash
Grep -n "TargetClass\|TargetMethod" -o content
Grep -n "DependencyClass" -o content
Grep -n "targetData\|SetData\|UpdateData" -o content
```

#### 2. Understanding
**Mandatory**: Read all discovered files and include necessary parts in context:
- Caller's purpose and context
- Dependency direction
- Data flow: generation -> modification -> reference

#### 3. Identification
Structured impact report (mandatory):
```
## Impact Analysis
### Direct Impact: ClassA, ClassB (with reasons)
### Indirect Impact: SystemX, ComponentY (with integration paths)
### Processing Flow: Input -> Process1 -> Process2 -> Output
```

**Important**: Do not stop at search; execute all 3 stages

### Unused Code Deletion Rule

When unused code is detected -> Will it be used?
- Yes -> Implement immediately (no deferral allowed)
- No -> Delete immediately (remains in Git history)

Target: Code, documentation, configuration files

## Red-Green-Refactor Process (Test-First Development)

**Recommended Principle**: Always start code changes with tests

**Development Steps**:
1. **Red**: Write test for expected behavior (it fails)
2. **Green**: Pass test with minimal implementation
3. **Refactor**: Improve code while maintaining passing tests

**NG Cases (Test-first not required)**:
- Pure configuration file changes (.env, config, etc.)
- Documentation-only updates (README, comments, etc.)
- Emergency production incident response (post-incident tests mandatory)

## Test Design Principles

### Test Case Structure
- Tests consist of three stages: "Arrange," "Act," "Assert"
- Clear naming that shows purpose of each test
- One test case verifies only one behavior

### Test Data Management
- Manage test data in dedicated directories
- Define test-specific environment variable values
- Always mock sensitive information
- Keep test data minimal, using only data directly related to test case verification purposes

### Mock and Stub Usage Policy

**Recommended: Mock external dependencies in unit tests**
- Merit: Ensures test independence and reproducibility
- Practice: Mock DB, API, file system, and other external dependencies

**Avoid: Actual external connections in unit tests**
- Reason: Slows test speed and causes environment-dependent problems

### Test Failure Response Decision Criteria

**Fix tests**: Wrong expected values, references to non-existent features, dependence on implementation details, implementation only for tests
**Fix implementation**: Valid specifications, business logic, important edge cases
**When in doubt**: Confirm with user

## Test Granularity Principles

### Core Principle: Observable Behavior Only
**MUST Test**: Public APIs, return values, exceptions, external calls, persisted state
**MUST NOT Test**: Private methods, internal state, algorithm implementation details
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