# Clean Code

> Applies principles from Robert C. Martin's 'Clean Code'. Use when writing, reviewing, or refactoring code to ensure high quality, readability, and maintainability. Covers naming, functions, comments, formatting, error handling, unit tests, classes, and code smells. Language-agnostic — applies to Java, TypeScript, Python, Dart, and all workspace stacks.

- Skill: `kumaran-is/clean-code` (Agent Skill)
- Install (CLI): `npx skillmds@latest add kumaran-is/clean-code`
- Raw SKILL.md: https://api.skillmd.com/api/skills/kumaran-is/clean-code/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: kumaran-is (https://skillmd.com/u/kumaran-is)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/kumaran-is/clean-code

---


# Clean Code Skill

> **Iron Law:** Before applying any principle, READ the relevant source file first.
> Do not refactor or flag issues based on memory — verify the actual code (file:line) before making any claim.

This skill embodies the principles of "Clean Code" by Robert C. Martin (Uncle Bob). Use it to transform "code that works" into "code that is clean."

## Core Philosophy
> "Code is clean if it can be read, and enhanced by a developer other than its original author." — Grady Booch

## When to Use
- **Writing new code**: Ensure high quality from the start.
- **Reviewing Pull Requests**: Provide constructive, principle-based feedback.
- **Refactoring legacy code**: Identify and remove code smells.
- **Improving team standards**: Align on industry-standard best practices.

## 1. Meaningful Names
- **Use Intention-Revealing Names**: `elapsedTimeInDays` instead of `d`.
- **Avoid Disinformation**: Don't use `accountList` if it's actually a `Map`.
- **Make Meaningful Distinctions**: Avoid `ProductData` vs `ProductInfo`.
- **Use Pronounceable/Searchable Names**: Avoid `genymdhms`.
- **Class Names**: Use nouns (`Customer`, `WikiPage`). Avoid `Manager`, `Data`.
- **Method Names**: Use verbs (`postPayment`, `deletePage`).

## 2. Functions
- **Small**: Functions should be shorter than you think.
- **Do One Thing**: A function should do only one thing, and do it well.
- **One Level of Abstraction**: Don't mix high-level business logic with low-level details (like regex).
- **Descriptive Names**: `isPasswordValid` is better than `check`.
- **Arguments**: 0 is ideal, 1-2 is okay, 3+ requires a very strong justification — introduce a parameter object.
- **No Side Effects**: Functions shouldn't secretly change global state.

## 3. Comments
- **Don't Comment Bad Code — Rewrite It**: Most comments are a sign of failure to express in code.
- **Explain Yourself in Code**:
  ```python
  # BAD: comment explains intent that code should express
  if employee.flags & HOURLY and employee.age > 65:

  # GOOD: code expresses intent directly
  if employee.isEligibleForFullBenefits():
  ```
- **Good Comments**: Legal, informative (regex intent), clarification (external library behavior), TODOs (with ticket).
- **Bad Comments**: Mumbling, redundant, misleading, mandated, noise, position markers.

## 4. Formatting
- **The Newspaper Metaphor**: High-level concepts at the top, details at the bottom.
- **Vertical Density**: Related lines should be close to each other.
- **Indentation**: Essential for structural readability.

## 5. Objects and Data Structures
- **Data Abstraction**: Hide the implementation behind interfaces.
- **The Law of Demeter**: A module should not know about the innards of objects it manipulates.
  ```java
  // BAD — violates Law of Demeter
  String city = order.getCustomer().getAddress().getCity();

  // GOOD — delegate to the object
  String city = order.getCustomerCity();
  ```
- **Data Transfer Objects (DTO)**: Classes with public variables and no functions — already enforced in NestJS/Spring patterns.

## 6. Error Handling
- **Use Exceptions instead of Return Codes**: Keeps logic clean.
- **Write Try-Catch-Finally First**: Defines the scope of the operation.
- **Don't Return Null**: Forces the caller to null-check every time — throw or return an error state instead.
- **Don't Pass Null**: Leads to `NullPointerException` / `NullError`. Validate at boundaries.

> This section reinforces `code-standards.md` error handling rules ("No Silent Failures"). These are consistent — not conflicting.

## 7. Unit Tests
- **The Three Laws of TDD**:
  1. Don't write production code until you have a failing unit test.
  2. Don't write more of a unit test than is sufficient to fail.
  3. Don't write more production code than is sufficient to pass the failing test.
- **F.I.R.S.T. Principles**: Fast, Independent, Repeatable, Self-Validating, Timely.

## 8. Classes
- **Small**: Classes should have a single responsibility (SRP).
- **The Stepdown Rule**: Code should read like a top-down narrative — callers before callees.

## 9. Smells and Heuristics
- **Rigidity**: Hard to change because every change cascades.
- **Fragility**: Breaks in many places when changed.
- **Immobility**: Hard to reuse in another context.
- **Viscosity**: Hard to do the right thing; wrong things are easier.
- **Needless Complexity/Repetition**: More code than needed.

## Anti-Patterns to Avoid

> **Don't do these** — they are the most common clean code violations in this codebase.

- **Don't** use single-letter variable names outside of loop counters (`i`, `j`) — name by intent
- **Don't** write functions longer than ~20 lines — extract until each function does one thing
- **Don't** return `null` or an empty list on error — throw or return an explicit error state
- **Don't** write a comment that restates what the code does — rewrite the code to be self-explanatory
- **Don't** pass more than 2–3 arguments to a function — introduce a parameter object instead
- **Don't** mix abstraction levels in one function (e.g., high-level business logic + raw SQL in the same method)

## Verify Step

After applying any clean code refactor, run the following to confirm nothing regressed:

```bash
# Run all tests — refactors must not change behavior
# Java
mvn test

# NestJS / Angular
npm test

# Python
uv run pytest

# Flutter / Dart
flutter test
```

Also check:
- `git diff` — confirm no behavior-changing lines snuck into a "rename only" refactor
- `grep -n "as any\|: any\b" <changed files>` — zero new `any` types in TypeScript
- `flutter analyze` — zero new warnings in Dart

## Implementation Checklist
- [ ] Is this function smaller than 20 lines?
- [ ] Does this function do exactly one thing?
- [ ] Are all names searchable and intention-revealing?
- [ ] Have I avoided comments by making the code clearer?
- [ ] Am I passing too many arguments (3+ = needs parameter object)?
- [ ] Is there a failing test for this change?