---
description: Code quality standards - SOLID, DRY, clean code practices, naming, error handling, immutability. Universal across languages and frameworks.
alwaysApply: true
---

# Code Quality Standards

## SOLID Principles

- **Single Responsibility**: One function, one job. One class, one purpose.
- **Open/Closed**: Extend via composition, not modification.
- **Liskov Substitution**: Subtypes must be substitutable for base types.
- **Interface Segregation**: Small, focused interfaces over large, general ones.
- **Dependency Inversion**: Depend on abstractions; inject dependencies, don't import concrete implementations.

## DRY

- Flag repetition aggressively — abstract repeated patterns into utilities
- One source of truth for validation logic, constants, configs
- Don't over-abstract — three similar lines often beats a premature abstraction

## Clean Code

### Naming
- Intention-revealing names; avoid abbreviations; be consistent and searchable
- ❌ `const e = getE(u)` → ✅ `const userEmail = getEmail(user)`

### Functions
- Small (<20 lines), single-purpose, 0-3 args, return early to reduce nesting
- Avoid side effects when possible
- Explicit over clever — bias toward readable, intentional code

### Comments
- Code should be self-documenting; comments explain *why*, not *what*
- Delete commented-out code (use git)

## Error Handling

- Never swallow errors silently
- Provide meaningful error messages with context
- Fail fast with clear diagnostics
- Handle errors at the appropriate level

## Immutability

- Prefer immutable updates: `{ ...user, name: newName }` over `user.name = newName`
- Prefer pure functions

## "Engineered Enough"

- No hacky or fragile code, but no over-engineered premature abstractions either
- Only make changes directly requested or clearly necessary
- Don't add features or abstractions beyond scope
- Don't design for hypothetical future requirements