design-principles

---

name: design-principles description: Code design patterns: pure functions, immutability, composition, and async. Use when designing code or functions.

Code Design Principles

Patterns for writing clean, maintainable, and testable code.

Quick Start

// Pure function with immutability
function updateUser(user: User, age: number): User {
  return { ...user, age }; // new object, no mutation
}

Key Principles

• Pure functions: no side effects, same input = same output
• Immutability: create new objects/arrays instead of mutating
• Single responsibility: one function = one task
• Early returns: reduce nesting, improve readability
• Composition over inheritance: inject dependencies
• Promises over callbacks for async code
• Template strings over concatenation
• Plain objects for data, classes only for stateful services
• Avoid global variables and getters/setters
• Lazy initialization: no module-level side effects, wrap in getter functions

References

See rules.md for complete guidelines with examples.