Code design patterns: pure functions, immutability, composition, lazy initialization, and side-effect management. Use when designing function signatures, structuring modules, or choosing between mutable and immutable patterns.
Design
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Updated Jan 17, 2026, 11:52 PM
Why Use This
This skill provides specialized capabilities for eser's codebase.
Use Cases
Developing new features in the eser repository
Refactoring existing code to follow eser standards
Understanding and working with eser's codebase structure
---
name: design-principles
description: "Code design patterns: pure functions, immutability, composition, lazy initialization, and side-effect management. Use when designing function signatures, structuring modules, or choosing between mutable and immutable patterns."
---
# Code Design Principles
Patterns for writing clean, maintainable, and testable code.
## Quick Start
```typescript
// 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: see `coding-practices` for details
- 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](references/rules.md) for complete guidelines with examples.
