Composition Over Inheritance

Kind: principle

Layer: Structural Core

Aliases: Composite Reuse Principle

Record: architecture:composition-over-inheritance

Severity: recommended

Scope: class, component

Canonical: Ontology

A design rule that behavior is reused by holding and delegating to another object instead of extending its class.

Listed in Architecture principles, after Composability and before Reusability.

Requires

• Delegation
• Interface-Based Design

Reinforces

• Low Coupling
• Replaceability

Enables

• Strategy Pattern
• Decorator Pattern

Conflicts with

• Deep Inheritance Hierarchy

In tension with

• Simplicity for Trivial Reuse

Tensions

• Composition Over Inheritance / Simplicity for Trivial Reuse

Violated by

• Deep Inheritance Hierarchy
• Broken Inheritance

Refactored by

• Replace Inheritance with Delegation
• Strategy Pattern

Severity

• recommended

Category

• Core Modular Design

Enforced by

• rules/eslint/closure-no-business-inheritance.eslint.rule.ts

Reinforced by

• Bridge Pattern

Linked from

• The structural domain
• Contracts / Interfaces / Compatibility
• Core Modular Design
• Behavioral Patterns
• Structural Patterns
• Core Modular Design
• SOLID / Object-Oriented Design
• Severity levels
• The resolutions