Composition Over Inheritance

Build behaviour by assembling small independent parts, not by extending a base class.

Rules for agents

• Default to composition. Use inheritance only for genuine "is-a" with no behaviour override surprises.
• Never inherit to reuse code — extract a collaborator and hold it as a field.
• Keep components small, single-purpose, and independently testable.
• Prefer passing behaviour (functions, strategies, middleware) over subclass hooks.
• Depth limit: if you are writing a third inheritance level, restructure.

Applies broadly

Functions composed into pipelines, middleware chains, React components, small services. Same principle at every scale.

Smells

Base classes with protected hook methods, super() calls that must occur in a specific order, "framework" base classes every class must extend, fragile changes to a base that break distant subclasses.