Prototype

Intent

Create new objects by cloning an existing object instead of constructing from scratch.

Problem

Repeatedly setting up similar objects duplicates configuration and risks inconsistent defaults.

Solution

Use a configured prototype as a template and clone it with explicit overrides.

TypeScript Implementation

CampaignPrototype clones campaign templates while copying mutable arrays safely.

npm run prototype

Trade-offs

• Reduces repetitive setup.
• Clone semantics must be explicit.

Practical Perspective

Creational patterns are about controlling object creation so callers do not depend on concrete construction details.

For Prototype, the important question is not “can I draw the UML diagram?” but “what dependency or decision becomes easier to change after I introduce this pattern?” In production code, the pattern should make ownership clearer, reduce accidental coupling, and give tests a natural seam.

Real-World Use Cases

• Framework integration points where Prototype keeps responsibilities separated.
• Configuration-driven runtime behavior where Prototype keeps responsibilities separated.
• Test fixture creation where Prototype keeps responsibilities separated.
• Infrastructure objects whose setup should be centralized where Prototype keeps responsibilities separated.

Decision Questions

• Who owns object creation?
• Which concrete type should callers be allowed to know?
• Can invalid combinations be prevented at construction time?
• Use it when a configured baseline object should be copied with focused overrides.
• Be explicit about deep copy vs shallow copy semantics.

Design Checklist

• Start with the client code: define the interface you want callers to depend on.
• Keep concrete classes small and named after one responsibility.
• Make creation, selection, delegation, or notification rules explicit instead of hidden in conditionals.
• Prefer composition roots for wiring objects together.
• Document the reason for using the pattern so future contributors do not cargo-cult it.

Common Mistakes

• Adding the pattern before the code has a real variation point.
• Creating abstractions that only rename concrete classes.
• Hiding important runtime behavior so debugging becomes harder.
• Letting examples stay toy-sized without showing where the pattern boundary sits in real code.
• Forgetting tests for negative paths, invalid states, or fallback behavior.

Testing Guidance

• Test through the public abstraction, not private implementation details.
• Use fakes or test doubles for collaborators so the pattern seam is verified.
• Add one integration-style test proving the objects are wired correctly.
• Cover edge cases that motivated the pattern: missing strategy, rejected state transition, failed handler, invalid factory family, stale proxy cache, or similar.
• Keep tests named after behavior and business outcome rather than pattern terminology.

Refactoring Signals

• The pattern is useful when adding a new variation no longer requires editing stable caller code.
• It is probably overdesigned when every new class has only one trivial method and no independent reason to exist.
• If contributors cannot explain the runtime flow quickly, simplify the wiring or improve names.
• If tests must mock too many layers, the abstraction boundary is likely in the wrong place.