Bridge

Intent

Separate an abstraction from its implementation so both can evolve independently.

Problem

Two independent dimensions of variation can create a subclass explosion.

Solution

Keep one dimension as an abstraction and delegate the other to an implementation interface.

TypeScript Implementation

InfoAlert and CriticalAlert are alert abstractions. EmailChannel and SlackChannel are delivery implementations. Either alert can be combined with either channel without creating classes such as CriticalSlackAlert or InfoEmailAlert.

npm run bridge

Trade-offs

• Avoids subclass explosion.
• Adds composition and interfaces.

Practical Perspective

Structural patterns are about shaping relationships between objects so systems can evolve without rewriting every caller.

For Bridge, 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

• Third-party API boundaries where Bridge keeps responsibilities separated.
• Legacy migration layers where Bridge keeps responsibilities separated.
• UI component composition where Bridge keeps responsibilities separated.
• Cross-cutting wrappers such as caching, logging, or access checks where Bridge keeps responsibilities separated.

Decision Questions

• Which interface should client code depend on?
• Where should translation, composition, or access control live?
• Does this abstraction reduce coupling or just rename it?
• Use it when two independent variation axes would otherwise create class explosion.
• Keep abstraction and implementation interfaces from leaking into each other.

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.