State

Intent

Allow an object to change behavior when its internal state changes.

Problem

Lifecycle-heavy objects often collect large conditional blocks based on state.

Solution

Represent each state as an object. The context delegates behavior to the current state and controls transitions.

TypeScript Implementation

Order delegates pay() and ship() to explicit state objects.

npm run state

Trade-offs

• Reduces state conditionals.
• Adds more objects.
• State transitions need careful tests.

Practical Perspective

Behavioral patterns are about distributing responsibilities between objects so workflows stay understandable as rules grow.

For State, 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

• Business rules that vary by tenant or product where State keeps responsibilities separated.
• Workflow orchestration where State keeps responsibilities separated.
• Event-driven UI or domain flows where State keeps responsibilities separated.
• Validation, authorization, pricing, routing, or lifecycle logic where State keeps responsibilities separated.

Decision Questions

• Which object owns the decision?
• Can a rule change without editing stable workflow code?
• Is runtime behavior explicit enough to debug?
• Use it when behavior depends on lifecycle state and transitions matter.
• Keep allowed and forbidden transitions explicit.

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.