Command

Intent

Command encapsulates a request or action as an object.

Problem

Some systems need to queue, retry, log, audit, or undo actions. If callers directly invoke business operations, those capabilities become scattered and hard to standardize.

Solution

Represent each action as a command object with an execute() method. An invoker can run commands without knowing the receiver details.

TypeScript Implementation

This implementation models account operations:

• Command is the executable action interface.
• DepositCommand and WithdrawCommand encapsulate account actions.
• CommandBus executes commands and records an audit log.

Run it from the repository root:

npm run command

When To Use

• Queueing or scheduling work.
• Auditing user actions.
• Retrying failed operations.
• Implementing undo/redo workflows.

When Not To Use

• The action is simple and never needs to be passed around.
• Command classes add ceremony without useful behavior.
• A plain function would be clearer.

Benefits

• Decouples invoker from receiver.
• Makes actions easy to log, queue, and test.
• Gives behavior a first-class representation.

Trade-offs

• Can create many small classes.
• Flow may be less direct than method calls.
• Command payloads need careful validation.

Related Patterns

• Memento
• Chain of Responsibility
• Strategy

Practical Perspective

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

For Command, 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 Command keeps responsibilities separated.
• Workflow orchestration where Command keeps responsibilities separated.
• Event-driven UI or domain flows where Command keeps responsibilities separated.
• Validation, authorization, pricing, routing, or lifecycle logic where Command 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 actions need queuing, retrying, auditing, undo, or delayed execution.
• Keep command names aligned with business actions rather than technical methods.

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.