Visitor

Intent

Add operations to a stable object structure without changing the object classes.

Problem

Tree structures often need many operations; putting all operations on nodes bloats the node classes.

Solution

Nodes accept a visitor object, and each visitor implements operations for each node type.

TypeScript Implementation

SizeVisitor calculates total size for a file tree. NameListVisitor adds a second operation over the same nodes without changing FileNode or FolderNode.

npm run visitor

Trade-offs

• Great when structure is stable and operations change.
• Adding new node types requires updating visitors.

Practical Perspective

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

For Visitor, 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 Visitor keeps responsibilities separated.
• Workflow orchestration where Visitor keeps responsibilities separated.
• Event-driven UI or domain flows where Visitor keeps responsibilities separated.
• Validation, authorization, pricing, routing, or lifecycle logic where Visitor 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 object structures are stable but operations change often.
• Avoid it when element types change frequently.

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.