oop-principles

name

oop-principles

description

Evaluate TypeScript/JavaScript code against Dave Thomas's OO principles and the Tell Don't Ask rule. Trigger when: class has only static methods; class has a single public method + constructor; code uses GoF pattern names (Decorator/Factory/Strategy/Builder/Command/Observer); class is invalid until setters are called; data class with only constructor + properties; external code reads getters to make decisions (Tell Don't Ask); user asks "is this a code smell?", "should this be a class or a function?", or "why does my code feel procedural?".

OO Principles

Distinguish true object-oriented design from class-oriented anti-patterns in TypeScript/JavaScript.

Core rule: Use a class only when it creates multiple instances with their own state. Otherwise prefer functions, object literals, or types.

The Seven Rules

For each rule below, full code examples live in references/anti-patterns.md.

1. Not an object factory → don't use a class. Class with only static methods is just a namespace. → use standalone functions or const X = { ... } as const. (anti-patterns.md §1)

2. Named after a design pattern → don't use a class. GoF patterns (Decorator/Factory/Strategy/Command) collapse to functions or HOFs in TS. (anti-patterns.md §3)

3. Abstract base classes are rarely necessary. Prefer composition + structural typing over inheritance. (anti-patterns.md §5)

4. No state, no class. A class with one public method (perform/call/execute) instantiated only to call it once is just a function. (anti-patterns.md §2)

5. Invalid after construction → don't use a class. Builders requiring setters before use → take all required args in one function or constructor. (anti-patterns.md §4, §9)

6. Stop writing data classes. Constructor + properties + no behavior → use type/interface + object literal. (anti-patterns.md §6)

7. Test-first implications:

   Pattern	Setup	Testing
   Pure functions	Pass args	Isolated, no mocks
   Stateful classes	Factories, mocks	Tangled
   Inheritance chains	Heavy fixtures	Nightmare

Tell Don't Ask

When you do use a class, don't let callers query state and decide externally — move the decision into the object. Watch for getter chains, if (obj.getX()) obj.setY() patterns, and decision logic scattered across callers.

→ Full guide, refactoring steps, nuance, and review checklist: references/tell-dont-ask.md

Workflow

• Decision tree: references/refactoring-checklist.md
• Anti-pattern catalog with fixes: references/anti-patterns.md
• Tell Don't Ask: references/tell-dont-ask.md

"If it's not making multiple instances, it's not a class." — Dave Thomas