review-solid-clean-code

Review: SOLID Principles and Clean Code

When to Use

• Pull request review when code smells are present
• After detecting Bloater or Change Preventer smells (see detect-code-smells)
• A class or module is hard to test in isolation
• A change in one place forces changes in many others

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SOLID Principles

SRP -- Single Responsibility Principle

One class = one reason to change.

Red Flags: Class name contains "Manager"/"Utils", 10+ public methods spanning unrelated domains, unrelated imports.

// Violation: persistence, formatting, and email in one class
class UserManager {
  save(user: User): void { /* DB write */ }
  formatReport(user: User): string { /* HTML formatting */ }
  sendWelcomeEmail(user: User): void { /* SMTP call */ }
}

Fix: Extract Class -> UserRepository, UserReportFormatter, UserEmailService. See refactor-moving-features.

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OCP -- Open/Closed Principle

Open for extension, closed for modification.

Red Flags: switch/if-else chain on type tag that grows with each new type, adding features requires editing existing tested classes.

// Violation: every new shape requires modifying this function
function area(shape: Shape): number {
  if (shape.type === "circle") return Math.PI * shape.radius ** 2;
  if (shape.type === "square") return shape.side ** 2;
}

Fix: Interface with polymorphism (Shape.area()). See refactor-simplifying-conditionals, design-patterns-behavioral (Strategy).

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LSP -- Liskov Substitution Principle

Subtypes must be substitutable for their base types without altering correctness.

Red Flags: Subclass overrides with NotImplemented, instanceof checks in business logic, subclass narrows preconditions.

// Violation: Square breaks Rectangle contract
class Square extends Rectangle {
  setWidth(w: number): void { this.width = w; this.height = w; }
}

// Violation: instanceof checks
function process(animal: Animal): void {
  if (animal instanceof Dog) { /* dog-specific */ }
}