Liskov Substitution Principle

Subtypes must be substitutable for their base type without breaking behavior.

// dirty — Square breaks Rectangle's contract
class Rectangle {
  width = 0;
  height = 0;
  setWidth(w: number): void { this.width = w; }
  setHeight(h: number): void { this.height = h; }
  area(): number { return this.width * this.height; }
}
class Square extends Rectangle {
  setWidth(w: number): void { this.width = w; this.height = w; } // breaks LSP!
  setHeight(h: number): void { this.width = h; this.height = h; }
}

Refactor using factory functions so createRectangle and createSquare are independent — no inheritance. Both expose area().

solve('square', 5) → 25 (side² )
solve('rect', 4, 6) → 24 (width × height)