What is Open-Closed Principle (OCP)?

javascript

// Bad: Modifying existing code for new shapes
class AreaCalculator {
  calculate(shape) {
    if (shape.type === "circle") {
      return Math.PI * shape.radius ** 2;
    } else if (shape.type === "square") {
      return shape.side ** 2;
    }
    // Adding triangle? Must modify this function!
  }
}

// Good: Open for extension
class Shape {
  area() {
    throw new Error("Must implement");
  }
}

class Circle extends Shape {
  constructor(radius) {
    super();
    this.radius = radius;
  }
  area() {
    return Math.PI * this.radius ** 2;
  }
}

class Square extends Shape {
  constructor(side) {
    super();
    this.side = side;
  }
  area() {
    return this.side ** 2;
  }
}

// Can add Triangle without modifying existing code!
class Triangle extends Shape {
  constructor(base, height) {
    super();
    this.base = base;
    this.height = height;
  }
  area() {
    return (this.base * this.height) / 2;
  }
}

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