What is Liskov Substitution Principle (LSP)?

javascript

// Bad: Square breaks Rectangle behavior
class Rectangle {
  setWidth(w) { this.width = w; }
  setHeight(h) { this.height = h; }
  area() { return this.width * this.height; }
}

class Square extends Rectangle {
  setWidth(w) {
    this.width = w;
    this.height = w;  // Changes both!
  }
}

// Breaks LSP:
function test(rectangle) {
  rectangle.setWidth(5);
  rectangle.setHeight(4);
  console.log(rectangle.area());  // Expected: 20
}

test(new Rectangle());  // 20 ✅
test(new Square());     // 16 ❌ Broken!

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