rectangle.js

const calculateRectangleArea = function(length, width) {
  if(length < 0 || width < 0) {
    return undefined
  } else { 
    const rectangle = length * width
    return rectangle; 
  }
}
const calculateTriangleArea = function(base, height) {
  if(base < 0 || height < 0) {
    return undefined;
  } else {
    const triangle = base * height / 2
    return triangle;
  }
} 
const calculateCircleArea = function(radius) {
  if(radius < 0) {
    return undefined;
  } else {
    const circle = Math.PI * (radius * radius)
    return circle;
  }
}

console.log(calculateRectangleArea(10, 5));     // should print 50
console.log(calculateRectangleArea(1.5, 2.5));  // should print 3.75
console.log(calculateRectangleArea(10, -5));    // should print undefined

console.log(calculateTriangleArea(10, 5)); // should print 25
console.log(calculateTriangleArea(3, 2.5)); // should print 3.75
console.log(calculateTriangleArea(10, -5)); // should print undefined

console.log(calculateCircleArea(10)); // should print 314.159...
console.log(calculateCircleArea(3.5)); // should print 38.484...
console.log(calculateCircleArea(-1)); // should print undefined