在JS对象中引用自身属性

Question

如何获得centerY和centerX这个对象“区域”。两者均返回NaN，因为this.left和this.right返回NULL：

const area = {
  top: viewportHeight * 20 / 100,
  bottom: viewportHeight - (viewportHeight * 20 / 100),
  left: 300,
  right: (viewportWidth * 30 / 100) + 300,
  centerX: (this.left + this.right) / 2, // NaN
  centerY: (this.top + this.bottom) / 2, // NaN
};