最后为什么会显示NaN？

```class Shape {
  constructor(width , height , color) {
    this.width = width;
    this.height = height;
    this.color = color;
  }

  draw() {
    console.log(`drowing ${this.color} color of`);
  }
  getArea() {
    return this.width*this.height;
  } 
}

class Triangle extends Shape {    
  draw() {
    super.draw();  
    console.log(`drowing ${this.color} color!`);
  }
  getArea() {
    return (this.width* this.heigt) / 2;
  }
}
const triangle = new Triangle(20,20,'blue');
console.log(triangle.getArea());

return (this.width* this.heigt) / 2;

heigt 写错了，应该是height