这个为啥会报错啊 about just-now-qa HOT 3 CLOSED

[1,2,3].forEach(Array.prototype.push);

[1, 2, 3].forEach(() => stack.push())

const obj = {
  fn() {
    console.log('fn. this is:', this);
  }
};

const callback = obj.fn;
// this 指向 window
callback();

// this 指向 fn
obj.fn();