属性 event.key 给了我一个未定义的值。相反，我使用 event.keyCode ：

The property event.key gave me an undefined value. Instead, I used event.keyCode:

function alphaOnly(event) {
  var key = event.keyCode;
  return ((key >= 65 && key <= 90) || key == 8);
};

请注意，值8是退格键。

Note that the value of 8 is for the backspace key.