删除属性

• delete 操作符只删除自有属性，不删除继承属性；
• delete 不会删除 configurable 特性为 false 的属性；

delete book.author
delete book['main title']

删除不存在的也返回true

let o = { x: 1 }
delete o.x // =>true; 删除属性x
delete o.y // =>true; y不存在也返回true
delete o.toString // =>true; 什么也不做（toString 不是自有属性）
delete 1 // =>true; 无意义，但仍然返回true