JS的变量声明提升和函数声明提升

[geminate]

http://liuhuihao.com/js/2018/01/16/JS%E7%9A%84%E5%8F%98%E9%87%8F%E5%A3%B0%E6%98%8E%E6%8F%90%E5%8D%87%E5%92%8C%E5%87%BD%E6%95%B0%E5%A3%B0%E6%98%8E%E6%8F%90%E5%8D%87.html