var scope = 'global';
if(true){
var scope = 'local';
console.log(scope) --> local
}
console.log(scope); --> local
var scope = 'global';
function foo(){
var scope = 'local';
console.log(scope); --> local
}
foo();
console.log(scope); --> global
https://cnbin.github.io/blog/2017/04/11/jszhong-de-bi-bao/
http://carlos-studio.com/2013/09/06/js-%E9%97%9C%E6%96%BC%E5%87%BD%E5%BC%8F/