var a = 200;
function f()
{
console.log(a);
var a = 100;
console.log(a);
}
f();
console.log(a);
var a;
console.log(a);
var a = 300;
console.log(a);
结果：undefined
100
200
200
300
原文：https://www.cnblogs.com/luckyXcc/p/9153307.html