函数作用域

  var a=2;
  function foo(){
   var a=3;
   console.log(a,"函数");
  }
  foo();//3,"函数"
  console.log(a);//2

var 与 let

var a = '1'
a= '2'
console.log(a,"a")
 
const a = '1'
a= '2'
console.log(a,"a")