var let 在 for 循环中的区别

//使用var声明，得到3个3
var a = [];
for (var i = 0; i < 3; i++) {
  a[i] = function () {
    console.log(i);
  };
}
a[0](); //3
a[1](); //3
a[2](); //3
 
//使用let声明，得到0,1,2
var a = [];
for (let i = 0; i < 3; i++) {
  a[i] = function () {
    console.log(i);
  };
}
a[0](); //0
a[1](); //1
a[2](); //2

for(var i=0;i<5;i++){
   setTimeout(()=>{
        console.log(i);//5个5
    },100)
}
console.log(i);//5
console.log('=============')
 
for(let j=0;j<5;j++){
   setTimeout(()=>{
        console.log(j);//0,1,2,3,4
    },100)
}
console.log(j);//报错 j is not defined