03-04 变量声明 - var 声明

for (var i = 0; i < 10; i++) {
  setTimeout(function() {
    console.log(i)
  }, 10)
}
输出10个10
for (var i = 0; i < 10; i++) {
  (function(i){
    setTimeout(function() {
      console.log(i)
    }, 10)
  })(i)
}
输出0,1,2,3,4,5,6,7,8,9

for (let i = 0; i < 10; i++) {
  setTimeout(function() {
    console.log(i)
  }
}

输出0,1,2,3,4,5,6,7,8,9