let和const

"use strict";

{
  let a = 10;
  var b = 1;
}
console.log(a); // Uncaught ReferenceError: a is not defined
console.log(b);

function demo1(argument) {
  var a = [];
  for (var i = 0; i < 10; i++) {
    a[i] = function () {
      console.log(i);
    };
  }
  a[6](); // var命令 存在变量提升。let不存在变量提升。
}

demo1();