3.let应用

for循环：

var a = [];
for (let i = 0; i < 10; i++) {
  a[i] = function () {
    document.write(i);
  };
}
document.write(a[6]()); //6

    var a = [];
    for (var i = 0; i < 10; i++) {
      a[i] = function () {
        document.write(i);
      };
    }
    a[6](); // 10