闭包：

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Closures
闭包是函数和声明该函数的词法环境的组合。

let：

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/let
let 语句声明一个块级作用域的本地变量，并且可选的将其初始化为一个值。

例：闭包和let修正匿名函数访问的变量

function foo(){ var bar = []; for (var index = 0; index < 9; index++) { bar[index] = function(){ return index; } } return bar;}function foo_closures(){ var bar = []; for (let index = 0; index < 9; index++) { (bar[index] = function(){ return index; })() } return bar;}function foo_let(){ var bar = []; for (let index = 0; index < 9; index++) { bar[index] = function(){ return index; } } return bar;}for(var d of foo()){ console.log(d());// 9个9}for(var d of foo_closures()){ console.log(d());// 1,2,...8}for(var d of foo_let()){ console.log(d());// 1,2,...8}