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}JS-闭包（Closures）和let声明块级作用域变量
原文地址：https://www.cnblogs.com/jffun-blog/p/10211964.html