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
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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
}