关于 JavaScript 中的关键词 'let'
About the key word 'let' in JavaScript
var funcs=[];
for(let i=0;i<3;i++){
funcs[i]=function(){
return i;
}
}
alert(funcs[1]);
alert(funcs[1]());
window 警报 2 次。
第一个是这样的：
function (){
return i;
}
第二个像这样
1
但是我不知道为什么funcs[1]可以执行不报错'i is undefined';
因为在循环中创建的函数 关闭了 创建时处于活动状态的 lexical environment。该词法环境（概念上）是一个对象，其中包含在其中定义的局部变量（以及其他一些东西），包括 i 变量，在本例中是为循环体的特定迭代创建的变量（因为非常特殊的方式 for 在其初始值设定项中处理 let 声明）。这是"closure,"核心技术之一的概念JavaScript。即使执行超出给定词法环境关联的范围（函数 returns，我们继续循环的下一次迭代，等等），如果任何东西仍然引用该环境对象，就像它赖以生存的所有物体一样。
由于 for 在其初始化程序中如何处理 let，funcs 中的每个条目都有自己的词法环境，因此也有自己的 i 副本。
当您调用其中一个函数时，会创建一个新的环境对象，其 "outer" 环境设置为附加到该函数的环境。当您在函数代码中引用 i 时，它首先查看函数的环境，如果在那里找不到 i，它会查看外部环境——找到它的地方（在本例中）并使用它。
在您说过的评论中
If you use 'var' not 'let', it will always return '3'
完全正确。使用 var，i 将被提升到与 for 循环所在的函数关联的环境对象（或者全局函数，如果这是全局代码）。所以在循环中创建的所有函数共享相同的 i，当你调用它们时，它的值是 3.
这是let/const和var的本质区别之一：let和const有块作用域，而for 在其初始化程序中对 let 进行了特殊处理。
让我们来看看这些不同的环境对象是如何创建的。假设这段代码：
funtion example() {
const funcs = [];
for (let i = 0; i < 3; ++i) {
funcs[i] = function() {
return i;
};
}
funcs[1](); // 1
}
example();
当我们调用 example 时，在 const funcs = [] 之后但在 for 循环开始之前，当前环境对象是为调用 example 创建的对象，所以我们在内存中有这样的东西（忽略一些细节）：
+−−−−−−−−−−−−−−−−+
current env>−−−−−−−−−−−−−−−−−−−−−−−−−>| `example` Call |
| Env Object |
+−−−−−−−−−−−−−−−−+
| [[Outer]] |>−−−>(The env obj for when `example` was created.)
| funcs |>−+
+−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
+−>| (array) |
+−−−−−−−−−−−+
| length: 0 |
+−−−−−−−−−−−+
现在，for 循环开始：创建一个新的每迭代环境对象作为 "current" 对象，前一个作为其 "outer" 环境。 i 变量在新的每次迭代环境对象中创建，并赋予值 0:
+−−−−−−−−−−−−−−+
current env>−−>| Iteration 0 |
| Env Object |
+−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−+
| [[Outer]] |>−−−−−−−>| `example` Call |
| i: 0 | | Env Object |
+−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−+
| [[Outer]] |>−−−>(The env obj for when `example` was created.)
| funcs |>−+
+−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
+−>| (array) |
+−−−−−−−−−−−+
| length: 0 |
+−−−−−−−−−−−+
在循环迭代期间，我们创建一个函数并将其存储在funcs中。该函数获取对当前环境对象的引用，它保持为 [[Environment]]（这是一个实现的东西，如果您查看该函数，您实际上不会发现它在代码级别不可访问，只是在 JavaScript 引擎中）：
+−−−−−−−−−−−−−−+
current env>−+>| Iteration 0 |
/ | Env Object | +−−−−−−−−−−−−−−−−+
/ +−−−−−−−−−−−−−−+ | `example` Call |
+ | [[Outer]] |>−−−−−−−>| Env Object |
| | i: 0 | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | funcs |>−+
| +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| +−>| (array) |
| +−−−−−−−−−−−+
| | length: 1 | +−−−−−−−−−−−−−−−−−+
| | 0 |>−−−−−>| Function 0 |
| +−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−+
| | [[Environment]] |>−−−−−+
| +−−−−−−−−−−−−−−−−−+ |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
现在这就是 for 初始化器中对 let 的巧妙处理（事实上，let 和 const 在 for 循环体）：为 next 迭代创建了一个 new 环境对象，i 的值为从上一次迭代的i复制到下一次迭代的i。所以我们有：
+−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−>| Iteration 0 |
| | Env Object | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | `example` Call |
| | [[Outer]] |>−−−+−−−>| Env Object |
| | i: 0 | / +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ + | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | funcs |>−−−+
| | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | +−−>| (array) |
| +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| current env>−+>| Iteration 1 | | | length: 1 | +−−−−−−−−−−−−−−−−−+
| | Env Object | | | 0 |>−−−>| Function 0 |
| +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−+
| | [[Outer]] |>−+ | [[Environment]] |>−+
| | i: 0 | +−−−−−−−−−−−−−−−−−+ |
| +−−−−−−−−−−−−−−+ |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
然后新环境中的 i 增加到 1，并创建一个新函数并存储在 funcs 中，给我们：
+−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−>| Iteration 0 |
| | Env Object | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | `example` Call |
| | [[Outer]] |>−−−+−−−>| Env Object |
| | i: 0 | / +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ + | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | funcs |>−−−+
| | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | +−−>| (array) |
| +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| current env>−+>| Iteration 1 | | | length: 2 | +−−−−−−−−−−−−−−−−−+
| / | Env Object | | | 0 |>−−−>| Function 0 |
| / +−−−−−−−−−−−−−−+ | | 1 |>−+ +−−−−−−−−−−−−−−−−−+
| + | [[Outer]] |>−+ +−−−−−−−−−−−+ | | [[Environment]] |>−−−+
| | | i: 1 | | +−−−−−−−−−−−−−−−−−+ |
| | +−−−−−−−−−−−−−−+ | |
| | | +−−−−−−−−−−−−−−−−−+ |
| | +−>| Function 1 | |
| | +−−−−−−−−−−−−−−−−−+ |
| | | [[Environment]] |>−+ |
| | +−−−−−−−−−−−−−−−−−+ | |
| | | |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
然后在该迭代结束时，我们为最后一次迭代再次执行整个操作，并得到：
+−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−>| Iteration 0 |
| | Env Object | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | `example` Call |
| | [[Outer]] |>−−+−−+ −>| Env Object |
| | i: 0 | / / +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | | | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | | funcs |>−−−+
| | | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | | +−−>| (array) |
| +−−−−−−−−−−−−−−+ | | +−−−−−−−−−−−+
| +−−−−−−−−−−−−−−−>| Iteration 1 | | | | length: 3 | +−−−−−−−−−−−−−−−−−+
| | | Env Object | | | | 0 |>−−−−−>| Function 0 |
| | +−−−−−−−−−−−−−−+ | | | 1 |>−−−+ +−−−−−−−−−−−−−−−−−+
| | | [[Outer]] |>−+ | | 2 |>−+ | | [[Environment]] |>−−−−−+
| | | i: 1 | | +−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+ |
| | +−−−−−−−−−−−−−−+ | | | |
| | | | | +−−−−−−−−−−−−−−−−−+ |
| | +−−−−−−−−−−−−−−+ | | +−>| Function 1 | |
| | current env>−+>| Iteration 2 | | | +−−−−−−−−−−−−−−−−−+ |
| | / | Env Object | | | | [[Environment]] |>−−−+ |
| | + +−−−−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+ | |
| | | | [[Outer]] |>−−−+ | | |
| | | | i: 2 | | +−−−−−−−−−−−−−−−−−+ | |
| | | +−−−−−−−−−−−−−−+ +−−−>| Function 2 | | |
| | | +−−−−−−−−−−−−−−−−−+ | |
| | | | [[Environment]] |>−+ | |
| | | +−−−−−−−−−−−−−−−−−+ | | |
| | | | | |
| | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ | |
| | | |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
当我们调用funcs[1]()时，会创建一个调用环境，并将其[[Outer]]环境设置为函数的[[Environment]]。所以就在我们 return i 之前，我们有（省略一些细节）：
+−−−−−−−−−−−−−−+
current env>−−>| Call to |
| `funcs[1]()` |
| Env Object |
+−−−−−−−−−−−−−−+
| [[Outer]] |>−−+
+−−−−−−−−−−−−−−+ |
|
| +−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−+ | `example` call |
| +−−−>| Env Object |
| | +−−−−−−−−−−−−−−−−+
| | | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | funcs |>−+
| | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | +−>| (array) |
\ +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
+−−−−−−−−−−−+−−−>| Iteraton 1 | | | length: 3 | +−−−−−−−−−−−−−−−−−+
| | Env Object | | | 0 |>−−−−−>| (function) |
| +−−−−−−−−−−−−−−+ | | 1 |>−−−+ +−−−−−−−−−−−−−−−−−+
| | [[Outer]] |>−+ | 2 |>−+ | | [[Environment]] |>...
| | i: 1 | +−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | |
| | | +−−−−−−−−−−−−−−−−−+
| | +−>| (function) |
| | +−−−−−−−−−−−−−−−−−+
| | | [[Environment]] |>−−−−+
| | +−−−−−−−−−−−−−−−−−+ |
| | |
| | +−−−−−−−−−−−−−−−−−+ |
| +−−−>| (function) | |
| +−−−−−−−−−−−−−−−−−+ |
| | [[Environment]] |>... |
| +−−−−−−−−−−−−−−−−−+ |
| |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
当函数查找 i 时，它会在当前环境对象中查找。由于它不存在，它会查找 [[Outer]] 对象。它在那里找到它，其值为 1，因此这就是它使用的值。
相反，如果我们使用 var，则 i 被提升到环境对象以调用 example（其中 funcs 是），所以在循环我们有这个代替（注意 i 不再在每次迭代环境中）：
+−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−>| Iteration 0 |
| | Env Object | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | `example` Call |
| | [[Outer]] |>−−+−−+−>| Env Object |
| +−−−−−−−−−−−−−−+ / / +−−−−−−−−−−−−−−−−+
| | | | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | | i: 3 |
| | | | funcs |>−−−+
| | | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | | +−−>| (array) |
| +−−−−−−−−−−−−−−+ | | +−−−−−−−−−−−+
| +−−−−−−−−−−−−−−−>| Iteration 1 | | | | length: 3 | +−−−−−−−−−−−−−−−−−+
| | | Env Object | | | | 0 |>−−−−−>| Function 0 |
| | +−−−−−−−−−−−−−−+ | | | 1 |>−−−+ +−−−−−−−−−−−−−−−−−+
| | | [[Outer]] |>−+ | | 2 |>−+ | | [[Environment]] |>−−−−−+
| | +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+ |
| | | | | |
| | | | | +−−−−−−−−−−−−−−−−−+ |
| | +−−−−−−−−−−−−−−+ | | +−>| Function 1 | |
| | current env>−+>| Iteration 2 | | | +−−−−−−−−−−−−−−−−−+ |
| | / | Env Object | | | | [[Environment]] |>−−−+ |
| | + +−−−−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+ | |
| | | | [[Outer]] |>−−−+ | | |
| | | +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−−−−−−−+ | |
| | | +−−−>| Function 2 | | |
| | | +−−−−−−−−−−−−−−−−−+ | |
| | | | [[Environment]] |>−+ | |
| | | +−−−−−−−−−−−−−−−−−+ | | |
| | | | | |
| | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ | |
| | | |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
这意味着当我们调用funcs[1]()时，为它创建了一个新环境，它的[[Outer]]设置为函数的[[Environment]]，就在return i之前我们有：
+−−−−−−−−−−−−−−+
current env>−−>| Call to |
| `funcs[1]()` |
| Env Object |
+−−−−−−−−−−−−−−+
| [[Outer]] |>−−+
+−−−−−−−−−−−−−−+ |
|
|
+−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−+
| | `example` call |
| +−−−>| Env Object |
| | +−−−−−−−−−−−−−−−−+
| | | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | i: 3 |
| | | funcs |>−+
| | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | +−>| (array) |
\ +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
+−−−−−−−−−−−+−−−>| Iteration 1 | | | length: 3 | +−−−−−−−−−−−−−−−−−+
| | Env Object | | | 0 |>−−−−−>| (function) |
| +−−−−−−−−−−−−−−+>−+ | 1 |>−−−+ +−−−−−−−−−−−−−−−−−+
| | [[Outer]] | | 2 |>−+ | | [[Environment]] |>...
| +−−−−−−−−−−−−−−+ +−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+
| | |
| | | +−−−−−−−−−−−−−−−−−+
| | +−>| (function) |
| | +−−−−−−−−−−−−−−−−−+
| | | [[Environment]] |>−−−−+
| | +−−−−−−−−−−−−−−−−−+ |
| | |
| | +−−−−−−−−−−−−−−−−−+ |
| +−−−>| (function) | |
| +−−−−−−−−−−−−−−−−−+ |
| | [[Environment]] |>... |
| +−−−−−−−−−−−−−−−−−+ |
| |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
所以函数查找i时，在当前环境中没有找到，在第一个[[Outer]]环境中也没有找到，但是确实找到了在 second [[Outer]] 环境中，值为 3，所以这就是它使用的值。
var funcs=[];
for(let i=0;i<3;i++){
funcs[i]=function(){
return i;
}
}
alert(funcs[1]);
alert(funcs[1]());
function (){
return i;
}
第二个像这样
1
但是我不知道为什么funcs[1]可以执行不报错'i is undefined';
因为在循环中创建的函数 关闭了 创建时处于活动状态的 lexical environment。该词法环境（概念上）是一个对象，其中包含在其中定义的局部变量（以及其他一些东西），包括 i 变量，在本例中是为循环体的特定迭代创建的变量（因为非常特殊的方式 for 在其初始值设定项中处理 let 声明）。这是"closure,"核心技术之一的概念JavaScript。即使执行超出给定词法环境关联的范围（函数 returns，我们继续循环的下一次迭代，等等），如果任何东西仍然引用该环境对象，就像它赖以生存的所有物体一样。
由于 for 在其初始化程序中如何处理 let，funcs 中的每个条目都有自己的词法环境，因此也有自己的 i 副本。
当您调用其中一个函数时，会创建一个新的环境对象，其 "outer" 环境设置为附加到该函数的环境。当您在函数代码中引用 i 时，它首先查看函数的环境，如果在那里找不到 i，它会查看外部环境——找到它的地方（在本例中）并使用它。
在您说过的评论中
If you use 'var' not 'let', it will always return '3'
完全正确。使用 var，i 将被提升到与 for 循环所在的函数关联的环境对象（或者全局函数，如果这是全局代码）。所以在循环中创建的所有函数共享相同的 i，当你调用它们时，它的值是 3.
这是let/const和var的本质区别之一：let和const有块作用域，而for 在其初始化程序中对 let 进行了特殊处理。
让我们来看看这些不同的环境对象是如何创建的。假设这段代码：
funtion example() {
const funcs = [];
for (let i = 0; i < 3; ++i) {
funcs[i] = function() {
return i;
};
}
funcs[1](); // 1
}
example();
当我们调用 example 时，在 const funcs = [] 之后但在 for 循环开始之前，当前环境对象是为调用 example 创建的对象，所以我们在内存中有这样的东西（忽略一些细节）：
+−−−−−−−−−−−−−−−−+
current env>−−−−−−−−−−−−−−−−−−−−−−−−−>| `example` Call |
| Env Object |
+−−−−−−−−−−−−−−−−+
| [[Outer]] |>−−−>(The env obj for when `example` was created.)
| funcs |>−+
+−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
+−>| (array) |
+−−−−−−−−−−−+
| length: 0 |
+−−−−−−−−−−−+
现在，for 循环开始：创建一个新的每迭代环境对象作为 "current" 对象，前一个作为其 "outer" 环境。 i 变量在新的每次迭代环境对象中创建，并赋予值 0:
+−−−−−−−−−−−−−−+
current env>−−>| Iteration 0 |
| Env Object |
+−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−+
| [[Outer]] |>−−−−−−−>| `example` Call |
| i: 0 | | Env Object |
+−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−+
| [[Outer]] |>−−−>(The env obj for when `example` was created.)
| funcs |>−+
+−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
+−>| (array) |
+−−−−−−−−−−−+
| length: 0 |
+−−−−−−−−−−−+
在循环迭代期间，我们创建一个函数并将其存储在funcs中。该函数获取对当前环境对象的引用，它保持为 [[Environment]]（这是一个实现的东西，如果您查看该函数，您实际上不会发现它在代码级别不可访问，只是在 JavaScript 引擎中）：
+−−−−−−−−−−−−−−+
current env>−+>| Iteration 0 |
/ | Env Object | +−−−−−−−−−−−−−−−−+
/ +−−−−−−−−−−−−−−+ | `example` Call |
+ | [[Outer]] |>−−−−−−−>| Env Object |
| | i: 0 | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | funcs |>−+
| +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| +−>| (array) |
| +−−−−−−−−−−−+
| | length: 1 | +−−−−−−−−−−−−−−−−−+
| | 0 |>−−−−−>| Function 0 |
| +−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−+
| | [[Environment]] |>−−−−−+
| +−−−−−−−−−−−−−−−−−+ |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
现在这就是 for 初始化器中对 let 的巧妙处理（事实上，let 和 const 在 for 循环体）：为 next 迭代创建了一个 new 环境对象，i 的值为从上一次迭代的i复制到下一次迭代的i。所以我们有：
+−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−>| Iteration 0 |
| | Env Object | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | `example` Call |
| | [[Outer]] |>−−−+−−−>| Env Object |
| | i: 0 | / +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ + | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | funcs |>−−−+
| | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | +−−>| (array) |
| +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| current env>−+>| Iteration 1 | | | length: 1 | +−−−−−−−−−−−−−−−−−+
| | Env Object | | | 0 |>−−−>| Function 0 |
| +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−+
| | [[Outer]] |>−+ | [[Environment]] |>−+
| | i: 0 | +−−−−−−−−−−−−−−−−−+ |
| +−−−−−−−−−−−−−−+ |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
然后新环境中的 i 增加到 1，并创建一个新函数并存储在 funcs 中，给我们：
+−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−>| Iteration 0 |
| | Env Object | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | `example` Call |
| | [[Outer]] |>−−−+−−−>| Env Object |
| | i: 0 | / +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ + | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | funcs |>−−−+
| | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | +−−>| (array) |
| +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| current env>−+>| Iteration 1 | | | length: 2 | +−−−−−−−−−−−−−−−−−+
| / | Env Object | | | 0 |>−−−>| Function 0 |
| / +−−−−−−−−−−−−−−+ | | 1 |>−+ +−−−−−−−−−−−−−−−−−+
| + | [[Outer]] |>−+ +−−−−−−−−−−−+ | | [[Environment]] |>−−−+
| | | i: 1 | | +−−−−−−−−−−−−−−−−−+ |
| | +−−−−−−−−−−−−−−+ | |
| | | +−−−−−−−−−−−−−−−−−+ |
| | +−>| Function 1 | |
| | +−−−−−−−−−−−−−−−−−+ |
| | | [[Environment]] |>−+ |
| | +−−−−−−−−−−−−−−−−−+ | |
| | | |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
然后在该迭代结束时，我们为最后一次迭代再次执行整个操作，并得到：
+−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−>| Iteration 0 |
| | Env Object | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | `example` Call |
| | [[Outer]] |>−−+−−+ −>| Env Object |
| | i: 0 | / / +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | | | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | | funcs |>−−−+
| | | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | | +−−>| (array) |
| +−−−−−−−−−−−−−−+ | | +−−−−−−−−−−−+
| +−−−−−−−−−−−−−−−>| Iteration 1 | | | | length: 3 | +−−−−−−−−−−−−−−−−−+
| | | Env Object | | | | 0 |>−−−−−>| Function 0 |
| | +−−−−−−−−−−−−−−+ | | | 1 |>−−−+ +−−−−−−−−−−−−−−−−−+
| | | [[Outer]] |>−+ | | 2 |>−+ | | [[Environment]] |>−−−−−+
| | | i: 1 | | +−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+ |
| | +−−−−−−−−−−−−−−+ | | | |
| | | | | +−−−−−−−−−−−−−−−−−+ |
| | +−−−−−−−−−−−−−−+ | | +−>| Function 1 | |
| | current env>−+>| Iteration 2 | | | +−−−−−−−−−−−−−−−−−+ |
| | / | Env Object | | | | [[Environment]] |>−−−+ |
| | + +−−−−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+ | |
| | | | [[Outer]] |>−−−+ | | |
| | | | i: 2 | | +−−−−−−−−−−−−−−−−−+ | |
| | | +−−−−−−−−−−−−−−+ +−−−>| Function 2 | | |
| | | +−−−−−−−−−−−−−−−−−+ | |
| | | | [[Environment]] |>−+ | |
| | | +−−−−−−−−−−−−−−−−−+ | | |
| | | | | |
| | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ | |
| | | |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
当我们调用funcs[1]()时，会创建一个调用环境，并将其[[Outer]]环境设置为函数的[[Environment]]。所以就在我们 return i 之前，我们有（省略一些细节）：
+−−−−−−−−−−−−−−+
current env>−−>| Call to |
| `funcs[1]()` |
| Env Object |
+−−−−−−−−−−−−−−+
| [[Outer]] |>−−+
+−−−−−−−−−−−−−−+ |
|
| +−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−+ | `example` call |
| +−−−>| Env Object |
| | +−−−−−−−−−−−−−−−−+
| | | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | funcs |>−+
| | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | +−>| (array) |
\ +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
+−−−−−−−−−−−+−−−>| Iteraton 1 | | | length: 3 | +−−−−−−−−−−−−−−−−−+
| | Env Object | | | 0 |>−−−−−>| (function) |
| +−−−−−−−−−−−−−−+ | | 1 |>−−−+ +−−−−−−−−−−−−−−−−−+
| | [[Outer]] |>−+ | 2 |>−+ | | [[Environment]] |>...
| | i: 1 | +−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | |
| | | +−−−−−−−−−−−−−−−−−+
| | +−>| (function) |
| | +−−−−−−−−−−−−−−−−−+
| | | [[Environment]] |>−−−−+
| | +−−−−−−−−−−−−−−−−−+ |
| | |
| | +−−−−−−−−−−−−−−−−−+ |
| +−−−>| (function) | |
| +−−−−−−−−−−−−−−−−−+ |
| | [[Environment]] |>... |
| +−−−−−−−−−−−−−−−−−+ |
| |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
当函数查找 i 时，它会在当前环境对象中查找。由于它不存在，它会查找 [[Outer]] 对象。它在那里找到它，其值为 1，因此这就是它使用的值。
相反，如果我们使用 var，则 i 被提升到环境对象以调用 example（其中 funcs 是），所以在循环我们有这个代替（注意 i 不再在每次迭代环境中）：
+−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−>| Iteration 0 |
| | Env Object | +−−−−−−−−−−−−−−−−+
| +−−−−−−−−−−−−−−+ | `example` Call |
| | [[Outer]] |>−−+−−+−>| Env Object |
| +−−−−−−−−−−−−−−+ / / +−−−−−−−−−−−−−−−−+
| | | | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | | i: 3 |
| | | | funcs |>−−−+
| | | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | | +−−>| (array) |
| +−−−−−−−−−−−−−−+ | | +−−−−−−−−−−−+
| +−−−−−−−−−−−−−−−>| Iteration 1 | | | | length: 3 | +−−−−−−−−−−−−−−−−−+
| | | Env Object | | | | 0 |>−−−−−>| Function 0 |
| | +−−−−−−−−−−−−−−+ | | | 1 |>−−−+ +−−−−−−−−−−−−−−−−−+
| | | [[Outer]] |>−+ | | 2 |>−+ | | [[Environment]] |>−−−−−+
| | +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+ |
| | | | | |
| | | | | +−−−−−−−−−−−−−−−−−+ |
| | +−−−−−−−−−−−−−−+ | | +−>| Function 1 | |
| | current env>−+>| Iteration 2 | | | +−−−−−−−−−−−−−−−−−+ |
| | / | Env Object | | | | [[Environment]] |>−−−+ |
| | + +−−−−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+ | |
| | | | [[Outer]] |>−−−+ | | |
| | | +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−−−−−−−+ | |
| | | +−−−>| Function 2 | | |
| | | +−−−−−−−−−−−−−−−−−+ | |
| | | | [[Environment]] |>−+ | |
| | | +−−−−−−−−−−−−−−−−−+ | | |
| | | | | |
| | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ | |
| | | |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
这意味着当我们调用funcs[1]()时，为它创建了一个新环境，它的[[Outer]]设置为函数的[[Environment]]，就在return i之前我们有：
+−−−−−−−−−−−−−−+
current env>−−>| Call to |
| `funcs[1]()` |
| Env Object |
+−−−−−−−−−−−−−−+
| [[Outer]] |>−−+
+−−−−−−−−−−−−−−+ |
|
|
+−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−+
| | `example` call |
| +−−−>| Env Object |
| | +−−−−−−−−−−−−−−−−+
| | | [[Outer]] |>−−−>(The env obj for when `example` was created.)
| | | i: 3 |
| | | funcs |>−+
| | +−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
| | +−>| (array) |
\ +−−−−−−−−−−−−−−+ | +−−−−−−−−−−−+
+−−−−−−−−−−−+−−−>| Iteration 1 | | | length: 3 | +−−−−−−−−−−−−−−−−−+
| | Env Object | | | 0 |>−−−−−>| (function) |
| +−−−−−−−−−−−−−−+>−+ | 1 |>−−−+ +−−−−−−−−−−−−−−−−−+
| | [[Outer]] | | 2 |>−+ | | [[Environment]] |>...
| +−−−−−−−−−−−−−−+ +−−−−−−−−−−−+ | | +−−−−−−−−−−−−−−−−−+
| | |
| | | +−−−−−−−−−−−−−−−−−+
| | +−>| (function) |
| | +−−−−−−−−−−−−−−−−−+
| | | [[Environment]] |>−−−−+
| | +−−−−−−−−−−−−−−−−−+ |
| | |
| | +−−−−−−−−−−−−−−−−−+ |
| +−−−>| (function) | |
| +−−−−−−−−−−−−−−−−−+ |
| | [[Environment]] |>... |
| +−−−−−−−−−−−−−−−−−+ |
| |
| |
+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
所以函数查找i时，在当前环境中没有找到，在第一个[[Outer]]环境中也没有找到，但是确实找到了在 second [[Outer]] 环境中，值为 3，所以这就是它使用的值。