JS循环变量范围 [英] JS loop variable scope

问题描述

我正在尝试一些简单的JS代码。我知道我们应该使用 var 关键字在循环中声明循环变量，为循环声明以避免全局变量宣言。但是我意识到循环变量在执行循环之后也存在：

I was trying out some simple JS code. I was aware that we should use var keyword to declare a loop variable inside the loop say for loop to avoid global variable declaration. However I realized that the loop variable exists after the execution of the loop too:

var a = [1, 2, 3, 4, 5, 6];
for (var i = 0; i < a.length; i++)
    document.write(a[i]); //123456
document.write(i);  //6

这不是内联的（事实上它不需要，我知道）面向对象概念中循环的循环变量如何表现。我知道我不应该尝试将OO语言中的JavaScript与任何方面进行比较。但是，我只是猜测OO语言中的行为更好。

This is not inline (in fact it does not need to be, I know) with how loop variable of for loop in Object Oriented concepts behaves. I know I should not try to compare the JavaScript with in OO language for any aspect. However I am just guessing that the behavior in OO languages is better.

我认为，如果我们将此代码直接放在< script>中标签然后它实际上将循环变量添加到全局范围，这是不希望的。是这样的吗？或者我认为这是错误的方式？

I think, if we put this code in directly in <script> tag then it actually adds the loop variable to global scope which is not desired. Is it like that? or I am thinking it wrong way?

我们可以通过创建显式函数范围来实现这种行为：

We can achieve such behavior by creating explicit function scope:

var a1 = [1, 2, 3, 4, 5, 6];
(function () {
    for (var i1 = 0; i1 < a.length; i1++)
        document.write(a[i1]); //123456
})();

document.write(i1);  //undefined

但这是生产中遵循的标准方式吗？还有什么可以做的？

But is it the standard way / followed in production? What other things can be done?

推荐答案

我们可以通过创建显式函数范围来实现这种行为。但这是生产中遵循的标准方式吗？

We can achieve such behavior by creating explicit function scope. But is it the standard way / followed in production?

是的。函数是在JavaScript中引入新范围的唯一可能性（尽管在未来的版本中可能会改变 让 ）;并且 IIFEs 也在生产中大量使用。

Yes. Functions are the only possibility to introduce a new scope in JavaScript (though that might change in future versions with let); and IIFEs are heavily used in production as well.

还可以做些什么？

What other things can be done?

要将 i 的范围限制为for-loop的块？无（更不用说让）。但是，您几乎不需要这样做，因为没有任何东西会干扰它 - var i 仍然是您的函数的本地作用域。有时，您甚至可能希望在循环后使用 i 作为持续计数器。

To limit the scope of i to the for-loop's block? None (let alone let). However, you hardly will need to do that, since nothing will interfere with it - var i is still scoped local to your function. Sometimes, you even might want to use i after the loop as an ongoing counter.

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