为什么for循环计数器在javascript中退出循环后不会被破坏？

问题描述：

for(var i=0;i<5;i++){}
alert(i);

在javascript中这将获得5
其他语言，如C ++，java，c＃.. ..只会给出一个错误，即i变量没有在上下文中定义。

in javascript this will get us 5 other languages like C++, java, c# .... will simply give an error that the i variable isn't defined in the context.

那么为什么for循环计数器在退出循环之后不会被破坏javascript？

So why the for loop counter doesn't get destroyed after exiting the loop in javascript?

答

这是因为JavaScript引擎会将变量decalaration移动（提升）到函数顶部在函数¹中声明它的位置。 JavaScript没有块范围。

This is because the JavaScript engine will move ("hoist") the variable decalaration to the top of the function no matter where it is declared inside the function¹. JavaScript does not have block scope.

{
//Some code
    for(var i=0;i<5;i++){}
    alert(i);
//Some code
}

相当于：

{
  var i;
 //.. some code
 for(i=0;i<5;i++){}
    alert(i);
}

_{¹除非是使用 catch 子句捕获的异常;该变量的范围是 catch block。}

_{¹ Unless it's the exception being caught with a catch clause; that variable is scoped to catch block.}

用于定义块范围变量 ecmascript 6 specs （javascript 1.7）介绍让即可。目前，这只适用于最新版本的FireFox浏览器并处于共识阶段。

For defining block scope variables ecmascript 6 specs (javascript 1.7) introduces let. Currently this will work only in latest version of FireFox browser and in consensus stage.

<script type="application/javascript;version=1.7">
     //Some code

        for (let i = 0; i < 10; i++) {

            alert(i); // 1, 2, 3, 4 ... 9
        }

        alert(i); // Here you will get an error here saying ReferenceError: i is not defined.
    }
</script>

小提琴

Fiddle

为什么for循环计数器在javascript中退出循环后不会被破坏？

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