循环索引变量的Javascript成为全局范围的一部分？ [英] Javascript for loop index variables become part of global scope?

问题描述

也许我不知道 循环索引变量是如何得到范围的，但是当我的一个循环没有完成时我很惊讶，似乎是因为一个函数在循环中调用包含 i 的 for 循环索引。

Perhaps I'm not aware of how for loop index variables get scoped, but I was very surprised when one of my loops didn't complete, seemingly because a function called from within a loop contained an i for its for loop index as well.

这是我用来展示这种行为的一个小脚本：

Here's a little script I put together to demonstrate this behavior:

var loopOne = function(test) {
    for(i = 0; i < test.length; i++)
        console.log(getMask(test));
};

var getMask = function(pass) {      
    var s = "";
    for (i = 0; i < pass.length; i++) {
        s = s + "*";
    }
    return s;      
};

loopOne('hello');

如果我在Chrome中运行此操作并查看控制台日志，我应该看到 ***** 五次。但是，我只看过一次。经过进一步检查，如果我在Chrome javascript控制台中输入 i ，它将输出6（ ='hello'.length + 1 ）。这让我认为 i 已成为全球范围的一部分，并且不限于的范围需要它的循环。

If I run this in Chrome and look at the console log, I should see ***** five times. However, I only see it once. Upon further inspection, if I type i in the Chrome javascript console, it will output 6 ( = 'hello'.length + 1). This makes me think that i has become a part of the global scope and is not limited to the scope of the for loop for which it was needed.

这是正确的吗？如果是这样，在javascript中为 c>循环定义的索引变量有什么更好的做法？

Is this correct? If so, what's a better practice for defining the index variable of a for loop in javascript?

推荐答案

在Javascript中，变量的作用域是 var 关键字。使用 var 声明变量时，变量的范围限定为当前函数。在不使用 var 关键字的情况下分配给变量时，假设您正在讨论相同或更高范围内已定义的变量。如果没有找到，则变量在最高范围内创建。

In Javascript, variables are scoped with the var keyword. When declaring variables with var, the variable is scoped to the current function. When assigning to a variable without using the var keyword, it is assumed you're talking about an already defined variable in the same or a higher scope. If none is found, the variable is created in the highest scope.

底线：使用 var 。

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