for(in)循环索引是字符串而不是整数

问题描述

考虑以下代码:

var arr = [111, 222, 333];
for(var i in arr) {
    if(i === 1) {
        // Never executed
    }
}

它将失败，因为typeof i === 'string'.

有没有解决的办法?我可以将i显式转换为整数，但是这样做似乎无法使用更简单的for in而不是常规的for -loop达到目的.

Is there way around this? I could explicitly convert i to integer, but doing so seems to defeat the purpose using simpler for in instead of regular for-loop.

我知道比较时会使用==，但这不是一个选择.

I am aware of using == in comparison, but that is not an option.

推荐答案

您有几种选择

1. 转换为数字:

1. Make conversion to a Number:

parseInt(i) === 1
~~i === 1
+i === 1

不比较类型(使用==代替===):

Don't compare a type (Use == instead of ===):

i == 1 // Just don't forget to add a comment there

将for循环更改为(我会这样做，但这取决于您要实现的目标):

Change the for loop to (I would do this but it depends on what you are trying to achieve):

for (var i = 0; i < arr.length; i++) {
   if (i === 1) { }
}

// or

arr.forEach(function(item, i) {
   if (i === 1) { }
}

通过这种方式，您不应该使用for...in遍历数组.请参阅文档: https://developer. mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/for...in

By the way you should not use for...in to iterate through an array. See docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in

因为迭代的顺序取决于实现，所以进行迭代 数组上的元素可能无法以一致的顺序访问元素.所以 最好使用带有数字索引的for循环(或Array.forEach 或非标准for ... of循环)在数组上进行迭代时，其中 访问顺序很重要.

Because the order of iteration is implementation dependent, iterating over an array may not visit elements in a consistent order. Therefore it is better to use a for loop with a numeric index (or Array.forEach or the non-standard for...of loop) when iterating over arrays where the order of access is important.

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