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
    }
}

它会失败，因为typeof i === 'string'.

有没有办法解决这个问题?我可以显式地将 i 转换为整数，但这样做似乎违背了使用更简单的 for in 而不是常规的 for 循环的目的.

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.

我知道相比之下使用 == ，但这不是一个选项.

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

推荐答案

你有几个选择

1. 转换为数字:

1. Make conversion to a Number:

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

不要比较类型(使用 == 而不是 ===):

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) { }
}

顺便说一下，您不应该使用 for...in 来遍历数组.请参阅文档:https://developer.mozilla.org/en-US/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

for..in 不应用于迭代索引顺序的数组很重要.数组索引只是可枚举的属性整数名称，在其他方面与一般对象相同特性.不能保证 for...in 会返回任何特定顺序的索引，它将返回所有可枚举的属性，包括那些具有非整数名称的属性和那些继承.

因为迭代的顺序是依赖于实现的，迭代在一个数组上可能不会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)在数组上迭代时访问顺序很重要.

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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