for ... in循环，字符串数组输出索引 [英] for ... in loop with string array outputs indices

问题描述

当我写一些像这样的javascript：

When I write some javascript such as this:

var words = ['word1', 'word2', 'word3']

for (word in words) {
    console.log(word)
}

结果输出是相应单词的数字索引。我在Google上进行了一些搜索，但我找不到这种行为的确切原因。我猜这是完全预期的行为，但我想知道原因。

The resulting output is the numeric indices of the respective word. I did some searching on Google and I couldn't find the exact reason for this behavior. I'm guessing that this is completely expected behavior but I would like to know the reason why.

谢谢！

推荐答案

为什么没有人提供正确答案？你永远不会用 for / in 循环迭代数组 - 这只是为了迭代普通对象及其键，而不是迭代数组中的项。

Why is nobody providing the correct answer? You NEVER iterate arrays with a for/in loop - that is only for iterating plain objects and their keys, not for iterating the items in an array.

你为这样的for循环迭代数组：

You iterate arrays for a for loop like this:

var words = ['word1', 'word2', 'word3'];

for (var i = 0, len = words.length; i < len; i++) {
    // here words[i] is the array element
}

或者，在现代浏览器中，您可以使用 .forEach（）数组方法：

Or, in a modern browser, you can use the .forEach() method of arrays:

var words = ['word1', 'word2', 'word3'];

words.forEach(function(value, index, array) {
    // here value is the array element being iterated
});

参见此处在MDN 获取有关 .forEach（）的更多信息。

See here at MDN for more info on .forEach().

ndp对这篇文章的引用显示了一些使用数组使用 for / in 可能出错的好例子。你可以使它有时工作，但它不是编写Javascript数组迭代的聪明方法。

ndp's reference to this post shows some good examples of things that can go wrong using for/in with arrays. You can make it works sometimes, but it is not the smart way to write Javascript array iteration.

或者更多现代，您可以使用ES6 for / of 构造，它是专为迭代迭代而构建的（数组是可迭代的）：

Or, in more modern times, you can use the ES6 for/of construct which is specially built for iterating an iterable (an array is an iterable):

var words = ['word1', 'word2', 'word3'];

for (const w of words) {
    console.log(w);
}

或者，如果你想要价值和指数，你可以这样做：

Or, if you want both the value and the index, you can do this:

var words = ['word1', 'word2', 'word3'];

for (const [index, w] of words.entries()) {
    console.log(index, ": ", w);
}

有几个优点 over .forEach（）。首先，你有更多的循环控制，因为你可以使用 break ，继续，返回控制 .forEach（）没有给你的循环流。此外，回调函数没有额外的开销，因此在某些环境中，它可以更快。

There are several advantages of for/of over .forEach(). To start with, you have more loop control as you can use break, continue, return to control the loop flow which .forEach() does not give you. In addition, there's no additional overhead for a callback function so, in some environments, it can be faster.

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