( for... in ) 和 ( for... of ) 语句之间有什么区别? [英] What is the difference between ( for... in ) and ( for... of ) statements?

问题描述

我知道什么是 for... in 循环(它遍历键)，但我第一次听说 for... of(它遍历值).

I know what is a for... in loop (it iterates over the keys), but I have heard about for... of for the first time (it iterates over values).

我对 for... of 循环感到困惑.

I am confused about for... of loop.

var arr = [3, 5, 7];
arr.foo = "hello";
    
for (var i in arr) {
  console.log(i); // logs "0", "1", "2", "foo"
}
    
for (var i of arr) {
  console.log(i); // logs "3", "5", "7"
  // it doesn't log "3", "5", "7", "hello"
}

我知道 for... of 迭代属性值.那么为什么它不记录 3"、5"、7"、hello" 而不是 3"、5"、?7"?

I understand that for... of iterates over property values. Then why doesn't it log "3", "5", "7", "hello" instead of "3", "5", "7"?

与 for... in 循环不同，它遍历每个键(0"、1"、2"、foo") 并且还迭代 foo 键，for... of 不会迭代 foo 的值> 属性，即 hello".为什么会这样?

Unlike for... in loop, which iterates over each key ("0", "1", "2", "foo") and also iterates over the foo key, the for... of does not iterate over the value of foo property, i.e., "hello". Why it is like that?

在这里我控制台 for... of 循环.它应该记录 3"、5"、7"、hello" 但它记录 3"、5"、7".为什么?

Here I console for... of loop. It should log "3", "5", "7","hello" but it logs "3", "5", "7". Why?

示例链接

推荐答案

for in 循环遍历对象的可枚举属性名称.

for in loops over enumerable property names of an object.

for of(ES6 中的新功能)确实使用了 特定于对象的迭代器 并遍历由此生成的值.

for of (new in ES6) does use an object-specific iterator and loops over the values generated by that.

在您的示例中，数组迭代器 确实产生数组中的所有值(忽略非索引属性).

In your example, the array iterator does yield all the values in the array (ignoring non-index properties).

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