forEach() 是否通过引用绑定? [英] Does forEach() bind by reference?

问题描述

var arr = ['Foo'];

arr.forEach(function(item){
  console.log(item);
  item = 'Lorem';
  console.dir(arr[0]);

});

for (var item in arr){
  arr[item] = 'Ipsum';
  console.dir(arr[0]);
}

如上面的代码所示，我注意到更改传递给 forEach() 回调的项的值不会导致迭代对象发生更改.

Like the code above shows, I noticed that changing the value of an item passed to the callback of forEach() does not cause the iterated object to alter.

使用 for...in 当然可以.

为什么会这样 &我应该如何更改数组中的值?

我发现该主题在 MDN

推荐答案

使用 for...in 当然可以.

Using for...in certainly does.

不，不是.你的 forEach 循环相当于这个 for...in 循环(除了顺序):

No it doesn't. Your forEach loop is equivalent to this for...in loop (apart from the order):

for (var index in arr) {
  var item = arr[index];
  console.log(item);
  item = 'Lorem';
  console.dir(arr[0]);
}

你看到数组也没有被修改吗?这是因为 JavaScript 总是传值，并且有一个非常简单的规则要记住:

Do you see that the array isn't modified either? That's because JavaScript is always pass-by-value, and there is a very simple rule to keep in mind:

为变量赋值从不改变另一个变量或数据结构的值.

Assigning a value to a variable never changes the value of another variable or data structure.

这意味着，为item 分配一个新值，不能改变arr 的元素.如果你想修改数组，你必须通过给一个索引赋值来直接改变它，即

That means, assigning a new value to item, cannot change an element of arr. If you want to to modify the array, you have to mutate it directly by assigning a value to an index, i.e.

arr[index] = 'foo';

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