执行 foreach 时更改数组中的值 [英] change values in array when doing foreach

问题描述

示例:

var arr = ["one","two","three"];

arr.forEach(function(part){
  part = "four";
  return "four";
})

alert(arr);

数组仍然是它的原始值，有什么方法可以从迭代函数中写入数组元素吗?

The array is still with it's original values, is there any way to have writing access to array's elements from iterating function ?

推荐答案

回调传递元素、索引和数组本身.

The callback is passed the element, the index, and the array itself.

arr.forEach(function(part, index, theArray) {
  theArray[index] = "hello world";
});

edit — 如注释中所述，.forEach() 函数可以接受第二个参数，该参数将用作 this<的值/code> 在每次调用回调中:

edit — as noted in a comment, the .forEach() function can take a second argument, which will be used as the value of this in each call to the callback:

arr.forEach(function(part, index) {
  this[index] = "hello world";
}, arr); // use arr as this

第二个例子显示 arr 本身在回调中被设置为 this.有人可能认为 .forEach() 中涉及的数组code> 调用可能是 this 的 default 值，但无论出于何种原因，它都不是；如果没有提供第二个参数，this 将是 undefined.

That second example shows arr itself being set up as this in the callback.One might think that the array involved in the .forEach() call might be the default value of this, but for whatever reason it's not; this will be undefined if that second argument is not provided.

(注意:如果回调是一个=>函数，上面关于this的内容不适用，因为this永远不会被绑定调用此类函数时的任何内容.)

(Note: the above stuff about this does not apply if the callback is a => function, because this is never bound to anything when such functions are invoked.)

此外，重要的是要记住，Array 原型上提供了一整套类似的实用程序，并且 Stackoverflow 上会弹出许多关于一个或另一个函数的问题，因此最好的解决方案是简单地选择一个不同的工具.你有:

Also it's important to remember that there is a whole family of similar utilities provided on the Array prototype, and many questions pop up on Stackoverflow about one function or another such that the best solution is to simply pick a different tool. You've got:

• forEach 用于对数组中的每个条目进行处理；
• filter 用于生成仅包含符合条件的条目的新数组；
• map 用于通过转换现有数组来创建一对一的新数组；
• some 检查数组中是否至少有一个元素符合某种描述；
• every 检查数组中的所有条目是否与描述匹配；
• find 在数组中查找值

• forEach for doing a thing with or to every entry in an array;
• filter for producing a new array containing only qualifying entries;
• map for making a one-to-one new array by transforming an existing array;
• some to check whether at least one element in an array fits some description;
• every to check whether all entries in an array match a description;
• find to look for a value in an array

等等.MDN 链接

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