JavaScript: .forEach()和.map()的区别

我知道有很多这样的话题。我知道基本原理:. foreach()操作原始数组，.map()操作新数组。

在我的例子中:

function practice (i){
    return i+1;
};

var a = [ -1, 0, 1, 2, 3, 4, 5 ];
var b = [ 0 ];
var c = [ 0 ];
console.log(a);
b = a.forEach(practice);
console.log("=====");
console.log(a);
console.log(b);
c = a.map(practice);
console.log("=====");
console.log(a);
console.log(c);

这是输出:

[ -1, 0, 1, 2, 3, 4, 5 ]
=====
[ -1, 0, 1, 2, 3, 4, 5 ]
undefined
=====
[ -1, 0, 1, 2, 3, 4, 5 ]
[ 0, 1, 2, 3, 4, 5, 6 ]

我不明白为什么使用practice会改变b的值为undefined。如果这是一个愚蠢的问题，我很抱歉，但我对这门语言很陌生，到目前为止我找到的答案并不能让我满意。

当前回答

	forEach()	map()
Functionality	Performs given operation on each element of the array	Performs given "transformation" on a "copy" of each element
Return value	Returns `undefined`	Returns new array with transformed elements, leaving back original array unchanged.
Preferrable usage scenario and example	Performing non-tranformation like processing on each element. For example, saving all elements in the database.	Obtaining array containing output of some processing done on each element of the array. For example, obtaining array of lengths of each string in the array

forEach()例子

chars = ['Hello'， 'world!!'); var retVal = char . foreach(函数(字){ console.log(" save to db: " + word) }) console.log (retVal) / /定义

map()例子

chars = ['Hello'， 'world!!'); Var长度= char .map(函数(单词){ 返回word.length }) console.log(长度)/ /(5、8)

2018-03-29 06:56:47

其他回答

forEach()和map()的区别

forEach()只是循环遍历元素。它会丢弃返回值并且总是返回undefined。此方法的结果不提供输出。

Map()循环遍历元素，通过迭代主数组分配内存并存储返回值

例子:

   var numbers = [2,3,5,7];

   var forEachNum = numbers.forEach(function(number){
      return number
   })
   console.log(forEachNum)
   //output undefined

   var mapNum = numbers.map(function(number){
      return number
   })
   console.log(mapNum)
   //output [2,3,5,7]

map()比forEach()快

2019-09-23 11:49:04

性能分析随着数组中元素数量的增加，For循环比map或foreach执行得更快。

Let array = []; For (var I = 0;I < 20000000;我+ +){ array.push(我) ｝ console.time(“映射”); 数组中。映射(num => { 返回num * 4; })； console.timeEnd(“映射”); console.time (' forEach '); 数组中。forEach((num, index) => { 返回数组[index] = num * 4; })； console.timeEnd (' forEach '); console.time(“的”); For (i = 0;I < array.length;我+ +){ 数组[i] =数组[i] * 2; ｝ console.timeEnd(“的”);

2019-05-23 12:20:50

不同之处在于他们的回报。执行后: