.map不用于循环吗? [英] is `.map` not for looping?

问题描述

我之前在这里回答了有关的问题如何获得JSON的响应数?，我建议他们使用 map 函数，而不要使用 for 循环，但是有人评论了.map 不用于循环，而应使用 forEach .

I've answered a question here before about How to get number of response of JSON? and I suggested for them to use the map function instead of using a for loop but someone commented that .map is not for looping and to use forEach instead.

在 for 循环上使用 map 是否有不利之处?

Are there any downsides to using map over a for loop?

我也对此进行了研究，发现了网站，说明该 map > forEach .

I also researched this and found a site stating that map > forEach .

推荐答案

Map用于将数组中的每个元素转换为另一个表示形式，并以新的顺序返回结果.但是，由于对每个项目都调用了该函数，因此您可能可以进行任意调用而什么也不返回，因此尽管严格说来它们并不相同，但它的行为就像 forEach 一样.

Map is used to transform each element in an array into another representation, and returns the results in a new sequence. However, since the function is invoked for each item, it is possible that you could make arbitrary calls and return nothing, thus making it act like forEach, although strictly speaking they are not the same.

正确使用map(将值的数组转换为另一种表示形式):

Proper use of map (transforming an array of values to another representation):

var source = ["hello", "world"];
var result = source.map(function(value) {
                return value.toUpperCase();
             });
console.log(result); // should emit ["HELLO, "WORLD"]

偶然使用 .map 进行迭代(语义错误):

Accidentally using .map to iterate (a semantic error):

var source = ["hello", "world"];

// emits:
// "hello"
// "world"
source.map(function(value) {
          console.log(value);
       });

第二个示例在技术上是有效的，它将编译并运行，但这不是 map 的预期用途.

The second example is technically valid, it'll compile and it'll run, but that is not the intended use of map.

谁在乎，是否满足我的要求?"可能是您的下一个问题.首先， map 会忽略索引中具有指定值的项目.另外，由于 map 期望返回结果，因此它正在做额外的事情，从而为一个简单的进程分配了更多的内存和处理时间(尽管很短).更重要的是，这可能会使您自己或其他开发人员混淆您的代码.

"Who cares, if it does what I want?" might be your next question. First of all, map ignores items at an index that have an assigned value. Also, because map has an expectation to return a result, it is doing extra things, thus allocating more memory and processing time (although very minute), to a simple process. More importantly, it may confuse yourself or other developers maintaining your code.

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