了解箭头函数参数 [英] Understanding arrow function parameters

问题描述

我刚刚练习了一个标准的基本白板问题:创建一个由 0 到 n 的整数填充的数组.我的解决方案有效，但还有其他人发布了非常不寻常的语法.我真的很想理解它，但 MDN 文档对我帮助不大.我可以把 {length: n} 的工作原理放在一起，但是 (_, i) =>我 看起来很奇怪._ 是未命名的函数，它接受 i 并返回 i?但为什么会这样呢?我希望得到任何帮助.

I just practiced a standard basic whiteboard problem: Create an array filled with integers from 0 through n. My solution worked but there were others posted that had very unusual syntax. I am really trying to understand it but the MDN docs aren't helping me very much. I can put together how {length: n} works but (_, i) => i seems strange. _ is the unnamed function and it takes in i and returns i? but why is that there? I would love any help.

我的解决方案:

function arr(n){
  var newArr = [];
  for(var i = 0; i < n; i++){
    newArr.push(i);
  }
  return newArr;
}

新的语法解决方案:

const arr = n => Array.from({length: n}, (_, i) => i);

推荐答案

映射器函数 Array.from 可以接受的第一个参数指示正在迭代的当前元素.也就是说，例如，从长度为 3 的类数组集合中，该参数将是 arrLike[0] 或 arrLike[1]> 或 arrLike[2].

The first argument to the mapper function Array.from can accept indicates the current element being iterated over. That is, from an array-like collection of length 3, for example, that argument will be arrLike[0], or arrLike[1], or arrLike[2].

如果集合中那个点没有任何元素，比如这里，那么访问这些索引将返回undefined:

If there aren't any elements at that point in the collection, like here, then accessing those indicies will return undefined:

const arr = n => Array.from({length: n}, (_, i) => {
  console.log(_);
  return i;
});

arr(3);

下划线只是一个变量名.您可以随意定义它.未使用的参数被称为_是一个常见的约定，但这只是一个约定，而不是语法规则.

The underscore is just a variable name. You can define it however you want. It's a common convention for an unused argument to be called _, but it's only a convention, not a syntax rule.

由于映射器函数只关心当前的索引被迭代以构建新数组，因此它定义了第二个参数i，然后立即返回它.(Array.from 可以迭代具有 length 属性的任何对象，从 0 到 length - 1.即使如果对象上实际上不存在这些属性，则第二个参数 index 仍将从 0 递增到 length - 1.)

Since the mapper function only cares about the current index being iterated over in order to construct the new array, it defines the second argument i and then returns it immediately. (Array.from can iterate over any object with a length property, from 0 to length - 1. Even if none of those properties actually exist on the object, the index, the second argument, will still be incremented from 0 up to length - 1.)

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