Javascript，地图返回未定义 [英] Javascript, map returns undefined

问题描述

因此，我有24个人"对象，这些对象是使用名称"数组创建的，因此花花公子"名称在重复； 接下来，我创建了一个函数，用于将一些随机数与对象id-s进行比较，如果它们相等，那么我将尝试创建与该id一起使用的新的人员姓名数组.

So I have 24 "person" objects, which I created using "names" array, so that "dude" names are repeating; Next I made a function that compares some random numbers to object id-s, and if they are equal,then I am trying to make new array of person names that go with that id.

(对不起，如果听起来太复杂，但是我真的不明白为什么我的 map方法无法正常工作.)

(I'm sorry if this sounds too complicated, but I really don't understand why my map method doesn't work.)

这里有2个版本的代码. 第一个作品可以满足我的要求.

Here 2 versions of my code. The first one works, and does what I want.

var names = ["jim", "jack", "aaron", "hugh", "jeff", "cameron", "allen", "charlie"];
var len = 3,
  arr1 = [],
  counter = 1;

for (var i = 0; i < len; i++) {
  names.forEach(name => {
    arr1.push({
      id: counter,
      dude: name
    });
    counter++;
  });
}

console.log(arr1);

function checkName(nums) {
  var namesarr = [];
  for (var i = 0; i < arr1.length; i++) {
    nums.forEach(function(num) {
      if (num === arr1[i].id) {
        namesarr.push(arr1[i].dude);
      }
    });
  }
  return (namesarr);
};

console.log(checkName([1, 3, 6]));

但是在第二版中，我的地图函数返回未定义，我真的不明白为什么?！

But in second version, my map function returns undefined, and I really don't understand why?!

var names = ["jim", "jack", "aaron", "hugh", "jeff", "cameron", "allen", "charlie"];
var len = 3,
  arr1 = [],
  counter = 1;


for (var i = 0; i < len; i++) {
  names.forEach(name => {
    arr1.push({
      id: counter,
      dude: name
    });
    counter++;
  });
}

console.log(arr1);

function checkName(nums) {
  var namesarr;
  for (var i = 0; i < arr1.length; i++) {
    namesarr = nums.map(function(num) {
      if (num === arr1[i].id) {
        return arr1[i].dude;
      }
    });
  }
  return (namesarr);
};
console.log(checkName([1, 3, 6]));

推荐答案

您的map函数不会为数组中的所有项目返回任何内容(由于if)，因此结果数组中的某些值将是undefined.

Your map function doesn't return something for all the items in the array (because of the if) so some values in the result array will be undefined.

此外，您的map处于循环遍历arr1的循环内，因此对于该循环的每次迭代，数组namesarr都会被覆盖.因此，map就像仅应用于arr1中的最后一个元素，因此，如果nums包含N元素，则namesarr至少将具有N - 1 undefined值( N - 1如果arr1的最后一个对象与nums匹配，则N如果不匹配).

Plus, your map is inside a loop that loops over arr1, so for each iteration of that loop, the array namesarr get overridden. So the map will be as if it was applied only for the last element in arr1, thus if nums contain N elements then namesarr will have at least N - 1 undefined values in it (N - 1 if the last object of arr1 matches nums, N if not).

使用 reduce 而不是map:

function checkName(nums) {
    return arr1.reduce(function(namesarr, obj) {     // for each object obj in the array arr1
        if(nums.indexOf(obj.id) !== -1) {            // if the object's id is in the array nums
            namesarr.push(obj.dude);                 // then add the object's dude to the array namesarr
        }
        return namesarr;
    }, []);                                          // the empty array to initialize namesarr
}

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