“空"和“未定义"之间的区别在 Javascript 中? [英] Difference between "empty' and "undefined" in Javascript?

问题描述

根据 ECMAScript，除了其他数据类型之外，还有 null 和 undefined 数据类型.但是最近我看到了一个叫做 empty 的东西.让我们先重新生成它(我将从基本开始):

According to ECMAScript there are null and undefined datatypes apart from others. But recently I got to see something called empty. Let's regenerate it first (I'll go from basic):

let a = new Array() //返回[] length=0

let a = new Array() //Returns [] length=0

let b = new Array(3) //返回[empty*3] length=3

let b = new Array(3) //Returns [empty*3] length=3

let c = new Array(3).fill() //返回[undefined,undefined,undefined] length=3

let c = new Array(3).fill() //Return [undefined,undefined,undefined] length=3

现在呢?查看所有定义的元素都是未定义的，它们应该是

Now, what? See all the elements defined are undefined, which they should be

• a[0]=undefined
• b[0]=undefined
• c[0]=undefined

主要内容:如果假设在 c 的位置 5 处添加一个元素，即 a[5]=4那么 c 将是 [undefined,undefined,undefined,empty,4]

Main thing: If suppose add an element into c at position 5 i.e a[5]=4 then c will be [undefined,undefined,undefined,empty,4]

这中间的空东西是什么?如果一切都相同，b 和 c 的开始初始化有什么区别(我说的是 [empty*3] &[未定义，未定义，未定义]).

What is this empty thing in between? And if everything is same what is the difference in starting initialization of b and c ( I'm talking about [empty*3] & [undefined,undefined,undefined]).

答案建议[此处]:JavaScrip 数组中的空项和未定义之间有什么区别? 没有正确解释它到底是什么.是的，它不能是 undefined，因为 undefined 是一个值，但是 null 怎么样，如果不是 null 为什么为空不是数据类型?

The Answer suggested [here]: What's the difference between empty items in a JavaScrip array and undefined? Doesn't explains the properly about what exactly it is. Yes it can't be undefined as undefined is a value but what about null and if not null why empty is not a data-type?

推荐答案

数组是对象.这意味着你的数组

Arrays are objects. That means that your array

  [undefined,undefined,undefined,empty,4]

可以写成这样一个对象:

could be written as such an object:

 {
  0: undefined,
  1: undefined,
  2: undefined,
  // 3 doesnt exist at all
  4: 4,
  length: 5,
 }

所以 undefined 实际上是一个存在并保存值的槽，而 3 甚至不是一个键值对.由于访问不存在的键会导致 undefined，因此在现实世界中没有区别:

So undefined is actually a slot that exists and holds a value while 3 is not even a key-value pair. As accessing a non existing key results in undefined there is no real world difference:

    array[2] // undefined
    array[3] // undefined

但是使用 map 和其他方法遍历数组会跳过空数组，因此您应该在迭代之前使用 .fill.

But iterating over an array with map and others will skip empty ones, so then you should use .fill before iterating.

检测差异的唯一方法是检查密钥是否存在:

The only way to detect a difference is to check if the key exists:

  2 in array // true
  3 in array // false

要将empty 变为undefined，您可以设置密钥:

To turn empty to undefined you could set the key:

  array[3] = undefined;

并且要将 undefined 变成 empty 您必须删除密钥:

and to turn undefined into empty you have to remove the key:

  delete array[3]

然而，这很少重要.

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