JavaScript数组中的空项目和undefined之间有什么区别? [英] What's the difference between empty items in a JavaScript array and undefined?

问题描述

考虑以下JavaScript代码(在节点REPL中):

Consider the following JavaScript code (in a node REPL):

> let a = new Array(10)
undefined
> a
[ <10 empty items> ]
> a.map(e => 1)
[ <10 empty items> ]
> let b = new Array(10).fill(undefined)
undefined
> b
[ undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined ]
> b.map(e => 1)
[ 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 ]
> 

当我创建一个空数组时，我会得到空项目"，它们的行为似乎与undefined不同.有人可以解释有什么区别吗?

When I create an empty array, I'll get 'empty items' which seem to behave differently from undefined. Can someone explain what is the difference?

推荐答案

当我创建一个空数组时，我会得到空项目"，似乎 行为与未定义的行为不同.有人可以解释什么是 有什么区别?

When I create an empty array, I'll get 'empty items' which seem to behave differently from undefined. Can someone explain what is the difference?

这是因为Array(10)不会填充数组.但这只是将 length 属性设置为10.因此，使用Array构造函数创建的数组只是一个具有length属性的对象，而没有填充任何项.

That's because Array(10) doesn't populate the array. But it just set the length property to 10 .So, the array created using Array constructor is simply an object with a length property, but with no items populated.

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