Why `new Array(5).map((x,y)=>y*2)` didn't work?

Question

问题描述

I was trying to create an array which is [1,2,3,...,n], so I used code new Array(10).map((value,index)=>index+1), but I got [empty × 5];

Meanwhile, new Array(10).fill(1).map((value,index)=>index+1), why?

How did Array.prototype.map() work?

相关代码

// 请把代码文本粘贴到下方（请勿用图片代替代码）

// [empty × 10]
new Array(10).map((value,index)=>index+1)

// [1,2,...,10]
new Array(10).fill(1).map((value,index)=>index+1)

你期待的结果是什么？实际看到的错误信息又是什么？

Why?

How did Array.prototype.map() work?

Answer 1

new Array() 创建的是一个稀疏数组，对于稀疏数组 map、filter、foreach 等方法并不会调用传入的第一个参数（函数）

Answer 2

It is not called for missing elements of the array (that is, indexes that have never been set, which have been deleted or which have never been assigned a value)

参考：https://developer.mozilla.org...

Answer 3

how about

Array.apply(null,Array(4)).map((x, y)=>y*2)

keep your code like a function