JavaScript "new Array(n)" and "Array.prototype.map" weirdness

I had a task that I only knew the length of the array and needed to transform the items.I wanted to do something like this:

let arr = new Array(10).map((val,idx) => idx);

To quickly create an array like this:

[0,1,2,3,4,5,6,7,8,9]

But it didn't work because:(see Jonathan Lonowski's answer a few answers above)

The solution could be to fill up the array items with any value (even with undefined) using Array.prototype.fill()

let arr = new Array(10).fill(undefined).map((val,idx) => idx);

console.log(new Array(10).fill(undefined).map((val, idx) => idx));