Array construction with spread syntax and map doesn't work

[codeKonami]

I use this syntax quite a lot to create my Arrays

const foos =
    [...Array(4)]
        .map(x => 'foo');
// ['foo', 'foo', 'foo', 'foo']

But on your platform the array has the good size but of "undefined" element.

See here : https://stackblitz.com/edit/js-mtffex

[seveves]

Your pattern should definitely work but how about using Array.from({ length: 4 }, x => 'foo')); 😎

[codeKonami]

@seveves That's a really nice way to do it too :)
Thanks

[gdlm91]

I'm having the same problem while trying to create a range of numbers using this pattern:

• https://stackblitz.com/edit/js-v83uva

const minNumber = 1;
const maxNumber = 10;

const rangeArr = [...Array(maxNumber - minNumber + 1)].map(
  (item, index) => index + minNumber
);

console.log(rangeArr);

returns

[undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined]

Instead of

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Tried the same pattern in other platforms and they work correctly:

• https://jsbin.com/sutiqunowa/1/edit?js,console
• https://jsfiddle.net/41xhbr6q/
• https://codepen.io/gdlm91/pen/bGGxaJx?editors=0011

Using @seveves pattern works correctly:

const minNumber = 1;
const maxNumber = 10;

const rangeArr = Array.from(
  { length: maxNumber - minNumber + 1 }, 
  (item, index) => index + minNumber
);

console.log(rangeArr);

[jayphelps]

A seemingly related issue is how spreading works in general. It seems that however Stackblitz is doing compilation, it's in some sort of very loose mode that IMO is way too lose for something like Stackblitz.

e.g. [...something] is always compiled to something.slice() which is a big no-no because something can be anything iterable, including a Set which doesn't have slice, concat, etc.

My guess is its maybe its using an old version of babel's loose mode--the latest versions of loose mode don't seem to do this but it might have in the past.

[sulco]

Good catch everyone, thanks for reporting this!
Yeah, it was confusing...

Thankfully it should no longer be an issue :)