Yet another attempt at typed JS data

Michael Haufe tno at thenewobjective.com
Mon Feb 10 21:12:59 UTC 2020


Given your history I know better than to assume what you know…

The definition of sparse in the spec (while not explicitly in its own section) is straightforward.

V8’s inability or unwillingness to perform a safe “upcast” internally to an appropriate tag doesn’t seem to provide enough weight to introduce a new construct.


From: Andrea Giammarchi <andrea.giammarchi at gmail.com>
Sent: Monday, February 10, 2020 2:26 PM
To: Michael Haufe <tno at thenewobjective.com>
Cc: Bergi <a.d.bergi at web.de>; es-discuss at mozilla.org
Subject: Re: Yet another attempt at typed JS data

Great, now maybe you also read how it works behind the scene, and debug properly to understand that every array is holey, including the latter one, to date.

https://v8.dev/blog/elements-kinds

Please, let's assume for a second I knew what I was talking about, when I've said it's a mess to not have holey arrays, thanks.

On Mon, Feb 10, 2020 at 9:21 PM Michael Haufe <tno at thenewobjective.com<mailto:tno at thenewobjective.com>> wrote:
Array(3)
//  [empty × 3]

Array(3).fill()
// [undefined, undefined, undefined]

Array(3).fill('whatever')
// ["whatever", "whatever", "whatever"]


-----Original Message-----
From: es-discuss <es-discuss-bounces at mozilla.org<mailto:es-discuss-bounces at mozilla.org>> On Behalf Of Bergi
Sent: Monday, February 10, 2020 1:27 PM
To: es-discuss at mozilla.org<mailto:es-discuss at mozilla.org>
Subject: Re: Yet another attempt at typed JS data

Hello!

> Unfortunately, `Array.from({ length: 4 }, () => whatever)` produces a
> holey array

Does it? But really, if the performance difference betweeen HOLEY and PACKED arrays were large enough to be relevant[1], the engine programmers would certainly already have optimised all those trivial cases where an array is filled gradually to produce the more efficient representation.

kind regards,
 Bergi

[1]: it probably isn't:
https://stackoverflow.com/questions/54481918/#comment95848513_54485509
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