Declare TypedArray length

[EnderShadow8]

Suggestion

🔍 Search Terms

typedarray length tuple typed array

✅ Viability Checklist

My suggestion meets these guidelines:

• [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [x] This wouldn't change the runtime behavior of existing JavaScript code
• [x] This could be implemented without emitting different JS based on the types of the expressions
• [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [x] This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

A way to declare a fixed length TypedArray similar to Tuples.

📃 Motivating Example

Consider a hypothetical vector library, similar glmatrix:

function addVector2(out: Float32Array, v1: Float32Array, v2: Float32Array) {
  out[0] = v1[0] + v2[0]
  out[1] = v1[1] + v2[1]
  return out
}

There's currently no way to type check that the arguments are indeed Vector2's and not some other Float32Arrays. This is possible with Tuple types though using regular arrays. Since TypedArrays are inherently fixed length even at runtime, it makes sense for there to be some way of declaring length. Even if this is just a minimum length, that would be a huge improvement since that guarantees out of bounds indexing safety.

💻 Use Cases

Many uses of TypedArrays in JS make assumptions about length. But the main one would probably be vectors, matrices, quarternions, etc.

[andrewbranch]

Some real-world examples might be helpful here. It feels to me like TS provides 95% of what you need, and you can fake the last 5% with an as type assertion: playground. Are there severe limitations to an approach like this when brought into the real world?

[andrewbranch]

Not exactly recommended, but here’s some hacks to make it work for plain old new Float32Array(length). I don’t think we’d ever add something like this to standard type definitions, but the fact that there are workarounds for cases like these means we’re probably less likely to add something specific to the language for it.

[fatcerberus]

Not directly related to the issue, but why does the error message in that playground say "Object is possibly undefined"? I was confused for a while because I couldn't figure out why it thought v1 could be undefined. I finally realized that it actually meant v1[0] might be undefined, but there's no object type involved there - the result of the indexing is number | undefined. It should probably say "value might be undefined" instead.

[amw]

I tried to hack something like this, but it doesn't work the way I prayed it would:

type RGBATuple = [number, number, number, number]
type TypedRGBATuple = Uint8ClampedArray & RGBATuple
const data = new Uint8ClampedArray(4) as TypedRGBATuple
const color = new Color(...data)
// produces error
// A spread argument must either have a tuple type or be passed to a rest parameter. ts(2556)

It's a bit odd since TypeScript should know that data is also a tuple.

[GoToLoop]

I can't pass a spread TypedArray to a function! =(

A spread argument must either have a tuple type or be passed to a rest parameter.(2556)

import type p5 from "p5"

export const ALL_COLORS = 0xff << 0o30

const arr8Bit = new Uint8Array(Int32Array.BYTES_PER_ELEMENT)
const bufView = new DataView(arr8Bit.buffer)

export function colorFrom32bitTo8Bits(p: p5, c: number) {
    bufView.setInt32(0, c)
    return p.color(...arr8Bit.slice(1), arr8Bit[0])
}

export function randomColor(p: p5) {
    return colorFrom32bitTo8Bits(p, p.random(ALL_COLORS))
}

Can't neither coerce it to a tuple nor make TS know its length.

https://www.TypeScriptLang.org/play?target=9#code/JYWwDg9gTgLgBDAnmApnMBWOAzKERwBEmhAUKSgB6SxwDGEAdgM7wCCAMhwPoDCA8h34AlAMpwAvHAAMlbNjgAeRTIgBmaeQYt4AQyhQAHACFg8KYxQB3OAFVgjGIbYHdiABQBJR2oBMLqDcAOmMATQAVAFFRbgAFSOFuSI5IgFlIgDlwgEpSbVY4ACMAV2wANWBrSThLGwARXRhdCut3fSNTGCCS+RQoXIpqaHhsYsY6GGAmeggAG2gAMTwQP0KzcIgTM2Z3MAAudAwAGnoDxmKQQr7suABvUjhHotKWqyDmFBhvGD93aRO6LknnAoJ9ilBGOgggx5lB3EEEe0tl1mLNgHQUO4AIzZE5IzoAbWkAF1cgBfchUGgjMYTKaQwKMAAm+F4c2guwOmBu92BoJg4MhMMWy1W602nR2YBOYCCjJZIHcnB4AiEYmy5NIQA

[GoToLoop]

Well, as a workaround I've had to use Array destructuring assignment for the TypedArray:

import type p5 from "p5"

export const ALL_COLORS = 0xff << 0o30

const arr8Bit = new Uint8Array(Int32Array.BYTES_PER_ELEMENT)
const bufView = new DataView(arr8Bit.buffer)

export function colorFrom32bitTo8Bits (p: p5, c: number) {
    bufView.setInt32(0, c)
    const [ a, r, g, b ] = arr8Bit
    return p.color(r, g, b, a)
}

export function randomColor (p: p5) {
    return colorFrom32bitTo8Bits(p, p.random(ALL_COLORS))
}

Not ideal though. But I can't see any fix in TS for it any time soon.

https://www.TypeScriptLang.org/play?target=9#code/JYWwDg9gTgLgBDAnmApnMBWOAzKERwBEmhAUKSgB6SxwDGEAdgM7wCCAMhwPoDCA8h34AlAMpwAvHAAMlbNjgAeRTIgBmaeQYt4AQyhQAHACFg8KYxQB3OAFVgjGIbYHdiABQBJR2oBMLqDcAOmMATQAVAFFRbgAFSOFuSI5IgFlIgDlwgEpSbVY4ACMAV2wANWBrSThLGwARXRhdCut3fSNTGCCS+RQoXIpqaHhsYsY6GGAmeggAG2gAMTwQP0KzcIgTM2Z3MAAudAwAGnoDxmKQQr7suABvUjhHotKWqyDmFBhvGD93aRO6LknjMdHAANpwXQnKAnADmJ0KcAAutV2lsYA8nlBPsUoIx0EEGPMoO4YXB4UUTrpcgBfchUGgjMYTKb4wKMAAm+F4c2guwOmBu92B2JguPxRMWy1W602nR2YBOYCC7K5IHcnB4AiEYmytKAA