microsoft/TypeScript

Incorrect inference for Uint8Array's underlying source

Closed this issue · 3 comments

BlackAsLight commented

🔎 Search Terms

uint8array, arraylike, sharedarraybuffer

🕗 Version & Regression Information

  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about Uint8Arrays.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20251022#code/GYVwdgxgLglg9mABAUwB4EMC2AHANsgClQC5EBBAJwvQE8AhEYYZCgGRgGtlEAfcq2uy4AeMCEwAjFgD4AlKQCqMMFAAclajWEbaDJiyHJpiAN4AoRIgrIoICkjDIA7oiUr1AmkVlmAvkA

💻 Code

function example(x: ArrayBufferLike | ArrayLike<number>): Uint8Array<ArrayBufferLike> {
  return new Uint8Array(x)
}

🙁 Actual behavior

The example function can take an ArrayBuffer, SharedArrayBuffer or a number[] to wrap it in a Uint8Array, but due to ArrayLike<number>, the Uint8Array constructor incorrectly assumes the underlying source will be an ArrayBuffer and rejects x being passed in since it can also be a SharedArrayBuffer.

🙂 Expected behavior

The expected behaviour is for the Uint8Array constructor to assume the underlying source will be ArrayBufferLike and not ArrayBuffer.

Additional information about the issue

No response

jcalz commented

Seems like #14107 is the underlying issue here; TS has two separate overloads for ArrayBufferLike and ArrayLike<number>, and you cannot call both of them at once. I doubt there's much appetite to add even more complicated construct signatures to handle these edge cases. If you need this yourself you can always just use declaration merging in your own code base like

interface Uint8ArrayConstructor {
  new <T extends ArrayBufferLike | Iterable<number> | ArrayLike<number> | number>(buffer: T):
    Uint8Array<T extends ArrayBufferLike ? T : ArrayBuffer>;
}

and then your code will just work.
Playground link to code

  • 👍2
RyanCavanaugh commented

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typescript-bot commented

This issue has been marked as "Design Limitation" and has seen no recent activity. It has been automatically closed for house-keeping purposes.