Uint8Array assignable to ArrayBuffer despite having subtle differences leading to browser TypeErrors

[peaBerberian]

Bug Report

🔎 Search Terms

ArrayBuffer
Uint8Array

🕗 Version & Regression Information

At least in:

• typescript 4.1.3
• nightly

⏯ Playground Link

Playground link with relevant code

💻 Code

var a = new Uint8Array([1, 2, 3]);
new DataView(a);

Also, to be more explicit about the root issue in TypeScript:

function createDataView(buffer : ArrayBuffer) : DataView {
  return new DataView(buffer);
}

var a = new Uint8Array([1, 2, 3]);
createDataView(a);

🙁 Actual behavior

This is kind of continuation of the since closed due to inactivity #31311 issue, but with other arguments.

The sample codes presented here go through TypeScript's checks despite both leading to a browser TypeError because DataView's constructor expects an ArrayBuffer, not an Uint8Array (on FireFox, the error is: TypeError: DataView: expected ArrayBuffer, got Uint8Array).

From the linked issue, it seems that TypeScript considers an Uint8Array to be a valid ArrayBuffer because they are structurally the same. But there are subtle differences like this one.

I understand the point of view that TypeScript is not supposed to be perfectly sound, but I think that it would be nice if TypeScript caught those types of mistakes.

We had an issue related to this and I think that we wouldn't have it with plain JavaScript, because we would have been more careful with types! In the end, not always thinking about these types of mistakes is a major reason for switching to TS.

🙂 Expected behavior

I would expect that an ArrayBuffer and a Uint8Array to not be considered the same thing.

[MartinJohns]

As mentioned in #31311, this is a duplicate of #202 (or it would require #202 due to the nature of TypeScripts type system). Wesley even showed a workaround.

since closed due to inactivity

The main reason it was closed was not due to inactivity, but because it was marked a duplicate.

[peaBerberian]

OK thanks.

My bad, I have skipped over that one comment strangely.
I saw the last comment in the issue write about a "duplicate" but I didn't see which one it was speaking of.

The work-around should work and I saw it but this issue was more about integrating that logic into TS instead (as I didn't see the duplicates, I took that comment as a refusal to include it in TS).

I'm closing that issue now.