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ArrayBuffer is not eliminated from union as expected in narrowing of ArrayBuffer | Uint8Array #50714
Description
Bug Report
🔎 Search Terms
arraybuffer, narrow, union
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about ArrayBuffer
⏯ Playground Link
Playground link with relevant code
💻 Code
import fs from "node:fs/promises";
async function writeData(data: Uint8Array | ArrayBuffer): Promise<void> {
if (data instanceof ArrayBuffer) {
data = new Uint8Array(data);
}
// Argument of type 'Uint8Array | ArrayBuffer' is not assignable to parameter of type 'string | ArrayBufferView | Iterable<string | ArrayBufferView> | AsyncIterable<string | ArrayBufferView> | Stream'.
// Type 'ArrayBuffer' is not assignable to type 'string | ArrayBufferView | Iterable<string | ArrayBufferView> | AsyncIterable<string | ArrayBufferView> | Stream'.
// Type 'ArrayBuffer' is missing the following properties from type 'Float64Array': BYTES_PER_ELEMENT, buffer, byteOffset, copyWithin, and 24 more.(2345)
await fs.writeFile("./some-file", data);
}🙁 Actual behavior
Currently ArrayBuffer is not eliminated from the union, so data being passed to fs.writeFile is not of the right type.
🙂 Expected behavior
I expected the conditional to remove ArrayBuffer from the union, making data's type Uint8Array (after the conditional).
Activity
jcalz commentedon Sep 11, 2022
Duplicate of #42534, #31311, and ultimately #202. It's hard to fix this without breaking other things, apparently.
Jamesernator commentedon Sep 11, 2022
I saw those issues, but even with them, they don't explain why narrowing shouldn't work here. Like even if TS (incorrectly) assumes
Uint8Array <: ArrayBuffer, the conditionaldata instanceof ArrayBuffershould be recognized as always true, in which casedata = new Uint8Array(data)should always narrowdatatoUint8Arrayanyway.An even simpler case (that would be incorrect at runtime, but TS shouldn't notice) that currently fails is (and should be what TS effectively sees for the above conditional):
Like why is
datastill inferred asUint8Array | ArrayBufferonce it hits the second line of the function? It should be narrowed to the subtypeUint8Arrayas it's clearly been set to the subtype.jcalz commentedon Sep 11, 2022
Okay, I see. So maybe the minimal repro here should be something like
and the question is why the assignment fails to narrow. You might want to put the stripped down repro at the top of the issue and mention how this differs from #31311, although presumably interested parties would be reading these comments too.
jcalz commentedon Sep 11, 2022
So maybe this is a combination of #42534 (array buffers and typed arrays are erroneously compatible) and #47731 (subtype reduction doesn't consistently happen upon assignment)?
typescript-bot commentedon Sep 18, 2022
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
Param Names + ArrayBuffer Sources
Offroaders123 commentedon Dec 13, 2022
Came up with a great solution! I'm using this in my own library at the moment.
ArrayBuffer.toString()is simply on the prototype, inherited fromObject, so there's nothing directly defined onArrayBufferfortoString()itself.I tried a few different ways to make it work, like a facade property definition, but that was fairly ugly, as it added a property that didn't exist at all, and it wasn't real.
This on the other hand,
toString()already exists, and this simply changes the return type fromstringto a more specific"[object ArrayBuffer]". So happy this solved it! Hopefully it works in your use case too.Another note: I defined this inside of my project's
global.d.tsfile, so I think that means it won't leak out of your own codebase, and into other type definitions. If that's not the case, definitely let me know.