ArrayBuffer.isView not narrowing type

[SarahAnnTolsma]

🔎 Search Terms

ArrayBuffer, ArrayBuffer.isView, type narrow

🕗 Version & Regression Information

This is the behavior in every version I tried, including the nightly version.

I've reviewed the FAQ on type narrowing, but I do not believe it applies to the issue at hand. It could be related to https://github.com/Microsoft/TypeScript/wiki/FAQ#why-doesnt-isfoox-narrow-x-to-foo-when-isfoo-is-a-type-guard, but given that the section is not filled out ("TODO") it is unclear whether this is the same issue.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240327#code/GYVwdgxgLglg9mABMOcAUATAhlLAuRAZygCcYwBzRAH0QFVyoAOAQRJKwE8bE2POAQiGDAApiQCUBMCAC2AI3EAoAN5LEGxDGCI0UTgAdRcHdlyIAvFcQAiYmUo2J6zWs3vEZrJcRhRAd0QAFVEADygAUUg4DHE0CQA6UWjYzBwsCQBuFw0AXxzEUQAbQlEtHTQAQj4uIRFxBJhCADUYALTcCWd3Nw8NLx8-QIYwZhrODozs93z3AoB6ed4oRCgACybEAzhGABpEAAMvA8QAfUI1uBAijFPERXpGVnYufcUILBBS3hfBYTESI0Wm1-PFECRRFAQCQwIRDsAsCVRCcUCRDuM6gCEogAJKwqCiLAYfZHdInTbEGBFIqrQyiDCILBwg4jMa-HgY-7iA4JAoQqEwzzpBJFZIUdbZXJAA

💻 Code

function foo(data: string | Uint8Array | ArrayBuffer): number
{
    if (typeof data === "string")
    {
        data = new TextEncoder().encode(data);
    }
    else if (!ArrayBuffer.isView(data))
    {
        data = new Uint8Array(data);
    }
    
    // At this point, `data` _should_ be Uint8Array, because ArrayBuffer.isView() returns `false` for `ArrayBuffer. Instead, `data` is still typed as `Uint8Array | ArrayBuffer`.
    return data.length;
}

🙁 Actual behavior

The data variable is not being narrowed to a Uint8Array by the time it reaches the return statement. This is causing both an Intellisense error, as well as a build error.

🙂 Expected behavior

In the above code, I would expect data to be correctly narrowed to a UInt8Array by the time we reach the return line. Given that ArrayBuffer.isView() returns true for Uint8Array and false for ArrayBuffer, the if / else if guarantees that data must be Uint8Array at the return.

I can confirm this behavior by running the code as plain JS in the DevTools console:

function foo(data)
{
    if (typeof data === "string")
    {
        console.log("string");
        data = new TextEncoder().encode(data);
    }
    else if (!ArrayBuffer.isView(data))
    {
        console.log("arraybuffer");
        data = new Uint8Array(data);
    }
    else
    {
        console.log("uint8array");
    }
    
    return data.length;
}

< foo("hello")
> string
> 5
< foo(new Uint8Array())
> uint8array
> 0
< foo(new Uint8Array().buffer)
> arraybuffer
> 0

Additional information about the issue

No response

[RyanCavanaugh]

In the above code, I would expect data to be correctly narrowed to a UInt8Array by the time we reach the return line. Given that ArrayBuffer.isView() returns true for Uint8Array and false for ArrayBuffer,

The problem is that it's possible (from a structural typing point of view) that data is an ArrayBuffer & ArrayBufferView, in which case isView would return true for a value inhabiting the ArrayBuffer part of the union.

[fatcerberus]

@RyanCavanaugh Is that related to the typed-array types being structurally compatible with ArrayBuffer (a complaint for which there are several existing issues that I don't feel like looking up at present)

[typescript-bot]

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