Cannot assign Uint8Array type to Uint8Array variable created with constructor

[achingbrain]

🔎 Search Terms

uint8array

🕗 Version & Regression Information

• This changed between versions 5.7.3 and 5.6.3
• This changed in commit or PR Make typed arrays generic over ArrayBufferLike #59417
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Uint8Arrays
• I was unable to test this on prior versions because Uint8Array was not generic before 5.7.3

⏯ Playground Link

https://www.typescriptlang.org/play/?target=11&ts=5.8.3#code/GYVwdgxgLglg9mABAcwKZQIICcuIBQCUAXIgKoxhQAc2WAhgJ6IDeAUIoluiFkmKgHcyFarUZ4ADAVYBfVqwA26RHRyIAvIn5DylGjnFTWq3JrSYcheUqgqcAJhK7RBppu3C9YhpOkn7GijotFZAA

💻 Code

function getArr (): Uint8Array {
  return new Uint8Array(0)
}

let arr = new Uint8Array(0)
arr = getArr() // fails compilation with TS2322

let arr2: Uint8Array = new Uint8Array(0)
arr2 = getArr() // this is fine

🙁 Actual behavior

TS2322 is thrown during compilation.

🙂 Expected behavior

I should be able to assign a value of type Uint8Array to a variable initialised with the Uint8Array constructor without specifying extra generics or types.

Additional information about the issue

The compilation error is:

Type 'Uint8Array<ArrayBufferLike>' is not assignable to type 'Uint8Array<ArrayBuffer>'.
  Type 'ArrayBufferLike' is not assignable to type 'ArrayBuffer'.
    Type 'SharedArrayBuffer' is missing the following properties from type 'ArrayBuffer': resizable, resize, detached, transfer, transferToFixedLength(2322)

[snarbles2]

I'm filling in the inferred types/generic arguments to make what's happening more obvious.

// <ArrayBufferLike> is the default generic argument
function getArr (): Uint8Array<ArrayBufferLike> { 
  return new Uint8Array(0)
}

// <ArrayBuffer> is the generic argument applied with this
// constructor overload
let arr: Uint8Array<ArrayBuffer> = new Uint8Array(0)
// Assigning Uint8Array<ArrayBufferLike> to Uint8Array<ArrayBuffer>
arr = getArr()

// <ArrayBufferLike> is the default generic argument
let arr2: Uint8Array<ArrayBufferLike> = new Uint8Array(0)
// Assigning Uint8Array<ArrayBufferLike> to Uint8Array<ArrayBufferLike>
arr2 = getArr()

I agree it's worthy of a double-take but I couldn't say what could be changed here that would fix this without causing other problems.

[Renegade334]

This is one of the inevitable consequences of #59417. This was mini-discussed at DefinitelyTyped/DefinitelyTyped#72056 (comment) and DefinitelyTyped/DefinitelyTyped#72056 (comment), with the verdict being "intended behaviour".

The flip-side is that this is very easy to work around by explicitly typing the variable, as you've identified.