Constructor for DataView is too broad

[ngbrown]

lib Update Request

Configuration Check

My compilation target is ES2021 and my lib is ["DOM", "DOM.Iterable", "ES2021"]. TypeScript version is 4.9.5.

Missing / Incorrect Definition

The DataView constructor parameter type is incorrectly broad by accepting ArrayBufferLike because TypedArray's such as Uint8Array match the ArrayBufferLike type but are not valid parameters. I think the DataView constructor parameter should only accept ArrayBuffer | SharedArrayBuffer.

Sample Code

The following produces no error in TypeScript but does produce an error in Node 16, Chrome 109, and FireFox 111b1:

new DataView(new Uint8Array(32));

The runtime error is:

Uncaught TypeError: First argument to DataView constructor must be an ArrayBuffer
    at new DataView (<anonymous>)

The correct code that does run is:

new DataView(new Uint8Array(32).buffer)

Documentation Link

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView/DataView
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/buffer
• https://tc39.es/ecma262/multipage/structured-data.html#sec-dataview-constructor (specifically a TypeError is to be thrown if this is a detached buffer, i.e. without array buffer data)

[fatcerberus]

ArrayBufferLike is already defined as ArrayBuffer | SharedArrayBuffer. The problem is that TS is structurally typed and typed arrays are structurally compatible with ArrayBuffer:

const foo: ArrayBuffer = new Uint8Array(42);  // no error

[ngbrown]

I knew that to a degree (duck typing and all), and I suppose someone will declare this as duplicate of #202, which I didn't know about. But this seems like these types should be the exception to structural typing because JavaScript is much more strict about the types actually being instanceof.

With lib.es5.d.ts listing ArrayBufferLike in the constructor there's a bit of indirection when looking at the type hints in an IDE, so you aren't clearly told that it should only be ArrayBuffer | SharedArrayBuffer because what is ArrayBufferLike anyways?

[fatcerberus]

Yeah, I think there's a feature request somewhere to be able to expand types in function hints.

Apparently this is intentional because doing otherwise is a breaking change: #31311 (comment)

[ngbrown]

Interestingly, never and Exclude<T, U> can almost help:

interface DataViewConstructor {
    readonly prototype: DataView;
    new<T extends (ArrayBuffer | SharedArrayBuffer) = ArrayBufferLike>(buffer: T & Exclude<T, {BYTES_PER_ELEMENT: number}>, byteOffset?: number, byteLength?: number): DataView;
}
declare var DataView: DataViewConstructor;

But it is a breaking change for types that inherit from DataView... Maybe someone smarter can make it work right without a generic?

[RyanCavanaugh]

I think this is actually a lot easier to accomplish:

new(buffer: ArrayBufferLike & { BYTES_PER_ELEMENT?: never }, byteOffset?: number, byteLength?: number): DataView;