Bug Report

TypeScript can not differentiate between ArrayBuffer and Uint8Array (or other typed array types).

const t: ArrayBuffer = new Uint8Array();

🔎 Search Terms

• Structural typing
• Brand checking

Related #50714, #42534, #202

🙁 Actual behavior

TypeScript checks the program above, and reports no errors.

🙂 Expected behavior

An error should be thrown, because in many APIs, passing a Uint8Array rather than an ArrayBuffer will throw or result in runtime errors.

Cause and proposed fix

The cause for this issue is TypeScript's structural typing. Within the type structure, ArrayBuffer is a subset of structure of Uint8Array. Because of this, TypeScript considers Uint8Array a valid ArrayBuffer type.

To fix this, we should consider adding a brand to the Uint8Array and ArrayBuffer interfaces, such as the following:

interface ArrayBuffer {
  readonly [Symbol.toStringTag]: "ArrayBuffer";
}

This may be an issue when subclassing ArrayBuffer, because the subclass may have a different Symbol.toStringTag