DataTransferItem Discriminated Union

[Offroaders123]

Realized how to use this in my own projects recently, and now I see that this is what I was looking for a while back to have for the DataTransferItem type. It's API is set up to work great with discriminated unions, but for some reason the standard library doesn't discern the different values against the kind property, while it does mention it in the JSDoc comments also.

I made a demo to try and add this functionality myself, but the types don't seem to apply for some reason. It might be that I'm trying to re-assign DataTransferItem with a type declaration instead of an interface one, I'm not completely sure. Either way, this is essentially the shape of what it would look like to enable the discriminated union checking using the kind attribute on the DataTransferItem object.

type DataTransferItem = DataTransferFile | DataTransferString;

interface DataTransferFile {
  readonly kind: "file";
  readonly type: string;
  getAsFile(): File;
  getAsString(callback: null): void;
  webkitGetAsEntry(): FileSystemEntry | null;
}

interface DataTransferString {
  readonly kind: "string";
  readonly type: string;
  getAsFile(): null;
  getAsString(callback: FunctionStringCallback): void;
  webkitGetAsEntry(): null;
}

https://basarat.gitbook.io/typescript/type-system/discriminated-unions

The base type definitions for DataTransfer and it's related APIs are in the DOM API types lib (lib.dom.d.ts), so I think it also may not be applying because of that, since I'm using declare global {} to wrap my custom types. I haven't tried overriding types in other lib definitions yet, so I think it also could be that.

[BenjaminAster]

I don't really think it's possible to completely reassign existing types and interfaces in TypeScript; at least not without causing some error messages in either lib.dom.d.ts or in the custom ones. If you can somehow get it to work, show me and I can add it to Better-TypeScript.