Types appear to be only additive, not subtractive when compared with JS Fetch API types

[hansede]

The Issue

Some of the JS Fetch API is not implemented in Cloudflare Workers. One example of this is Body.blob() on a Request object, which is implemented in the JS Fetch API, but calling this function in a Cloudflare Worker will result in an error:

Failed to execute 'blob' on 'Body': the method is not implemented.

The Cloudflare Workers reference appears to agree (by omission) that Body.blob is not implemented:
https://developers.cloudflare.com/workers/reference/apis/request/

However, when using the Request type provided by the @cloudflare/worker-types repo, using Body.blob does not cause a type error and instead appears to be implemented:

Inspecting the types in this repo, it seems that the definition of Request is purely additive (adding the cf property) and does nothing to prohibit unimplemented properties:

interface Request {
  cf: IncomingRequestCfProperties;
}

A Possible Solution

Before discovering the official types repo for Cloudflare Workers, I implemented TypeScript types for my Workers from scratch. In order to make the types for Cloudflare's Fetch API omit properties from the JS Fetch API, I defined custom types prefixed with Cf, for example:

export default interface CfRequest {
  readonly clone: () => CfRequest;
  readonly cf: CfObject;
  readonly bodyUsed: boolean;
  readonly headers: Headers;
  readonly method: RequestMethod;
  readonly redirect: RequestRedirect;
  readonly text: () => Promise<string>;
  readonly url: string;
}

By generating a custom type that inherits nothing from the official Fetch API types, unimplemented properties like Body.blob can be omitted.