Change type definition with a condition

Question

Currently I have the following code:

declare const isBrowser: boolean;
declare const API: BrowserAPI & ElectronAPI;

allowing me to do something such as the following:

if (isBrowser) API.browserOnlyFunction();

which works great, but unfortunately because of the union, this makes it so I can use API.browserOnlyFunction anywhere.

What I want to do is the following:

API; // BrowserAPI | ElectronAPI

if (isBrowser) {
  API; // BrowserAPI
} else {
  API; // ElectronAPI
}

I've tried doing the following but with no avail:

declare const isBrowser: boolean;
declare const API: typeof isBrowser extends true ? BrowserAPI : ElectronAPI; // is always ElectronAPI

// having to pass API as an argument every time seems pretty ugly and repetitive to me
function browserAPI(API: BrowserAPI | ElectronAPI): API is BrowserAPI {
  if (isBrowser) return true;
  return false;
}

Is there any way I can do this with just the isBrowser constant or just a function that doesn't take any arguments?