TypeScript: How to set a new property on the window object

When merging global interfaces in TypeScript, you have to declare the interface in the global scope.

export declare global {
  interface Window {
    // add you custom properties and methods
    YT:(videoID: string, options?: {}) => Observable<YtResponse>
  }
}

TypeScript supports declaration merging, the premise is the same for global objects. The difference is to declare the interface as part of the global scope.