Declaration Files in TypeScript

// Ambient declarations (*.d.ts)
declare module "untyped-module" {
  export function doSomething(): void
  export const value: string
}

// Global declarations
declare const VERSION: string
declare function analyticsTrack(event: string): void

// Module declarations
declare module "*.css" {
  const content: { [className: string]: string }
  export default content
}

declare module "*.svg" {
  const content: React.FC<React.SVGProps<SVGSVGElement>>
  export default content
}