TypeScript | Declaration Merging

index.d.ts

/**
 * Why is any so important?
 * 
 * > Non-function members of the interfaces should be unique. If they are not unique,
 * they must be of the same type. The compiler will issue an error if the interfaces
 * both declare a non-function member of the same name, but of different types.
 * @see https://www.typescriptlang.org/docs/handbook/declaration-merging.html#merging-interfaces
 * 
 * Docs
 * @see https://www.typescriptlang.org/docs/handbook/declaration-merging.html
 */

// in the library
interface BaseImageSettings {
  // ATTENTION: must be any; or the app will not be able to properly augment this type
  imageVariation: any
}
interface IImageSettings extends BaseImageSettings {}

// in the app
interface IImageSettings {
  imageVariation: 'default' | 'custom'
}

const test: IImageSettings =  {
  imageVariation: 'default' // ✅
}

const test2: IImageSettings =  {
  imageVariation: 'custom' // ✅
}

const test3: IImageSettings =  {
  imageVariation: 'fails' // 🚨
}