Typescript Conditional Types - The simple example

typescript-conditional-types.ts

// full article https://medium.com/@martinratinaud/typescript-conditional-types-the-simple-example-8c971c04b6bc
// playground https://www.typescriptlang.org/play/#code/PTAEDEFcBttBDATgFwJYGNoFNQHsB2oAFssgA4DOAXCALZYAmqktAdOrrcAAK1Jr5E8AfEgNgyAJ5ksFdIlRlkAWg74maAvGjKpMirqJZlFVLTLZlWAB7xzlgBzoAnAHYAjOgAMAFgBGAGx+6ABQIXo4AJIAKtI4ALygAOTQkgroFEmgAD7JFBaoyEkA3OFxoJEUADJpGBQA8ogAygXIADzRAHygidGgNshY6hTJqemZoAD8oAAKiJyoFFhtAERjdSvdVKAr+dCFK6UhahTIoABmkPjoPaAAFBHbMXEAlE-VtRmNLfvtzzLdeLdADeIVA4NAqHO9wiPXiiRSn0yL1AoIh6NAiCwyEgiEIcwWS1YWIouGgADcsHdEeMki9SuiAL5giFYnF4vKtEohZlAA

type IType = 'lyrics' | 'split';
type IsLyricsOrSplit<T> = T extends 'lyrics' ? Promise<"lyrics"> : "split";

const func = (type: IType): IsLyricsOrSplit<IType> => {
    if (type === 'lyrics') {
        return Promise.resolve('lyrics');
    }
    return 'split';
}