typesafe i18n

i18n.ts

const translations = {
  translationOne: 'hello world',
  translationTwo: 'hello {{world}}',
  translationThree: "hello {{world}}, I'm {{name}}!",
  translationFour: '{{foo}} has a {{bar}}',
} as const;

type Translations = typeof translations;
type InterpolateInner<
  S extends string,
  U extends object = {}
  > = S extends `${string}{{${infer V}}}${infer Rest}`
  ? InterpolateInner<Rest, U & { [key in V]: string }>
  : U;

type Interpolate<S extends keyof Translations> = InterpolateInner<
  Translations[S]
>;

function t<T extends keyof Translations, Payload = Interpolate<T>>(
  ...args: keyof Payload extends never
    ? [translation: T]
    : [translation: T, payload: Interpolate<T>]
) { }

// Test cases

t('translationOne');
t('translationTwo'); // FAIL needs world
t('translationTwo', { world: 'Greece' });
t('translationThree', { world: 'Greece' }); // FAIL needs name
t('translationThree', { world: 'Greece',  name: 'Trash', }); 
t('translationFour', { foo: 'Greece', bar: 'Trash', });