useTranslation typings

useTranslation.ts

type Dict = typeof dict

type KeysUnion<T, Cache extends string = ''> =
  T extends PropertyKey ? Cache : {
    [P in keyof T]:
    P extends string
    ? Cache extends ''
    ? KeysUnion<T[P], `${P}`>
    : Cache | KeysUnion<T[P], `${Cache}.${P}`>
    : never
  }[keyof T]

type RemoveDot<T extends string> = T extends `.${infer Tail}` ? Tail : T;

type ExtractString<T extends string, U extends string, Result extends string = ''> =
  T extends `${infer Head}${infer Tail}` ? `${Result}${Head}` extends U ? Tail : ExtractString<Tail, U, `${Result}${Head}`> : Result

type ValidPrefix<T extends string, U extends string> = T extends `${U}${string}` ? Exclude<T, U> : never


type ConcatNamespaceWithPrefix<N extends string, P extends string> = `${N}.${P}`


type Values<T> = T[keyof T]

type Elem = string;

type Acc = Record<string, any>

// (acc, elem) => hasProperty(acc, elem) ? acc[elem] : acc
type Predicate<Accumulator extends Acc, El extends Elem> =
  El extends keyof Accumulator ? Accumulator[El] : Accumulator

type Reducer<
  Keys extends Elem,
  Accumulator extends Acc = {}
  > =
  Keys extends `${infer Prop}.${infer Rest}`
  ? Reducer<Rest, Predicate<Accumulator, Prop>>
  : Keys extends `${infer Last}`
  ? Predicate<Accumulator, Last>
  : never

type UseTranslationsProps<D = typeof dict> =
  (() => <Prefix extends KeysUnion<D>>(prefix: Prefix) => Reducer<Prefix, D>)
  & (
    <
      ValidKeys extends KeysUnion<D>,
      Namespace extends ValidKeys
      >(namespace?: Namespace) =>
      <Prefix extends RemoveDot<ExtractString<ValidPrefix<KeysUnion<Dict>, Namespace>, Namespace>>>(
        prefix: Prefix
      ) => Reducer<ConcatNamespaceWithPrefix<Namespace, Prefix>, D>
  )


declare const useTranslations: UseTranslationsProps;

const dict = {
  one: {
    two: {
      three: "3",
      foo: "bar"
    }
  },
  bar: {
    baz: 2
  }
} as const

{
  const t = useTranslations() // ok
  const ok = t('one') // {two: ....}
}
{
  const t = useTranslations('one.two') // ok
  const ok = t('three') // 3
}

{
  const t = useTranslations() // ok
  const ok = t('three') // expected error
}