Strongly typed sprintf (allows placeholders and named placeholders)

sprintf.d.ts

type Specifiers = {
    's': string,
    'd': number,
    'b': boolean,
    'D': Date
};
type S = keyof Specifiers;

type ExtractNamedPlaceholders<T extends string> =
    T extends `${infer Start}%(${infer Key})${infer Spec}${infer Rest}`
        ? Spec extends S
            ? { [K in Key]: Specifiers[Spec]} & ExtractNamedPlaceholders<Rest>
            : never
        : {};

type ExtractUnnamedPlaceholders<T extends string> =
    T extends `${infer Start}%${infer Spec}${infer Rest}`
        ? Spec extends S
            ? [Specifiers[Spec], ...ExtractUnnamedPlaceholders<Rest>]
            : never
        : [];

type SprintfArgs<T extends string> =
    ExtractUnnamedPlaceholders<T> extends never
        ? [values: ExtractNamedPlaceholders<T>]
        : ExtractUnnamedPlaceholders<T>;

declare function sprintf<T extends string>(
    format: T,
    ...values: SprintfArgs<T>
): string;

sprintf('this is a %s and it is %d years old, right?%b %D', 'erik', 20, true, new Date()); // ok, it checks for different types
sprintf('this is a %s and it is %b years old, right?%d %D', 'erik', 20, true, new Date()); // not ok, wrong type
sprintf('this is a %s and it is %b%b%b%byears old, right?%d %D', 'erik', 20, true, new Date()); // not ok, too many arguments
sprintf('Hello %s', 'John'); // ok
sprintf('Hello %s', 'John', 'Doe'); // not ok, too many arguments
sprintf('Hello %s', false); // not ok, wrong type
sprintf('Hello %s', false); // not ok, wrong type
sprintf("Ciao %(names)s", { names: "erik" }) // ok named placeholder
sprintf("Ciao %(names)s %(years)d", { names: "erik", years: 2 });// ok multiple named placeholder
sprintf("Ciao %(names)s %(years)d", { names: "erik", years: "2" });// not ok, wrong type