Type alias Key<TStringMap>

Key<TStringMap>: {
    [KeyPart in keyof TStringMap]: TStringMap[KeyPart] extends StringMap
        ? `${FormatKey<KeyPart>}${KeyInner<TStringMap[KeyPart]>}`
        : FormatKey<KeyPart>
}[keyof TStringMap]

Union type of all keys for TStringMap