Vanilla JavaScript validators: the algorithms behind Spanish ID documents

Vanilla JavaScript validators: the algorithms behind Spanish ID documents This article walks through the validation algorithms for Spanish identity documents (DNI, NIE, CIF) and their Iberoamerican counterparts (RFC, RUT, RUC, NIT, CUIT). All implementations are vanilla JavaScript with zero dependencies. They run client-side at haz.tools/c/validar . Why these algorithms matter Identity documents in Spanish-speaking countries embed mathematical structure to detect transcription errors. A single t