PHP

function formatCnpjCpf($value)
{
  $cnpj_cpf = preg_replace("/\D/", '', $value);
  
  if (strlen($cnpj_cpf) === 11) {
    return preg_replace("/(\d{3})(\d{3})(\d{3})(\d{2})/", "\$1.\$2.\$3-\$4", $cnpj_cpf);
  } 
  
  return preg_replace("/(\d{2})(\d{3})(\d{3})(\d{4})(\d{2})/", "\$1.\$2.\$3/\$4-\$5", $cnpj_cpf);
}

JS

function formatCnpjCpf(value)
{
  const cnpjCpf = value.replace(/\D/g, '');
  
  if (cnpjCpf.length === 11) {
    return cnpjCpf.replace(/(\d{3})(\d{3})(\d{3})(\d{2})/g, "\$1.\$2.\$3-\$4");
  } 
  
  return cnpjCpf.replace(/(\d{2})(\d{3})(\d{3})(\d{4})(\d{2})/g, "\$1.\$2.\$3/\$4-\$5");
}