spanishAccountNumberValidation.js

/**
* Spanish Bank Account Number validation
*/
function saneaFormato(str, num) {
    return '' + new Array((num - str.toString().length)+1).join('0') + str;
}

function mod11(x) {
    var pesos = [ 1, 2, 4, 8, 5, 10, 9, 7, 3, 6 ], control = 0, i, offset = pesos.length - x.length;
    for (i = 0; i < x.length; i++)  control += x.charAt(i) * pesos[i+offset];
    control = 11 - (control % 11);
    if (control == 11) control=0;
    if (control == 10) control=1;
    return control;
}

function validaCC( banco, oficina, dcontrol, ncuenta){
    var entoficina = 0, ccorriente = 0, i;
    entoficina = mod11(saneaFormato(banco, 4) + '' + saneaFormato(oficina, 4));
    ccorriente = mod11(saneaFormato(ncuenta, 10));
    if (entoficina*10+ccorriente != saneaFormato(dcontrol, 2)) return false;
    return true;
}

It lacks some checks but as long as you keep the numbers in the range it will work