Javascript-based validation of Swiss AHV numbers AHVN13 / EAN13 [Validierung von Schweizer AHV Nummern mit Javascript]

ahvn13-validation.js

/**
 * Javascript-based validation of Swiss AHV numbers AHVN13 / EAN13
 * [Validierung von Schweizer AHV Nummern mit Javascript]
 */

var inp = document.getElementById('ahvn13');
inp.addEventListener('change', _validateAhvn13);

var _validateAhvn13 = function(input) {
    var pattern = /^756\.(\d{4})\.(\d{4})\.(\d{2})$/; // format is 756.0000.0000.00
    var inp = input.target;
    if(!inp.value.match(pattern)) {
        inp.setCustomValidity('Wrong format. Expected format: 756.0000.0000.00');
    } else {
        if(_computeAhvn13CheckDigit(inp.value) != parseInt(inp.value.substr(-1))) {
            inp.setCustomValidity('Number not valid');
        } else {
            inp.setCustomValidity('');
        }
    }
}

var _computeAhvn13CheckDigit = function(ahvn13str)
{
    // EAN13: remove non-digits, remove last character, reverse the order of the string
    var ahvn13str = ahvn13str.replace(/\D/g,'');
    chars = ahvn13str.substring(0, ahvn13str.length-1).split("").reverse();

    // EAN13: first  + second * 3 + third + fourth * 3 etc.
    var crosssum = 0;
    for(i = 0; i < chars.length; i++) {
        if(0 == i % 2 ) { // even
            crosssum += parseInt(chars[i]) * 3;
        } else { // odd
            crosssum += parseInt(chars[i]);
        }
    }

    // EAN13: checkdigit is the difference of crosssum to the next multiple of 10
    return (10 - (crosssum % 10));
}

Sorry. You may try without the reverse(). I am not sure that this step is necessary according to EAN-13 standard.