Bank Routing Checksum; ABA Routing checksum; Clean

bankRoutingChecksum.js

function isValidRoutingNumber(routingNum) {
    var ABA_CHECKSUM_CONSTANTS = [3, 7, 1];
    var ABA_CHECKSUM_DIVIDER = 10;
    var ABA_LENGTH = 9;
    var i = 0;
    var checksum = 0;

    if (isNaN(routingNum) || routingNum.length !== ABA_LENGTH) {
        return false;
    }

    for (i = 0; i < routingNum.length; i++) {
        checksum += parseInt(routingNum[i].toString()) * ABA_CHECKSUM_CONSTANTS[i % ABA_CHECKSUM_CONSTANTS.length];
    }

    return checksum !== 0 && checksum % ABA_CHECKSUM_DIVIDER === 0;
}

Checksum reasoning https://www.azcode.com/aba/aba.htm