Determines if a credit card number is valid and returns the name of the issuing network.

cardInfo() - JavaScript Credit Card Validity Check Function (Luhn and length checks)

// cardInfo()
//
// Determines if a credit card number is valid and returns the name of the issuing network.  When
// passed a purported card number as a candidate if the card number is valid, the cardInfo()
// function returns a boolean flag ('isValid') indicating if the card is a valid number and a 
// string ('network') containing the name of the issuing network.  Card validity is determined
// based upon the card number length and Luhn check digit value.  Valid card lengths are based
// upon the card lengths valid for a given issuing network.  The Luhn check digit is calculated
// using the Luhn algorithm.  The function contains a static table ('IINData') that defines the
// supported issuing network.  The table is not a complete list--it is only meant to contain
// the networks supported by the system using this function.  But, it can easily be expanded
// too support additional networks.  cardInfo() expects that the card number be passed as a 
// string and will log a console error and return 'isValid' of false if this is not the case. The
// function does not return the card number itself in the object for safety, but does return the
// last four digits and a masked version as a conveinence. Also, anytime 'isValid' is false an
// error reason is returned in 'errMessage'
//

function cardInfo(cardNum) {

    // Issuer identification number (IIN) look-up table
    // c.f. https://en.wikipedia.org/wiki/Payment_card_number
    // c.f. http://www.iinbase.com/
    // Expand this table to support additional issuing networks.  'startIIN' and 'endIIN' define
    // the range of IINs supported by an issuing network.  'name' defines the network's common
    // name.  And, 'lengths' is an array containing the supported card number lengths valid
    // for the IIN range.
    const IINData = [
        {startIIN:"222100", endIIN:"272099", name:"MasterCard", lengths:[16]},
        {startIIN:"340000", endIIN:"349999", name:"American Express", lengths:[15]},    
        {startIIN:"370000", endIIN:"379999", name:"American Express", lengths:[15]}, 
        {startIIN:"400000", endIIN:"499999", name:"Visa", lengths:[13, 16, 19]},
        {startIIN:"510000", endIIN:"559999", name:"MasterCard", lengths:[16]},    
        {startIIN:"601100", endIIN:"601199", name:"Discover", lengths:[16]},    
        {startIIN:"622126", endIIN:"622925", name:"Discover", lengths:[16]},    
        {startIIN:"644000", endIIN:"649999", name:"Discover", lengths:[16]},    
        {startIIN:"650000", endIIN:"659999", name:"Discover", lengths:[16]},    
    ];
    
    var cardInfo = {
        isValid: false
    }
    var IIN, IINIndex, lengthOk, luhnOk, sum, parity;
  
    // Set card primary account number or exit if card number is not a string
    if (typeof cardNum !== "string") {
        console.error("'cardInfo()' passed an unexpected type (" + (typeof cardNum) + ") instead of a string");
        cardInfo.errMessage = "Internal error: 'cardInfo()' passed an unexpected type";
        return cardInfo;
    }

    // Get IIN or exit if PAN is less then absolute minimum length (6)
    if (cardNum.length < 6) {
        cardInfo.errMessage = "Card number is not long enough";
        return cardInfo;
    }
    IIN = cardNum.substring(0,6);
  
    // Save masked and last-four-digits version of number
    cardInfo.fourDigits = cardNum.slice(-4);
    cardInfo.masked = Array(cardNum.length - 3).join('*') + cardNum.slice(-4);
    
    // Search IIN table and exit if an entry is not found
    for (IINIndex = 0; IINIndex < IINData.length; IINIndex++) {
        if (IINData[IINIndex].startIIN <= IIN && IIN <= IINData[IINIndex].endIIN) {
            break;
        }
    }
    if (IINIndex === IINData.length) {
        cardInfo.errMessage = "Issuing network is not supported"
        return cardInfo;
    }
    cardInfo.network = IINData[IINIndex].name;
  
    // Check length requirements and exit if length is too short for issuing network
    for (var i = 0; i < IINData[IINIndex].lengths.length && !lengthOk; i++) {
        lengthOk = (IINData[IINIndex].lengths[i] === cardNum.length);
    }
    if (lengthOk === false) { // Avoiding not-truthy (!lengthOk) for paranoid safety
        cardInfo.errMessage = IINData[IINIndex].name + " card numbers must be " + IINData[IINIndex].lengths + " digits in length";
        return cardInfo;
    }

    // Check Luhn 
    // c.f. https://en.wikipedia.org/wiki/Luhn_algorithm
    sum = 0;
    parity = cardNum.length % 2;
    for (var i = 0; i < cardNum.length - 1; i++) {
        let doubleDigit = (((i % 2) === parity) ? 2 : 1) * cardNum[i];
        let sumOfDigits = (Math.floor(doubleDigit / 10) + doubleDigit % 10);
        sum += sumOfDigits;
    }
    luhnOk = (String(9 * sum).slice(-1) === cardNum.slice(-1));
    if (!luhnOk) {
        cardInfo.errMessage = "Card number is invalid (incorrect Luhn value)";
        return;
    }
    
    // Card is valid if both the length is correct and the Luhn check is valid
    cardInfo.isValid = lengthOk && luhnOk;
  
    // Return card
    return cardInfo;

} // cardInfo()

function testValidity(cardNum, expectedResult) {
    card = cardInfo(cardNum);
    return (card.isValid === expectedResult);
} // testValidity()

function runTests() {
  
    // Sources:
    //    [1] http://testcreditcardnumbers.com/ (note "311" and "61" are not a valid IINs, see http://www.iinbase.com/)
    //    [2] Internal test cards
    tests = [
        {card:"378282246310005",  isValid:true},   // American Express [1]
        {card:"371449635398431",  isValid:true},   // American Express [1]
        {card:"378734493671000",  isValid:true},   // Amercian Express Corporate [1]
        {card:"371144371144376",  isValid:true},   // American Express [2]
        {card:"341134113411347",  isValid:true},   // American Express [2]
        {card:"30569309025904",   isValid:false},  // Diners Club [1]
        {card:"38520000023237",   isValid:false},  // Diners Club [1]
        {card:"6011111111111117", isValid:true},   // Discover [1]
        {card:"6011000990139424", isValid:true},   // Discover [1]
        {card:"6011016011016011", isValid:true},   // Discover [2]
        {card:"6559906559906557", isValid:true},   // Discover [2]
        {card:"3530111333300000", isValid:false},  // JCB [1]
        {card:"3566002020360505", isValid:false},  // JCB [1]
        {card:"5111111111111118", isValid:true},   // MasterCard [1]
        {card:"5555555555554444", isValid:true},   // MasterCard [1]
        {card:"5105105105105100", isValid:true},   // MasterCard [1]
        {card:"5111005111051128", isValid:true},   // MasterCard [2]
        {card:"5112345112345114", isValid:true},   // MasterCard [2]
        {card:"5115915115915118", isValid:true},   // MasterCard II [2]
        {card:"5116601234567894", isValid:true},   // MasterCard III [2]
        {card:"5610591081018250", isValid:false},  // Austrian Bankcard [1]
        {card:"4111111111111111", isValid:true},   // Visa [1]
        {card:"4012888888881881", isValid:true},   // Visa [1]
        {card:"4222222222222",    isValid:true},   // Visa [1]
        {card:"4000300611112224", isValid:true},   // Visa [2]
        {card:"4110144110144115", isValid:true},   // Visa Commercial Card [2]
        {card:"4114360123456785", isValid:true},   // Visa Commercial Card II [2]
        {card:"4061724061724061", isValid:true},   // Visa Commercial Card III [2]
        {card:"76009244561",      isValid:false},  // Dankort (PBS) [1]
        {card:"5019717010103742", isValid:false},  // Dankort (PBS) [1]
        {card:"6331101999990016", isValid:false},  // Switch/Solo (Paymenttech) [1]
    ];
  
    var globalPass = true;
  
    console.log("Validity checks:")
    for (var i = 0; i < tests.length; i++) {
        testNum = (1000+i).toString().substring(1);
        if (testValidity(tests[i].card, tests[i].isValid)) {
            console.log("\t" + testNum + ': "' + tests[i].card + '": Pass');
        } else {
            console.log("\t" + testNum + ': "' + tests[i].card + '": Fail');
            globalPass = false;
        }
    }
  
    if (globalPass) {
        console.log("All test cases passed");
    } else {
        console.error("One or more tests failed");
    }

} // runTests()

runTests();
console.log(cardInfo("371449635398431"));