Javascript, validation, for-loops

Question

I am writing a validator for credit card numbers in Javascript. I want it to be check against the LUHN-variable and I want to write the steps myself and I try to do this with help of for loops in order to practice them. However, I am stuck and I could use some friendly advice on what to do from here on. I am also slightly troubled with the entire syntax of the for loop, any tips on how to improve them is welcome.

The project is here: http://jsfiddle.net/tomasantonj/eZKMa/12/

Here is the code I am having trouble with:

else if (filter.test(cardnumber.value))
{

  // 1. Begin with second to last number iterate, every second number with 2 until start.
    for(i = 0; i < cardnumber.length;i += 1){
    newnumbers = (i * 2);

  // 2. The result of the previous string to be added i+i+i.
    for (i = 0; i < newnumbers.length; i += 1){   
    sum1 = (i + i);


  // 3. Then add the remaining numbers together but skip the last which is a control number.
    for (i = 0; i < cardnumber.length; i += 1){
    sum2 = (i + i);

  // 4. Add sum1 and sum2 together

    var checksum = (sum1 + sum2);

    alert('the sum of x and y is: ' + checksum);
    // 5. Mod10 out of the checksum + cardnumber control number to get validity.
    if ((checksum + cardnumber[-0]) % 10 === 0){
            alert('LUHN checks out!');
    }
    else {
    alert('not yet');
    }
   }  
  } 
 }

I am unsure how to ask the proper question, but I am guessing my problems are variable scope, the for loops and getting the for loops to do index based queries. I am aware of this code being ugly and long, this is my second ever javascript attempt so please don't mind it too much.

When this code is run, checksum is undefined- anyone know why this is? Something to do with strings and integers maybe? :S

Any response is highly appreciated. Thank you, Tomas

If you can use jQuery there is an excellent credit card validation plugin that checks the Luhn checksum as well as determining the card type: github.com/PawelDecowski/jQuery-CreditCardValidator — James Allardice
– James Allardice, Commented Apr 18, 2012 at 11:14
Ah, I cannot, this is an assignment for a school project I got and it supposed to be in JavaScript. No libraries etc. But thanks! — tomasantonj
– tomasantonj, Commented Apr 18, 2012 at 11:16

gabitzish · Accepted Answer · 2012-04-18 11:08:52Z

1

Is it ok that you are using the same variable "i" in all your nested for loops? that doesn't look good to me!

answered Apr 18, 2012 at 11:08

gabitzish

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2 Comments

tomasantonj Over a year ago

Most likely very true! I was trying some refactoring of loops just a minute ago, the loops were at first not nested and was using the same variable "i"- that is ok to do though?

gabitzish Over a year ago

If the loops were not nested, reusing the variable would be ok, but in nested loops you shouldn't do that, unless you are reffering to the same things (you want to skip or repeat some stept in a higher level loop)

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