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

Answer 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!