Javascript Credit Card Checker Help

Hi, hope that whomever is reading this is doing well! All tests, whether they are valid or invalid, return as valid. I know this is because the value of my sum isnt updating, instead it is staying equal to 0. Does anyone know why the value of sum isnt updating, what am I doing wrong?

I am aware that this is probably something very simple LOL thank you in advance to anyone that helps!

// All valid credit card numbers
const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8];
const valid2 = [5, 5, 3, 5, 7, 6, 6, 7, 6, 8, 7, 5, 1, 4, 3, 9];
const valid3 = [3, 7, 1, 6, 1, 2, 0, 1, 9, 9, 8, 5, 2, 3, 6];
const valid4 = [6, 0, 1, 1, 1, 4, 4, 3, 4, 0, 6, 8, 2, 9, 0, 5];
const valid5 = [4, 5, 3, 9, 4, 0, 4, 9, 6, 7, 8, 6, 9, 6, 6, 6];

// All invalid credit card numbers
const invalid1 = [4, 5, 3, 2, 7, 7, 8, 7, 7, 1, 0, 9, 1, 7, 9, 5];
const invalid2 = [5, 7, 9, 5, 5, 9, 3, 3, 9, 2, 1, 3, 4, 6, 4, 3];
const invalid3 = [3, 7, 5, 7, 9, 6, 0, 8, 4, 4, 5, 9, 9, 1, 4];
const invalid4 = [6, 0, 1, 1, 1, 2, 7, 9, 6, 1, 7, 7, 7, 9, 3, 5];
const invalid5 = [5, 3, 8, 2, 0, 1, 9, 7, 7, 2, 8, 8, 3, 8, 5, 4];

// Can be either valid or invalid
const mystery1 = [3, 4, 4, 8, 0, 1, 9, 6, 8, 3, 0, 5, 4, 1, 4];
const mystery2 = [5, 4, 6, 6, 1, 0, 0, 8, 6, 1, 6, 2, 0, 2, 3, 9];
const mystery3 = [6, 0, 1, 1, 3, 7, 7, 0, 2, 0, 9, 6, 2, 6, 5, 6, 2, 0, 3];
const mystery4 = [4, 9, 2, 9, 8, 7, 7, 1, 6, 9, 2, 1, 7, 0, 9, 3];
const mystery5 = [4, 9, 1, 3, 5, 4, 0, 4, 6, 3, 0, 7, 2, 5, 2, 3];

// An array of all the arrays above
const batch = [valid1, valid2, valid3, valid4, valid5, invalid1, invalid2, invalid3, invalid4, invalid5, mystery1, mystery2, mystery3, mystery4, mystery5];


// Add your functions below:
//J is used as a counter to double the correct numbers
//Sum is created so that it can be modified within the loop

const validateCred = arr => {
  let j = 0;
  let sum = 0;
  for(let i = arr.size; i <= 0; i--) {
    j += 1;
    let k = j % 2;
    if(k === 0) {
      arr[i] = arr[i] * 2;
    }
    if(arr[i] > 9) {
      arr[i] = arr[i] - 9;
    }
    return sum = sum + arr[i];
  }

//Checking to see value of sum
  console.log(sum);
  //Checks to see if card is valid or invalid by using modulo
    if(sum % 10 === 0){
      return 'Valid';
    } else {
      return 'Invalid';
    }
  }
  
  console.log(validateCred(invalid2));

Hiya!
This line…
return sum = sum + arr[i];

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Thank you for pointing that out! What do you suggest that I change it to so that the sum stops being equal to 0? I tried substituting it with sum += arr[i]; , but still get the same results

Delete the return part. That’s ending your function early and stopping your loop from working.
The addition bit looks fine (either way).

Ah I stepped away for a while and figured it out. The main cause of all my problems was that i was using arr.size instead of arr.length, rookie mistake!

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