Credit Card Checker : some issue with my code

My code seem to be working for some arrays. I couldn’t find what is wrong with it.
Any help is much appreciated .Credit card tracker

// 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];
//const test=[4,5,3,9,6,8,9,8,8,7,7,0,5,7,9,8];

// Add your functions below:
const validateCred=arr=>{
let newArr=;
let numPos=1;
for (let i=arr.length-1;i>=0;i–){
if(numPos%2===0){
let newNum=(arr[i]*2);
if(newNum>9){
newNum=newNum-9;
}
}
else{
newNum=arr[i];
}
newArr.push(newNum);
numPos++;
}
const totalSum=newArr.reduce((a,b)=>a+b);
if(totalSum%10===0){
return true;
}
else{
return false;
}

}
console.log(validateCred(invalid1));

Hello, and welcome to the forums!

The bug you’re experiencing is one of those quirky problems that can be very difficult to spot. You’re really really close to a solution.

You have 2 variables named newNum even though it doesn’t look like it. By using let newNum inside your if block, it’s scoped to that block. In your else block, you assign a value to newNum without declaring it, so JavaScript treats it like a var and the scope of that goes beyond that block of code.

Here’s your validateCred() function with some console.log() statements added to it so you can see what’s happening:

const validateCred = (arr) => {
  let newArr = [];
  let numPos = 1;
  for (let i = arr.length - 1; i >= 0; i--) {
    if (numPos % 2 === 0) {
      let newNum = arr[i] * 2;
      if (newNum > 9) {
        newNum = newNum - 9;
      }
      console.log(`${numPos}: 'if' newNum = ${newNum}`);
    } else {
      newNum = arr[i];
      console.log(`${numPos}: 'else' newNum = ${newNum}`);
    }
    console.log(`${numPos}: 'push' newNum = ${newNum}`);
    newArr.push(newNum);
    numPos++;
  }
  const totalSum = newArr.reduce((a, b) => a + b);
  if (totalSum % 10 === 0) {
    return true;
  } else {
    return false;
  }
};

This would be the output of running validateCred(invalid1)

1: 'else' newNum = 5
1: 'push' newNum = 5
2: 'if' newNum = 9
2: 'push' newNum = 5
3: 'else' newNum = 7
3: 'push' newNum = 7
4: 'if' newNum = 2
4: 'push' newNum = 7
5: 'else' newNum = 9
5: 'push' newNum = 9
6: 'if' newNum = 0
6: 'push' newNum = 9
7: 'else' newNum = 1
7: 'push' newNum = 1
8: 'if' newNum = 5
8: 'push' newNum = 1
9: 'else' newNum = 7
9: 'push' newNum = 7
10: 'if' newNum = 7
10: 'push' newNum = 7
11: 'else' newNum = 7
11: 'push' newNum = 7
12: 'if' newNum = 5
12: 'push' newNum = 7
13: 'else' newNum = 2
13: 'push' newNum = 2
14: 'if' newNum = 6
14: 'push' newNum = 2
15: 'else' newNum = 5
15: 'push' newNum = 5
16: 'if' newNum = 8
16: 'push' newNum = 5

Notice that the value of newNum in the if block doesn’t always match what ends up getting pushed to the array.

That’s because it’s the block-scoped newNum.

All of this explanation is so you’d get some background information of the importance of variable scopes.

To fix your code, you just need to put a let newNum; before the if condition and remove the let keyword you have for newNum already. That way each branch of your if / else assigns the value it calculates to the same variable.

PS. For future posts, be sure to format your code so we can see it properly in the forums:

Oh,got it now.
Thank you so much for the detailed explanation. :slightly_smiling_face: