JSFiddle - React, Tailwind, and code Playground

JavaScript

var ccNumbers = ['1111', '888-222', '21-11', '9989', '9918'];

function addDigits(numbers) {

    var sumArray = []; //store several sums to print

    for (x = 0; x < numbers.length; x++) { //loop through ccNumbers array 
        var ccNmbr = numbers[x]; //pick out one array item and store it in ccNmbr
        ccNmbr = ccNmbr.replace("-", 0);
        ccNmbr = ccNmbr.replace(" ", 0);

        let sum = 0;
        for (y = 0; y < ccNmbr.length; y++) { //loop through each digit in ccNmbr 
            sum += parseInt(ccNmbr.charAt(y), 10); //add each digit to the sum
        } //close adding digits loop
        sumArray.push(sum); //add sum to an array

    } //close looping through array loop

    var largest = Math.max(...sumArray); //finds the largest added up number 

    let index = 0;
    for (i = 0; i < sumArray.length; i++) {
        if (largest === sumArray[i]) { //if there is a match... This isn't working. It matches itself.
            index = i;
        }
    }

    return ccNumbers[index];
}

alert(`The largest credit card number is ${addDigits(ccNumbers)}`);