your company assigns each customer a membership id
//your company assigns each customer a membership id
function createCheckDigit(membershipId) {
let arr = membershipId.split('');
while (arr.length > 1) {
arr = arr.reduce((count, item) => {
return count + parseInt(item);
},0);
arr = arr.toString().split('');
}
return arr[0];
}
console.log(createCheckDigit("55555"));