var sum = 0;

function addNum(total, newVal) {
  total += newVal;
}

addNum(sum, 5);

console.log(sum); // 0

function add(newVal) {
  sum += newVal
}

add(5);

console.log(sum); // 5

var nums = [1, 2, 3];

function addElem(arr, newElem) {
    arr.push(newElem);
}

addElem(nums, 4);

console.log(nums); // [1, 2, 3, 4]