The default value without assignment is always undefined, and undefined + a number is NaN. Therefore, you need to initialize nemo value. In your case, it should be

var nemo = 0;

Full code

function newSum(elem) {

  console.log(elem.value)
  var sum = elem.parentElement.parentElement.children[5];

  var price = elem.parentElement.parentElement.children[2].textContent;

  sum.textContent = Number(price) * Number(elem.value);

  var sum = document.querySelectorAll(".summing");

  for (num of sum) {
    tota_sum.push(Number(num.textContent));
  }

  console.log(tota_sum);

  var overall = document.querySelector("#overall");

  var nemo = 0;

  for (i = 0; i < tota_sum.length; i++) {
    nemo += Number(tota_sum[i]);
    console.log(nemo);
  }

}