let currentNum = 0;
let storedNum = 0;

let history = [];
let historyIndex = null;

function addHistory() {
  let historyObject = {lastNum: currentNum,
                       value: storedNum};
  history.unshift(historyObject);
  historyIndex = null;

function historyBack() {
  if (historyIndex === history.length - 1) {
    return;
  }
  if (historyIndex === null) {
    historyIndex = 0;
  } else {
    historyIndex++;
  }
  setHistoryValues();
}

function setHistoryValues() {
  let historyObject = history[historyIndex];
  currentNum = historyObject.lastNum;
  storedNum = historyObject.value;
}