View difference between Paste ID: aKh31pVw and MfWJ68CB

 /* I am getting "TypeError: historyObject is undefined" at the line
    "currentNum = historyObject.lastNum;"
    Any help would be appreciated */
 
 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;
 }