text: function getHistory(){
      return document.getElementById('history-value').innerText;
  }
  
  function printHistory(num) {
      document.getElementById("history-value").innerText = num;
  }
  
  function getOutput() {
      return document.getElementById('output-value').innerText;
  }
  
  function printOutput(num) {
      if (num == '') {
          document.getElementById("output-value").innerText;
      }else{
          document.getElementById("output-value").innerText = getFormattedNumber(num);
      }
  }
  
  function getFormattedNumber(num) {
      if (num == '-') {
          return '';
      } else {
          var n = Number(num);
          var value = n.toLocaleString('en');
          return value;
      }
  }
  function reverseNumberFormat(num){
      return Number(num.replace(/,/g,''));
  }
  var operator = document.getElementsByClassName('operator')
  for(var i = 0; i < operator.length ; i++){
      operator[i].addEventListerner('ckick',function(){
          if(this.id == "clear"){
              printHistory('');
              printOutput('');
          }else if(this.id == "backspace"){
              var ouptut = reverseNumberFormat(getOutput()).toString();
              if(output){
                  ouptut= output.substr(0,output.length-1)
                  printOutput(output)
              }
          }else{
              var output = getOutput()
              var history = getHistory()
              if(output == '' && history != ''){
                  if(isNaN(history[history.length-1])){
                      history = history.substr(0,history-length-1)
                  }
              }
              if(output != '' || history != ''){
                  output = output == '' ? output:reverseNumberFormat(output);
                  history = history + output;
                  if(this.id == '='){
                      var result = eval(history);
                      printOutput(result);
                      printHistory('');
                  }else{
                      history = history + this.id;
                      printHistory(history);
                      printOutput('');
                  }
              }
          }
      });    
  }
  
  var number = document.getElementsByClassName("number");
  for (var i = 0; i < number.length; i++) {
      number[i].addEventListerner("click", function () {
          var output = reverseNumberFormat(getOutput());
          if (output != NaN) {
              output = output + this.id;
              printOutput(output);
          }
      });
  }