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 = num;

  } else {

    document.getElementById("output-value").innerText = getFormattedNumber(num);

  }

}

function getFormattedNumber(num) {

  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].addEventListener('click',function(){

    if(this.id == "lear"){

      printHistory("");

      printOutput("");

    }

    else if(this.id == "backspace"){

      var output = reverseNumberFormat(getOutput).toString();

      if(output){

        output = output.substr(0,output.length -1);

        printOutput(output);

      }

    }

  });

}

var number = document.getElementsByClassName("number");

for(var i = 0; i < number.length; i++){

  number[i].addEventListener('click',function(){

    var output = reverseNumberFormat(getOutput());

    if(output!=NaN){ //if output is number

      output = output +this.id;

      printOutput(output);

    }

  });

}

​