calculator.js

function getHistory() {
  return document.getElementById('history-value').innerText;
}
function printHistory(num) {
  return 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 = getFormatedNumber(num);
  }
}
function getFormatedNumber(num) {
  if (num == "-") {
    return "";
  }
  var n = Number(num);
  var value = n.toLocaleString('en');
  return value;
}
function reversNumberFormat(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 == "clear") {
      printHistory("");
      printOutput("");
    }
    else if (this.id == "backspace") {
      var output = reversNumberFormat(getOutput()).toString();
      if (output) {
        output = 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 : reversNumberFormat(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].addEventListener('click', function () {
    var output = reversNumberFormat(getOutput());
    if (output != NaN) {
      output = output + this.id;
      printOutput(output)
    }
  })
}