calculator.js

var button = document.getElementsByClassName("button");
var display = document.getElementById("resultBox");
var operand1 = 0;
var operand2 = null;
var operator = null;

for(var i=0; i<button.length; i++){
  button[i].addEventListener('click',function(){
     var value = this.getAttribute("data-value");
     if(value=="AC"){
       display.innerText = "";
     }else if(value == '+'){
       operator = '+';
       operand1 = parseFloat(display.textContent);
       display.innerText = "";
     } else if(value == '-'){
       operator = '-';
       operand1 = parseFloat(display.textContent);
       display.innerText = "";
      } else if(value == '*'){
        operator = '*';
        operand1 = parseFloat(display.textContent);
        display.innerText = "";
      } else if(value == '/'){
         operator = '/';
         operand1 = parseFloat(display.textContent);
         display.innerText = "";
       } else if(value == '%'){
           operator = '%';
           operand1 = parseFloat(display.textContent);
           display.innerText = "";
         }else if(value == '='){
           operand2 = parseFloat(display.textContent);
           display.innerText = "";
          if(operand1 != 0 && operand2 == 0){
            display.innerText = "Zero Division Is Not Possible";
            return;
          }

           var ans = eval(operand1 + operator + operand2);
           display.innerText = ans;
          }else{
            display.innerText += value;
     }
  });
}