main.js

var selectedOperator = "";
var leftPart = "";
var rightPart = "";

var numbers = document.querySelectorAll(".number");
for (var i = 0; i < numbers.length; i++) {
  numbers[i].addEventListener(
    "click",
    function () {
      document.getElementById("resultInner").innerHTML += this.value;

      if (selectedOperator === "") {
        leftPart += this.value;
      } else if (selectedOperator !== "") {
        rightPart += this.value;
      }
    },
    false
  );
}

var operators = document.querySelectorAll(".sign");
for (var i = 0; i < operators.length; i++) {
  operators[i].addEventListener(
    "click",
    function () {
      document.getElementById("resultInner").innerHTML += this.value;

      selectedOperator = this.value;
    },
    false
  );
}
document.getElementById("equal").addEventListener(
  "click",
  function () {
    if (selectedOperator == "+") {
      document.getElementById("resultInner").innerHTML =
        parseInt(leftPart) + parseInt(rightPart);
    } else if (selectedOperator == "-") {
      document.getElementById("resultInner").innerHTML =
        parseInt(leftPart) - parseInt(rightPart);
    } else if (selectedOperator == "*") {
      document.getElementById("resultInner").innerHTML =
        parseInt(leftPart) * parseInt(rightPart);
    } else if (selectedOperator == "/") {
      document.getElementById("resultInner").innerHTML =
        parseInt(leftPart) / parseInt(rightPart);
    }
  },
  false
);

document.getElementById("delete").addEventListener(
  "click",
  function () {
    selectedOperator = "";
    leftPart = "";
    rightPart = "";
    document.getElementById("resultInner").innerHTML = "";
  },
  false
);