Untitled

<!DOCTYPE html>
<html>
<body>
<fieldset>
  <label for="">Number 1:</label>
  <input type="text" id="firstNumber">
  <br><br>
  <label for="">Number 2:</label>
  <input type="text" id="secondNumber">
  <br><br>
  <button type="button" onclick="execute(add)"> + </button>
  <button type="button" onclick="execute(substract)"> - </button>
  <button type="button" onclick="execute(multiply)"> * </button>
  <button type="button" onclick="execute(divide)"> / </button>
  <h2 id="operationResult">
  </h2>
</fieldset>

<script>
  function add() {
    this.result = this.numberOne + this.numberTwo;
  }

  function substract() {
    this.result = this.numberOne - this.numberTwo;
  }

  function multiply() {
    this.result = this.numberOne * this.numberTwo;
  }

  function divide() {
    this.result = this.numberOne / this.numberTwo;
  }

  function receiverValues() {
    this.numberOne = Number(document.getElementById("firstNumber").value);
    this.numberTwo = Number(document.getElementById("secondNumber").value);
    this.result = 0;
    }

    function execute(operation) {
      receiverValues();
      operation();
      document.getElementById("operationResult").innerHTML = this.result;
  }
</script>
</body>
</html>