calc_js2.html

<script>

var result
function calc() {
  let num1 = document.getElementById("n1");
  let num2 = document.getElementById("n2");
  let op = document.getElementById("op");

  if(op.value === '+'){
    result = parseFloat(num1.value) + parseFloat(num2.value);
  }else if(op.value === '-'){
    result = parseFloat(num1.value) - parseFloat(num2.value);
  }else if(op.value === '*'){
    result = parseFloat(num1.value) * parseFloat(num2.value);
  }else if(op.value === '/' && num2.value == 0){
    alert('The second number cannot be zero');    
    return;
  }else{
    result = parseFloat(num1.value) / parseFloat(num2.value);
  }
  alert(result);
}

// Adapted from https://stackoverflow.com/questions/60346902/i-cant-do-a-simple-calculator-with-javascript
// https://medium.com/@singhamritpal49/creating-simple-addition-calculator-with-javascript-563ede3527e2
</script>

<h3>Simple Javascript Calculator</h3>
<input type="text" id="n1">
<select name="select" id="op">
  <option value="+" selected>+</option>
  <option value="-">-</option>
  <option value="*">*</option>
  <option value="/">/</option>
</select>
<input type="text" id="n2">
<button id="btn" onclick="return calc();">Sum</button>