JSFiddle - React, Tailwind, and code Playground

HTML

<div class="form1">
  <input type="number" step="0.01" id="number1"><br>
  <select id="operation1">
    <option value="1">sin</option> 
    <option value="2" selected>cos</option> 
  </select><br>
  <button id="rax">порахувати</button>
</div>

JavaScript

function do_calc(){
  //let number_value2 = parseInt(number1.value);
  //let number_value2 = parseFloat(number1.value);
  let number_value = number1.value;
  let number_value2 = Number(number_value);
  //console.log(number_value2);
  if((number_value === "") || isNaN(number_value2)){ alert('Число не число !!'); return }
  let number_value3 = number_value2 * Math.PI / 180;
  
  let operation_value = operation1.value;
  //console.log(operation_value);
  
  if(operation_value == "1"){
    // sin
    var result = Math.sin(number_value3);
  }else if(operation_value == "2"){
    // cos
    var result = Math.cos(number_value3);
  }else{ alert("Операція не операція !!"); return}
  alert("Результат: " + result + " !");
}

window.addEventListener("load", function(){
  rax.addEventListener("click", do_calc);
});