JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="input" id="input">
   <input type="button"  id="btn" value="Calc">
   <div id="res"></div>

JavaScript

var plus=/\d+\+\d+/;
var minus=/\d+\-\d+/;
var mul=/\d+\*\d+/;
var div=/\d+\/\d+/;
var text;
var result;
var btn;
window.onload=function () {
   text=document.getElementById('input');
   btn=document.getElementById('btn');
   result=document.getElementById('res');
   btn.onclick=function () {
      result.innerHTML=calc();
   }
}
function calc() {
   var val;
   if(plus.test(text)){
      val=text.split('+');
      return val[0]+val[1];
   }else if (minus.test(text)) {
      val=text.split('-');
      return val[0]-val[1];
   } else if (mul.test(text)){
      val=text.split('*');
      return val[0]*val[1];
   }
   else if (div.test(text)){
      val=text.split('/');
      if (val[1]==0){
         return "Деление на ноль";
      }
      return val[0]/val[1];
   }
}