text: function insert(num){
      var numExisting = document.querySelector('#result').innerHTML;
      document.querySelector('#result').innerHTML = numExisting + num
  };
  
  function clean(){
      document.querySelector('#result').innerHTML = "";
  };
  
  function back(){
      var result = document.querySelector('#result').innerHTML;
      document.querySelector('#result').innerHTML = result.substring(0, result.length - 1);
  }
  
  function result(){
      var result = document.querySelector('#result').innerHTML;
      if(result){
          document.querySelector('#result').innerHTML = eval(result);
      } else {
          document.querySelector('#result').innerHTML = "";
      }
  }