How would I stop receiving a new RandomNumber within this code? [closed]

I am trying to encode a string of words the user gives using ASCII. Doing it by each character. But the random number keeps changing insetead of using the same number to encode.

I want all the Uppercase letters to be the same number/character.

    const rand = generateRandom(65, 90);
    const randomNum = rand;
    function generateRandom(min, max)
    {
         let rand = Math.floor(Math.random() * (max - min) + 1) + min;
         return rand;
    }
      function encodeIt()
      {
           document.getElementById("message").innerHTML = ("<h2>&nbsp;</h2>");
           var msg = prompt("Enter your message." , " ");
           let newmsg = " ";
           var upCaseCode = 155;
           var newCode = 0;
           var lowCaseCode = 219;
           var specialCode = 3;
     
       
           //the loop encodes each letter in the message string
              for (var j = 0; j < msg.length; j++)
              {
  
           //check for lowercase letters and encode them
         if ((msg.charCodeAt(j)>=97) && (msg.charCodeAt(j)<=122)) 
                 {
                     newcode = (lowCaseCode - msg.charCodeAt(j));
                 }
                else
   //check for numbers and special characters and encode them33.                 
   if (((msg.charCodeAt(j)>90) && (msg.charCodeAt(j)<97)) || (msg.charCodeAt(j)<65))
                 {
                      newcode = (msg.charCodeAt(j) + specialCode);
                 }
  //add each encoded character to the new message
       newmsg = newmsg + " " + String.fromCharCode(newcode);
       }
  //display the encoded message on the web page
       document.getElementById("secret").innerHTML = ("<h2>" + newmsg + "</h2>");
  //decide if original message should be shown
       var choice = prompt("Do you want the special key? Yes or No?", " ");
       if ((choice.charAt(0) == 'y') || (choice.charAt(0) == 'Y'))
       {
            document.getElementById("message").innerHTML  = ("<h2>" + randomNum + "</h2>");
        
        }
  }

  //check for upppercase letters and encode them
            if ((msg.charCodeAt(j)>=65) && (msg.charCodeAt(j)<=90))
            {
                 newcode = (**randomNum **- msg.charCodeAt(j));      
            }
            else