Create a Typewriter Effect with another Variable

I am looking to make a typewriter effect using a Variable. My Variable will select a random string listed to write. I want it to list the string the same way, but i want it to use the typewriter effect to do it!

No matter where i look, i cannot find an answer on how to make the typewriter effect, it will not type the function! I want it to type a random text based on the variable! Thanks!

this is my code.

Finalising Deletion...
<progress></progress>
<!DOCTYPE html>
<html>
<body>

<br><br>

<button onclick="typeWriter()">View Status</button>

<p id="demo"></p>

<script>
var i = 0;
var txt = randomText();
var speed = 50;

function typeWriter() {
  if (i < txt.length) {
    document.getElementById("demo").innerHTML += (i);
    i++;
    setTimeout(typeWriter, speed);
  }
}
  function randomText() {
              //array splashes
              var say = [];
              say[0] = "Moving Core Files...";
              say[1] = "Deleting JavaScript";
              say[2] = "Uploading Data...";
              say[3] = "Editing HTML...";
              say[4] = "Disabling Server.js";
              say[5] = "Deleting Cookies...";
              say[6] = "Deleted Files (3/754)";
              say[7] = "Attempting URL Shutdown";
              say[8] = "Uploading Deletion Program!";
              say[9] = "Deleting URL...";
              say[10] = "Changing CSS";
              say[11] = "Deleting Inside Fules...";
             
           
              
              //pick a random greeting
              var howmany = 11;
              var bRand = 0;
              bRand = Math.random();
              bRand = Math.floor(bRand * howmany);
              //prepare and docwrite the greeting
              sayWhat = say[bRand];
              document.write(sayWhat);
              document.close();
              //direct type in html p element
              //document.getElementById("splash").innerHTML ='javascript:alert("' + '");'
              // I tried to make this work but it says no.
            }
  
</script>

</body>
</html>
  <script style="color:white" language="JavaScript" type="text/javascript">
            //script to generate random greetings
            
    </script>
    <script style="color:white" type="text/javascript">
      
    </script>