You don’t “use” JavaScript variables in HTML. HTML is not a programming language, it’s a markup language, it just “describes” what the page should look like.
If you want to display a variable on the screen, this is done with JavaScript.
First, you need somewhere for it to write to:
<body> <p id="output"></p> </body>
Then you need to update your JavaScript code to write to that <p>
tag. Make sure you do so after the page is ready.
<script> window.onload = function(){ var name = prompt("What's your name?"); var lengthOfName = name.length document.getElementById('output').innerHTML = lengthOfName; }; </script>