JavaScript Key Presses

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>JavaScript Key Press</title>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <script>
      //wait for document to load
      document.addEventListener("DOMContentLoaded", function(){
        let output = document.querySelector("#output");
      });

      document.addEventListener('keydown', function( ev ) {
        output.innerHTML += ev.key + " DOWN<br>";
        //console.log(ev.key); //get keycode in console

        //clear on backspace
        if(ev.key == "Backspace"){
          output.innerHTML = "Cleared!<br>";
        }

        //scroll to bottom
        window.scrollTo(0,output.scrollHeight);
      });

      document.addEventListener('keyup', function( ev ) {
        output.innerHTML += ev.key + " UP<br>";
        //console.log(ev.key); //get keycode in console

        //scroll to bottom
        window.scrollTo(0,output.scrollHeight);
      });
    </script>
  </head>
  <body>
    <div id="output" style="font-size:50px"></div>
  </body>
</html>