old_contents: $(document).ready(function () {
    let bpmCounter = new BpmCounter();
  });
  
new_contents: $(document).ready(function () {
    let bpmCounter = new BpmCounter();
  
    $(document.body).on("keydown", handleKeyDown);
  });
  
  function handleKeyDown () {
    console.log("Key down");
  }
  
subject: Handle keydown