Cute... if this was 2008. Variables for nothing, tons of lines for nothing. Seriously, Object.assign is your friend.

document.body.appendChild(
  Object.assign(
    document.createElement("script"),
    {
      async : true,
      onerror : () => {
       console.log("Error occurred while loading script");
      },
      onload : () => {
        console.log("Script loaded successfuly");
        document.getElementById('box').textContent = "The script has loaded.";
      },
      src : "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.1/jquery.min.js"
    }
  )
);

No variables to worry about scope with, leveraging the fact that most of these object and element methods return what’s being manipulated.