如果已经选择了用户名，如何动态禁用提交按钮？

<input type="text" id="username" />
<button id="submit" disabled>提交</button>

<script>
  var usernameInput = document.getElementById('username');
  var submitButton = document.getElementById('submit');

  usernameInput.addEventListener('input', function() {
    if (usernameInput.value !== '') {
      submitButton.disabled = false;
    } else {
      submitButton.disabled = true;
    }
  });
</script>