This script (source) is a hack to highlight your username in yellow when there are new messages on your user talk page (since the in Timeless skin, the talk page link is hidden in a dropdown). This script should only be used with the Timeless skin – so add it to your timeless.js, not your common.js! For details on why this is not done by the skin, nor by the Echo extension, see tasks T215695 and T216046.
Without the script (left), the "new message" indication is only within the dropdown menu. With the script (right), you username above the dropdown is also highlighted in yellow.