If a JavaScript library such as jQuery is being used, look out for sinks that can alter DOM elements on the page. For instance, the attr() function in jQuery can change attributes on DOM elements. If data is read from a user-controlled source like the URL and then passed to the attr() function, then it may be possible to manipulate the value sent to cause XSS.

$(function(){
$('#backLink').attr("href",(new URLSearchParams(window.location.search)).get('returnUrl'));
});