Javascript added to theme header returns $ is not defined

Development
3

Thanks but if this javascript this works fine when used in the developer console
document.querySelector('.topic-list .topic-excerpt[href="/t/urltobechanged/8"]').setAttribute('href', "/t/newurltoset/17");

Somehow when it is loaded from the discourse header, it is returned as “Uncaught TypeError: document.querySelector(…) is null”, no matter if I wrap it with :

function doSomething() {
or
document.addEventListener("DOMContentLoaded", function(event)

Any idea how this single line to change the href attribute should be run as it works fine in the dev console?