Help, please! JQuery Widget

JavaScript
#1

Please tell me how to rewrite this code in the jQuery widget?

let scrollToTopBtn = document.getElementById('back_top');
let rootElement = document.documentElement;

function scrollToTop() {
rootElement.scrollTo({
    top: 0,
    behavior: 'smooth'
    });
}

scrollToTopBtn.addEventListener('click', scrollToTop);

By example:

$.widget( "backToTop", {
    //...
});
#2

You really do not need JS for that. You can use the CSS scroll-behavior property and a normal link with the href set to #.

Nor do you need jQuery if it has to be a button using JS

As for your question, I’m not sure I know what widget you are referring to.

https://api.jqueryui.com/jQuery.widget/