I'm doing an A/B test, but I noticed that I have to change everything via JS, I just don't understand why I can't just use something like this:

document.getElementById("myDIV").style.background = "url('smiley.gif')

Here is an example of a code used in VWO with JS

  function colpromo() {
      if (typeof (catalogpageloaded) == 'undefined') {
        window.catalogpageloaded = 'loaded';
        // javascript changes here
        /*Eliminating content*/
        const BANNER = document.querySelector(".col-promo-with-cta-module");
        BANNER.remove();
        const HERO = document.querySelector(".hero-content .hero-content-wrapper ");
        const HERO_CHILDREN = Array.from(HERO.children);
        HERO_CHILDREN[2].remove();
        HERO_CHILDREN[3].remove();

Is there any documentation on how to use JS to do an A/B test?