Learning Javascript OOP with a Collapsing Item Class
This seems over-engineered. If really all you need is this.content.classList.toggle(&quot;bb-collapsed&quot;); then I would simply go for
class Collapsible {
  constructor(link, content) {
    this.link = link;
    this.content = content;
    this.content.classList.toggle(&quot;bb-collapsed&quot;);
    this.link.addEventListener(&quot;click&quot;, e =&gt; content.classList.toggle(&quot;bb-collapsed&quot;));
  }
}

And from there, there really isn't much of a point in having a class. You could do that straight in your checkForCollapsibles.
Something like this
function wireCollapsibles() {
  const links = document.querySelectorAll(&quot;.bb-collapsible&quot;);
  const contents = document.querySelectorAll(&quot;.bb-collapsible-content&quot;);

  links.forEach((link, index) =&gt; {
    new bb_Collapsible(collapsibleLinks[index], collapsibleContents[index]);
    link.addEventListener(e =&gt; contents[i].classList.toggle(&quot;bb-collapsed&quot;));
  });
}

Note that I drop the console.log thing, ideally you dont have log statements in your production code.