best you may not need jquery - document ready

 I have long appreciated the site You might not need jQuery - it does a great job of bridging the gap from jQuery to vanilla.js. (The two are really quite similar... for my money the biggest similarity is "You don't need a build system".) 

One thing I look up a lot: document.ready

$(document).ready(function () {});

which can become

function ready(fn) {
  if (document.readyState !== 'loading') {
    fn();
  } else {
    document.addEventListener('DOMContentLoaded', fn);
  }
}

More as I end up using it...