you might not need jquery

What's the oldest version of IE you need to support? IE10

 /**json**/
 var request = new XMLHttpRequest();
 request.open('GET', '/my/url', true);

 request.onload = function() {
   if (this.status >= 200 && this.status < 400) {
     // Success!
     var data = JSON.parse(this.response);
   } else {
     // We reached our target server, but it returned an error

   }
 };

 request.onerror = function() {
   // There was a connection error of some sort
 };

 request.send();

 /**request**/
 var request = new XMLHttpRequest();
 request.open('GET', '/my/url', true);

 request.onload = function() {
   if (this.status >= 200 && this.status < 400) {
     // Success!
     var resp = this.response;
   } else {
     // We reached our target server, but it returned an error

   }
 };

 request.onerror = function() {
   // There was a connection error of some sort
 };

 request.send();

 /**post**/
 var request = new XMLHttpRequest();
 request.open('POST', '/my/url', true);
 request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
 request.send(data);

/**hide**/
el.style.display = 'none';

 /**fade in**/
 /**javascript**/
 <script>
     el.classList.add('show');
     el.classList.remove('hide');
 </script>
 <style>
     .show {
       transition: opacity 400ms;
     }
     .hide {
       opacity: 0;
     }
 </style>

/**show**/
el.style.display = '';

/**add class**/
el.classList.add(className);

/**after**/
el.insertAdjacentHTML('afterend', htmlString);

/**append**/
parent.appendChild(el);

/**before**/
el.insertAdjacentHTML('beforebegin', htmlString);

/**clone**/
el.cloneNode(true);

/**contains**/
el !== child && el.contains(child);

/**children**/
el.children

/**Contains Selector**/
el.querySelector(selector) !== null

/**Each**/
var elements = document.querySelectorAll(selector);
Array.prototype.forEach.call(elements, function(el, i){

});

/**Empty**/
el.innerHTML = '';