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 = '';