JS implements jQuery's append function

HTMLElement.prototype.appendHTML = function(html) {
	let divTemp = document.createElement("div");
	let nodes = null;
	let fragment = document.createDocumentFragment();
	divTemp.innerHTML = html;
	nodes = divTemp.childNodes;
	nodes.forEach(item => {
		fragment.appendChild(item.cloneNode(true));
	})
	// Insert to the end append
	this.appendChild(fragment);
	// Insert prepend at the front
	// this.insertBefore(fragment, this.firstChild);
	nodes = null;
	fragment = null;
};

<style>
.child {
  height: 50px;
  width: 50px;
  background: #66CCFF;
  margin-bottom: 1em;
}
</style>

<div id="app">
  <div class="child">
  <div class="child">
</div>

let app = document.getElementById('app');
let child = `<div class="child">down</div>`;
app.appendHTML(child);

var div2 = document.querySelector("#div2");
  div2.insertAdjacentHTML("beforebegin","<p>hello world</p>");//Add an element before the calling elementdiv2.insertAdjacentHTML("afterbegin","<p>hello world</p>");//Add a child element inside the calling element and replace the first child elementdiv2.insertAdjacentHTML("beforeend","<p>hello world</p>");//Add a child element after the calling element and replace the last child elementdiv2.insertAdjacentHTML("afterend","<p>hello world</p>");//Add an element after the calling element