How to insert an element after another element in JavaScript without using a library?

referenceNode.parentNode.insertBefore(newNode, referenceNode.nextSibling);

Where referenceNode is the node you want to put newNode after. If referenceNode is the last child within its parent element, that's fine, because referenceNode.nextSibling will be null and insertBefore handles that case by adding to the end of the list.

So:

function insertAfter(newNode, referenceNode) {
    referenceNode.parentNode.insertBefore(newNode, referenceNode.nextSibling);
}

You can test it using the following snippet:

function insertAfter(referenceNode, newNode) {
  referenceNode.parentNode.insertBefore(newNode, referenceNode.nextSibling);
}

var el = document.createElement("span");
el.innerHTML = "test";
var div = document.getElementById("foo");
insertAfter(div, el);

<div id="foo">Hello</div>

Straightforward JavaScript Would Be the Following:

Append Before:

element.parentNode.insertBefore(newElement, element);

Append After:

element.parentNode.insertBefore(newElement, element.nextSibling);

But, Toss Some Prototypes In There For Ease of Use

By building the following prototypes, you will be able to call these function directly from newly created elements.

• newElement.appendBefore(element);

• newElement.appendAfter(element);

.appendBefore(element) Prototype

Element.prototype.appendBefore = function (element) {
  element.parentNode.insertBefore(this, element);
},false;

.appendAfter(element) Prototype

Element.prototype.appendAfter = function (element) {
  element.parentNode.insertBefore(this, element.nextSibling);
},false;

And, To See It All In Action, Run the Following Code Snippet

/* Adds Element BEFORE NeighborElement */
Element.prototype.appendBefore = function(element) {
  element.parentNode.insertBefore(this, element);
}, false;

/* Adds Element AFTER NeighborElement */
Element.prototype.appendAfter = function(element) {
  element.parentNode.insertBefore(this, element.nextSibling);
}, false;


/* Typical Creation and Setup A New Orphaned Element Object */
var NewElement = document.createElement('div');
NewElement.innerHTML = 'New Element';
NewElement.id = 'NewElement';


/* Add NewElement BEFORE -OR- AFTER Using the Aforementioned Prototypes */
NewElement.appendAfter(document.getElementById('Neighbor2'));

div {
  text-align: center;
}
#Neighborhood {
  color: brown;
}
#NewElement {
  color: green;
}

<div id="Neighborhood">
  <div id="Neighbor1">Neighbor 1</div>
  <div id="Neighbor2">Neighbor 2</div>
  <div id="Neighbor3">Neighbor 3</div>
</div>

Run it on JSFiddle

insertAdjacentHTML + outerHTML

elementBefore.insertAdjacentHTML('afterEnd', elementAfter.outerHTML)

Upsides:

• DRYer: you don't have to store the before node in a variable and use it twice. If you rename the variable, on less occurrence to modify.
• golfs better than the insertBefore (break even if the existing node variable name is 3 chars long)

Downsides:

• lower browser support since newer: https://caniuse.com/#feat=insert-adjacent
• will lose properties of the element such as events because outerHTML converts the element to a string. We need it because insertAdjacentHTML adds content from strings rather than elements.