Get next / previous element using JavaScript?

Really depends on the overall structure of your document.

If you have:

<div></div>
<div></div>
<div></div>

it may be as simple as traversing through using

mydiv.nextSibling;
mydiv.previousSibling;

However, if the 'next' div could be anywhere in the document you'll need a more complex solution. You could try something using

document.getElementsByTagName("div");

and running through these to get where you want somehow.

If you are doing lots of complex DOM traversing such as this I would recommend looking into a library such as jQuery.

use the nextSibling and previousSibling properties:

<div id="foo1"></div>
<div id="foo2"></div>
<div id="foo3"></div>

document.getElementById('foo2').nextSibling; // #foo3
document.getElementById('foo2').previousSibling; // #foo1

However in some browsers (I forget which) you also need to check for whitespace and comment nodes:

var div = document.getElementById('foo2');
var nextSibling = div.nextSibling;
while(nextSibling && nextSibling.nodeType != 1) {
    nextSibling = nextSibling.nextSibling
}

Libraries like jQuery handle all these cross-browser checks for you out of the box.

Well in pure javascript my thinking is that you would first have to collate them inside a collection.

var divs = document.getElementsByTagName("div");
//divs now contain each and every div element on the page
var selectionDiv = document.getElementById("MySecondDiv");

So basically with selectionDiv iterate through the collection to find its index, and then obviously -1 = previous +1 = next within bounds

for(var i = 0; i < divs.length;i++)
{
   if(divs[i] == selectionDiv)
   {
     var previous = divs[i - 1];
     var next = divs[i + 1];
   }
}

Please be aware though as I say that extra logic would be required to check that you are within the bounds i.e. you are not at the end or start of the collection.

This also will mean that say you have a div which has a child div nested. The next div would not be a sibling but a child, So if you only want siblings on the same level as the target div then definately use nextSibling checking the tagName property.