Get child node index

ES—Shorter

[...element.parentNode.children].indexOf(element);

The spread Operator is a shortcut for that

I've become fond of using indexOf for this. Because indexOf is on Array.prototype and parent.children is a NodeList, you have to use call(); It's kind of ugly but it's a one liner and uses functions that any javascript dev should be familiar with anyhow.

var child = document.getElementById('my_element');
var parent = child.parentNode;
// The equivalent of parent.children.indexOf(child)
var index = Array.prototype.indexOf.call(parent.children, child);

ES6:

Array.from(element.parentNode.children).indexOf(element)

Explanation :

• element.parentNode.children → Returns the brothers of element, including that element.

• Array.from → Casts the constructor of children to an Array object

• indexOf → You can apply indexOf because you now have an Array object.

you can use the previousSibling property to iterate back through the siblings until you get back null and count how many siblings you've encountered:

var i = 0;
while( (child = child.previousSibling) != null ) 
  i++;
//at the end i will contain the index.

Please note that in languages like Java, there is a getPreviousSibling() function, however in JS this has become a property -- previousSibling.

Use previousElementSibling or nextElementSibling to ignore text and comment nodes.