Finding DOM node index

I want find the index of a given DOM node. It's like the inverse of doing

document.getElementById('id_of_element').childNodes[K]

I want to instead extract the value of K given that I already have the reference to the child node and the parent node. How do I do this?

The shortest possible way, without any frameworks, in all versions of Safari, FireFox, Chrome and IE >= 9:

var i = Array.prototype.indexOf.call(e.childNodes, someChildEl);

A little shorter, expects the element to be in elem, returns k.

for (var k=0,e=elem; e = e.previousSibling; ++k);

After a comment from Justin Dearing I reviewed my answer and added the following:

Or if you prefer "while":

var k=0, e=elem;
while (e = e.previousSibling) { ++k;}

The original question was how to find the index of an existing DOM element. Both of my examples above in this answer expects elem to be an DOM element and that the element still exists in the DOM. They will fail if you give them an null object or an object that don't have previousSibling. A more fool-proof way would be something like this:

var k=-1, e=elem;
while (e) {
    if ( "previousSibling" in e ) {
        e = e.previousSibling;
        k = k + 1;
    } else {
        k= -1;
        break;
    }
}	

If e is null or if previousSibling is missing in one of the objects, k is -1.

RoBorg's answer works... or you could try...

var k = 0;
while(elem.previousSibling){
    k++;
    elem = elem.previousSibling;
}
alert('I am at index: ' + k);

A modern native approach might include Array.from(e.children).indexOf(theChild)

No IE support, but Edge works: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from

As with the original poster, I was trying to

> find the index of a given DOM node

but one that I had just use a click handler on, and only in relation to its siblings. I couldn't end up getting the above to work (because of noobness undoubtably, i tried subbing in 'this' for elem but it didn't work).

My solution was to use jquery and use:

var index = $(this).parent().children().index(this);

It works without having to specify the type of the element ie:'h1' or an id etc.

I think the only way to do this is to loop through the parent's children until you find yourself.

var K = -1;
for (var i = myNode.parent.childNodes.length; i >= 0; i--)
{
    if (myNode.parent.childNodes[i] === myNode)
    {
        K = i;
        break;
    }
}

if (K == -1)
    alert('Not found?!');

using a framework like prototype you could use this :

$(el).up().childElements().indexOf($(el))