javascript - Fastest way to find the index of a child node in parent

Question

I want to find the index of the child div that has the id 'whereami'.

<div id="parent">
   <div></div>
   <div></div>
   <div id="whereami"></div>
   <div></div>
</div>

Currently I am using this function to find the index of the child.

function findRow(node){
    var i=1;
    while(node.previousSibling){
        node = node.previousSibling;
        if(node.nodeType === 1){
            i++;
        }
    }
    return i; //Returns 3
}

var node = document.getElementById('whereami'); //div node to find
var index = findRow(node);

fiddle: http://jsfiddle.net/grantk/F7JpH/2/

The Problem
When there are a thousands of div nodes, the while loop has to traverse through each div to count them. Which can take a while.

Is there any faster way to tackle this?

*Note that the id will change to different divs node, so it will need to be able to re-calculate.

Answer 1

Out of curiosity I ran your code against both jQuery's .index() and my below code:

function findRow3(node)
{
    var i = 1;
    while (node = node.previousSibling) {
        if (node.nodeType === 1) { ++i }
    }
    return i;
}

Jump to jsperf results

It turns out that jQuery is roughly 50% slower than your implementation (on Chrome/Mac) and mine arguably topped it by 1%.

Edit

Couldn't quite let this one go, so I've added two more approaches:

Using Array.indexOf

[].indexOf.call(node.parentNode.children, node);

Improvement on my earlier experimental code, as seen in HBP's answer, the DOMNodeList is treated like an array and it uses Array.indexOf() to determine the position within its .parentNode.children which are all elements. My first attempt was using .parentNode.childNodes but that gives incorrect results due to text nodes.

Using previousElementSibling

Inspired by user1689607's answer, recent browsers have another property besides .previousSibling called .previousElementSibling, which does both original statements in one. IE <= 8 doesn't have this property, but .previousSibling already acts as such, therefore a feature detection would work.

(function() {
    // feature detection
    // use previousElementSibling where available, IE <=8 can safely use previousSibling
    var prop = document.body.previousElementSibling ? 'previousElementSibling' : 'previousSibling';

    getElementIndex = function(node) {
        var i = 1;
        while (node = node[prop]) { ++i }
        return i;
    }

Conclusion

Using Array.indexOf() is not supported on IE <= 8 browsers, and the emulation is simply not fast enough; however, it does give 20% performance improvement.

Using feature detection and .previousElementSibling yields a 7x improvement (on Chrome), I have yet to test it on IE8.