[whatwg] Proposal: Adding methods like getElementById and getElementsByTagName to DocumentFragments

[Ojan Vafai]

On Thu, Jul 25, 2013 at 1:42 PM, Jonas Sicking <jonas at sicking.cc> wrote:

> On Thu, Jul 25, 2013 at 9:05 AM, Boris Zbarsky <bzbarsky at mit.edu> wrote:
> > On 7/24/13 10:42 PM, Jussi Kalliokoski wrote:
> >>
> >> Argh, I had forgotten about live NodeLists. OK, this is a reason that
> >> resonates with me and justifies calling these methods obsolete. Too bad
> >> these methods are so badly flawed
> >
> >
> > Fwiw, I think the performance impact of live NodeLists is ... unclear.
> Their
> > existence does mean that you have to deal with DOM mutations changing the
> > lists, but them being live also means you can make the list getters much
> > faster in cases when the caller doesn't actually want the entire list.
>
> And, as importantly, it also means that for multiple consecutive calls
> to get the list, say inside of a loop, can return the same result
> object. I.e. you don't have to re-walk the DOM for every iteration
> through the loop.
>

I think these are good points of what is lost by using static NodeLists. I
still feel pretty strongly though that these benefits don't outweigh the
costs. If we want to give people most of the benefits of live NodeLists
without the costs we could expose an iterator API:

var iterator = document.querySelectorAll('div').iterator(); <--- does some
magic to not precompute the whole list
while (let current = iterator.next()) { ... }

And next always does the walk from the current node. So, if you add a div
before the current node, this specific iterator won't hit it, but if you
add a div after it would. I'm not sure what should happen though if you
remove the node that's the current node. I think I'm OK with the iterator
just returning null for the next() call in that case.

This gets the performance benefits of live NodeLists, I think meets the
main use-cases of not wanting to walk the whole DOM, but doesn't require
the browser to do a lot of metadata tracking as you go.

Ojan