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JavaScript DocumentFragment

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One of the lessor known but incredibly useful gems within JavaScript is the DocumentFragment.  DocumentFragments allow developers to place child elements onto an arbitrary node-like parent, allowing for node-like interactions without a true root node.  Doing so allows developers to produce structure without doing so within the visible DOM -- an increase of speed is the true advantage.  Let me show you how DocumentFragments are used!

DocumentFragment Example

Let's start with a UL with which we'll inject LI's to:

<ul id="list"></ul>

DOM injections and modifications are taxing, so the fewer the interactions the better;  that's where the DocumentFragment comes in.  The first step is creating the DocumentFragment:

// Create the fragment
var frag = document.createDocumentFragment();

This DocumentFragment act like a pseudo-DOM node -- think of it as a virtual UL element, in this case.  Now it's time to add elements:

// Create numerous list items, add to fragment
for(var x = 0; x < 10; x++) {
	var li = document.createElement("li");
	li.innerHTML = "List item " + x;
	frag.appendChild(li);
}

Elements can be added to the DocumentFragment just as you could a normal DOM node.  Once the tree of DOM nodes is ready to hit the page, simply place the DocumentFragement into its parent (or other DOM placement functions):

// Mass-add the fragment nodes to the list
listNode.appendChild(frag);

Using DocumentFragments is faster than repeated single DOM node injection and allows developers to perform DOM node operations (like adding events) on new elements instead of mass-injection via innerHTML.  Keep DocumentFragment close by when performing lots of DOM operations -- it could speed up your app considerably!

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Discussion

  1. But test (http://jsperf.com/out-of-dom-vs-documentfragment/3) do not show “an increase of speed” …

  2. @sergey: Probably depends on your browser, I got about 5-6 % speed gain with your test. http://imgur.com/SjHWF

  3. David

    I ran that test too, and documentFragment was slower on all my browsers.

    I’m on a Mac… documentFragment was always slower on Firefox 16.0.2 and Safari 6.0.1 (3-4 tests); and mixed results (slower and faster) on Chrome 23.

    David

  4. I’ve created revision 7 to show the improvement of using docfragments. As for differences in creating a temp element using createElement vs createDocumentFragment,

    http://stackoverflow.com/questions/3397161/should-i-use-document-createdocumentfragment-or-document-createelement

    has some explanation.

  5. wouldn’t you want to just concat a string and append that once at the end?

    • Not if you want to add events or work with nodes DOM structure.

  6. fbender

    I “fixed” the jsperf test and had a better performance with Fx 17 Mac, Chrome did not show any significant performance difference between the two options.

    http://jsperf.com/out-of-dom-vs-documentfragment/10

    As long as some browsers profit from documentFragment, everybody should use it (well, at least if you do it correctly).

  7. I updated the speed test http://jsperf.com/out-of-dom-vs-documentfragment/11 by adding test without documentFragment, but still outside of DOM. documentFragment have a limitations and sometimes can not be used.

  8. seelts

    But why do so?
    I can do something like:


    var ul = document.createElement('ul');
    var li;
    for (...) {
    li = document.createElement('li');
    ul.appendChild(li);
    }

    smth.appendChild(ul);

    What is the difference?

    • That’s a good option, but sometimes the “main” element is already in the DOM… like your “ul”.

      So you wouldn’t be appending it into in a “phantom” element, but would be rendering it in the DOM itself. That may be a problem when we are thinking about reflow and thousands of elements.

  9. I too use the same method as @seelts, and am wondering the same about performance.

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