Is parsing JSON faster than parsing XML

I'm creating a sophisticated JavaScript library for working with my company's server side framework.

The server side framework encodes its data to a simple XML format. There's no fancy namespacing or anything like that.

Ideally I'd like to parse all of the data in the browser as JSON. However, if I do this I need to rewrite some of the server side code to also spit out JSON. This is a pain because we have public APIs that I can't easily change.

What I'm really concerned about here is performance in the browser of parsing JSON versus XML. Is there really a big difference to be concerned about? Or should I exclusively go for JSON? Does anyone have any experience or benchmarks in the performance difference between the two?

I realize that most modern web developers would probably opt for JSON and I can see why. However, I really am just interested in performance. If there's a proven massive difference then I'm prepared to spend the extra effort in generating JSON server side for the client.

JSON should be faster since it's JS Object Notation, which means it can be recognized natively by JavaScript. In PHP on the GET side of things, I will often do something like this:

<script type="text/javascript">
    var data = <?php json_encode($data)?>;
</script>

For more information on this, see here:

https://stackoverflow.com/questions/1743532/why-is-everyone-choosing-json-over-xml-for-jquery

Also...what "extra effort" do you really have to put into "generating" JSON? Surely you can't be saying that you'll be manually building the JSON string? Almost every modern server-side language has libraries that convert native variables into JSON strings. For example, PHP's core json_encode function converts an associative array like this:

$data = array('test'=>'val', 'foo'=>'bar');

into

{"test": "val", "foo": "bar"}

Which is simply a JavaScript object (since there are no associative arrays (strictly speaking) in JS).

Firstly, I'd like to say thanks to everyone who's answered my question. I REALLY appreciate all of your responses.

In regards to this question, I've conducted some further research by running some benchmarks. The parsing happens in the browser. IE 8 is the only browser that doesn't have a native JSON parser. The XML is the same data as the JSON version.

Chrome (version 8.0.552.224), JSON: 92ms, XML: 90ms

Firefox (version 3.6.13), JSON: 65ms, XML: 129ms

IE (version 8.0.6001.18702), JSON: 172ms, XML: 125ms

Interestingly, Chrome seems to have almost the same speed. Please note, this is parsing a lot of data. With little snippets of data, this isn't probably such a big deal.

Benchmarks have been done. Here's one. The difference in some of the earlier browsers appeared to be an entire order of magnitude (on the order of 10s of milliseconds instead of 100s of ms), but not massive. Part of this is in server response time - XML is bulkier as a data format. Part of it is parsing time - JSON lets you send JavaScript objects, while XML requires parsing a document.

You could consider adding to your public API a method to return JSON instead of modifying existing functions if it becomes and issue, unless you don't want to expose the JSON.

See also the SO question When to prefer JSON over XML?

Performance isn't really a consideration, assuming that you're not talking about gigabytes of XML. Yes, it will take longer (XML is more verbose), but it's not going to be something that the user will notice.

The real issue, in my opinion, is support for XML within JavaScript. E4X is nice, but it isn't supported by Microsoft. So you'll need to use a third-party library (such as JQuery) to parse the XML.

If possible, it would make sense to just measure it. By 'if possible' I mean that tooling for javascript (esp. for performance analysis) may not be quite as good as for stand-alone programming languages.

Why measure? Because speculation based solely on properties of data formats is not very useful for performance analysis -- developers' intuitions are notoriously poor at predicting performance. In this case it just means that it all comes down to maturity of respective XML and JSON parser (and generators) in use. XML has the benefit of having been around longer; JSON is bit simpler to process. This based on having actually written libraries for processing both. In the end, if all things are equal (maturity and performance optimization of libraries), JSON can indeed be bit faster to process. But both can be very fast; or very slow with bad implementations.

However: I suspect that you should not worry all that much about performance, like many have already suggested. Both xml and json can be parsed efficiently, and with modern browsers, probably are. Chances are that if you have performance problems it is not with reading or writing of data but something else; and first step would be actually figuring out what the actual problem is.

since JSON is native in and designed FOR Javascript, it's going to out-perform XML parsing all day long. you didn't mention your server-side language, in PHP there is the json_encode/json_decode functionality built into the PHP core...

the difference in performace will be so tiny, you wouldn't even notice it (and: you shouldn't think about performance problems until you have performance problems - there are a lot of more important points to care for - maintainable, readable and documented code...).

but, to answer ayou question: JSON will be faster to parse (because it's simple javascript object notation).

In this situation, I'd say stick with the XML. All major browsers have a DOM parsing interface that will parse well-formed XML. This link shows a way to use the DOMParser interface in Webkit/Opera/Firefox, as well as the ActiveX DOM Object in IE: https://sites.google.com/a/van-steenbeek.net/archive/explorer_domparser_parsefromstring

It also depends on how your JSON is structured. Tree-like structures tend to parse more efficiently than a list of objects. This is where one's fundamental understanding of data structures will be handy. I would not be surprised if you parse a list-like structure in JSON that might look like this:

{
        {
            "name": "New York",
            "country":"USA",
            "lon": -73.948753,
            "lat": 40.712784
        },
        {
            "name": "Chicago",
            "country":"USA",
            "lon": -23.948753,
            "lat": 20.712784
        },
        {
            "name": "London",
            "country":"UK",
            "lon": -13.948753,
            "lat": 10.712784
        }
}

and then compare it to a tree like structure in XML that might look like this:

<cities>
  <country name="USA">
     <city name="New York">
        <long>-73.948753</long>
        <lat>40.712784</lat>
     </city>
     <city name="Chicago">
        <long>-23.948753</long>
        <lat>20.712784</lat>
     </city>
  </country>
 <country name="UK">
     <city name="London">
        <long>-13.948753</long>
        <lat>10.712784</lat>
     </city>
  </country>
</cities>

The XML structure may yield a faster time than that of JSON since if I loop through the node of UK to find London, I don't have to loop through the rest of the countries to find my city. In the JSON example, I just might if London is near the bottom of the list. But, what we have here is a difference in structure. I would be surprised to find that XML is faster in either case or in a case where the structures are exactly the same.

Here is an experiment I did using Python - I know the question is looking at this strictly from a JavaScript perspective, but you might find it useful. The results show that JSON is faster than XML. However, the point is: how you structure is going to have an effect on how efficiently you are able to retrieve it.

Another reason to stick with XML is, that if you switch to JSON, you modify the "maintenance contract". XML is more typed than JSON is, in the sense that it works more naturally with typed languages (i.e. NOT javascript).

If you change to JSON, some future maintainer of the code base might introduce a JSON array at some point which has mixed type content (e.g. [ "Hello", 42, false ]), which will present a problem to any code written in a typed language.

Yes, you could do that as well in XML but it requires extra effort, while in JSON it can just slip in.

And while it does not seem like a big deal at first glance, it actually is as it forces the code in the typed language to stick with a JSON tree instead of deserializing to a native type.

best example i have found about these two is :

http://www.utilities-online.info/xmltojson/#.VVGOlfCYK7M

that means JSON is more human readable and understandable than XML.