Passing Data with JSON

JSON, or Javascript Object Notation, is a lightweight data interchange standard.   It can, in theory, be used to pass data between any two programming languages, but it has special advantages when used with Javascript.  JSON is fundementally just the Javascript array and object initializer syntax on its own.  So this array of objects in a Javascript program:

Roughly everything after the "=" is JSON.  Here's what the JSON packet would look like coming back from a web service:

"Excuse me," you might say, "but we already have a data interchange format.  You might have heard of it ... it's called XML."   So why not just pass it as:

The answer is performance, performance, performance!  JSON data is parsed up to 100 times faster than XML.  That makes sense because the parser is just eval().  For small portions of data, this doesn't mean much, but for large ones it's indispensible.  XML expressed in Javascript must carry the full weight of XML - including namespaces, DTD's and schemas.  Furthermore, the DOM representation of XML is much more memory-intensive than native Javascript.

Add to that the fact that XML is interpreted a little differently in each browser, and JSON is the preferred method for data interchange in dojo.  XML is supported, but you might not want to use it in simple scenarios. Where it really begins to make sense, however, is when large scale transformations are needed on the client side. Almost every modern browser today provides a client-side XSLT transform facility which can often outstrip JSON for speed in transforming large data sets from one structure to another since that transformation is handled in C or C++ and not in JavaScript. Whether your app chooses to use JSON or XML is often a foregone conclusion, but be aware that there are tradeoffs for each.

Accepting JSON Data in dojo.xhrGet

Provided your web service sends JSON (as the above example shows) Dojo pretty much handles the rest for you, including parsing the JSON into a JavaScript object. All you have to do is specify a JSON content handler: