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    <p>JSON came into popular use primarily because it offers a way to circumvent the same-origin policy used in web browsers and thereby allow mashups.</p> <p>Let's say you're writing a web service on domain A. You can't load XML data from domain B and parse it because the only way to do that would be XMLHttpRequest, and XMLHttpRequest was originally limited by the same-origin policy to talking to only URLs at the same domain as the containing page.</p> <p>It turns out that for a variety of reasons, you <em>are</em> allowed to request &lt;script&gt; tags across origins. Clever people realized this was a good way to work around the limitation with XMLHttpRequest. Instead of the server returning XML, it can return a series of JavaScript object and array literals.</p> <p>(bonus question left as an exercise to the reader: why is &lt;script src="..."&gt; allowed across domains without server opt-in but XHR isn't?)</p> <p>Of course, returning a &lt;script&gt; which consists of nothing more than object literals is not useful because without assigning the values to some variable, you can't do anything with it. Thus, most services use a variant of JSON, called JSONP (<a href="http://bob.pythonmac.org/archives/2005/12/05/remote-json-jsonp/" rel="nofollow noreferrer">http://bob.pythonmac.org/archives/2005/12/05/remote-json-jsonp/</a>).</p> <p>With the rise in popularity of mashups, people realized that JSON was a convenient data interchange format in general, especially when JavaScript is one end of the channel. For example, JSON is used extensively in Chromium, even in cases where C++ is on both sides. It's just a nice lightweight way to represent simple data, that good parsers exist for in many languages.</p> <p>Amusingly, using &lt;script&gt; tags to do mashups is incredibly insecure because it is essentially XSS'ing yourself on purpose. So native JSON (<a href="http://ejohn.org/blog/native-json-support-is-required/" rel="nofollow noreferrer">http://ejohn.org/blog/native-json-support-is-required/</a>) had to be introduced, which obviates the original benefits of the format. But by that time, it was already super popular :)</p>
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    1. POWhy is JSON important?
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    1. USAaron Boodman
    1. USAaron Boodman
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    1. COI don't understand this origin story. It's trivial to write a JavaScript function that returns an (unparsed) XML document. You could use this hack to provide XML just as easily as using it to provide an entirely new serialization format.
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      1. USRobert Rossney
    2. COIt's true, you could just return a string of XML and then parse it. But the string would have to be carefully escaped inside a JavaScript string. People found themselves dynamically generating: returnData("&lt;data foo=\"bar\"&gt;...&lt;/foo&gt;") ... and asking themselves why bother when they could just return objects and arrays directly. And once you go down that path, you realize that working with native objects and arrays with no separate parse phase is just a better idea in general. But I think that mashups are how JSON got started, or at least one of a couple primary reasons.
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      1. USAaron Boodman
 

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