JavaScript Expressions – Beyond JSON

September 22nd, 2008
JSON a powerful simple expressive format for data structures with a high level of interoperability; implementations exist in virtually every popular language. However, there are certainly situations where developers often want additional constructs for effectively representing data that are not natively supported in JSON. Perhaps the most common usage of JSON is for the consumption of data in the browser. Most JavaScript libraries, parse JSON by using eval, and consequently are actually capable of full JavaScript expression evaluation, of which JSON is a subset. JavaScript expressions support a much wider range of constructs than pure JSON. Usually a simple JSON/JavaScript expression parser looks like:

function parseJson(jsonText){
return eval(‘(‘ + jsonText + ‘)’);
One of the most oft-desired data type that JSON doesn’t provide is a date type. Numerous creative, bizarre, weird, and silly techniques have been proposed for expressing dates in JavaScript. These methods often require extra parsing or walking strategies. Douglas Crockford’s reference library for JavaScript JSON serialization, serializes dates to strings by ISO format. I have written about deserializing these ISO dates. But, on deserialization it is not possible to determine if a value is actually a string or is really intended to be an actual date object. However, if the recipient of JSON is known to support full JavaScript evaluation (like the browser with a library using an eval), the solution for delivering date type value is simple, we can just use the normal JavaScript constructor:

{“mydate”:new Date(1222057313264)}
There are also several numeric entities that JavaScript provides that are not available, including Infinite, -Infinite, and NaN. I am not sure why you would need it, but undefined can also be transferred with JavaScript. Functions can be included as well:

“reallyBig”: Infinite,
“parsedString”: NaN,
“whatValue?”: undefined,
“doSomething”:function(arg){return doSomethingElse{arg};}
The data representations thus demonstrated are not JSON, they are JavaScript using the same object/array literal that JSON is based on. However, certainly one of the biggest benefits of JSON is it’s interoperability. If your data is going to be consumed by more than just JavaScript-parsing JavaScript libraries, you must make your data available in pure JSON format as well. This is well-handled through content negotiation. If you are using JavaScript expressions to transfer data, you should make sure your requests from the browser actually are specifying they can handle JavaScript:

Accept: text/javascript
Your server should be prepared to handle requests from clients that indicate that they only understand pure JSON:

Accept: application/json
Persevere, the JavaScript/JSON application and storage server, also provides support for parsing and storing extra constructs including non-finite numbers (NaN, Infinite), functions. Persevere can output the data as JSON or JavaScript (expression).

JSON is certainly powerful, expressive data format. This post is by no means an attempt to expand or lobby for the modification JSON. However, when it is known that consumers are actually JavaScript capable clients, it can often be advantageous to use the full power of JavaScript to represent data, while still providing JSON as representation for non-JavaScript capable clients.


4 Responses to “JavaScript Expressions – Beyond JSON”

  1. unscriptable Says:

    Why not add a date type to JSON? Nobody — and I mean nobody — who’s using JSON is not dealing with dates.

    With ES5 coming, why not add date primitives while we’re at it? Hell, we’ve got native XML and RegExp, why not Dates????

    Then we could just use the same simple eval-based JSON parsers if we want to — or use the new native JSON parsers.

    — J

  2. Olly Says:

    Hmm, apart from dates, I’m not sure why you would want any of the other stuff. Passing NaN is bad enough, but passing a javascript function from the server to the front end via JSON seems downright evil; you’re exposing the client to potentially malicious code, mixing server-side and front-end logic, and potentially having your browser interpret the same functions over and over again.

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