JSON vs. JavaScript

January 8, 2008 at 12:09 am · Filed under Ajax, JavaScript, JSON ·Tagged difference, difference between json and javascript, JavaScript, JSON, json vs. javascript

JSON is not JavaScript (nor a JavaScript “subset”)

JSON is not only a syndication format (e.g. an alternative to XML), it is a language-independent data interchange format.

As such, while JSON objects are based on the JavaScript object literal, they are not the same. In fact, JSON objects can be invalid as JavaScript objects. Therefore, JSON should be transformed to JavaScript-compliant JSON before feeding Ajax or JavaScript applications.

This flexibility on the part of JSON increases its extensibility as an interchange format. Although JSON makes a strong competitor to XML for Ajax applications, it is meant to be suitable for other mediums as well (hence JSON RPC).

How they are Different—Property Names

Their difference is in what is permissible for property names, defined by the ECMA Standard for JavaScript, and JSON member (property) names, defined by RFC 4627.

JavaScript requires property names to start with an underscore (_), letter, or dollar sign ($), and contain only alphanumeric characters, underscores, and dollar signs:

var chris = {
	lastName: "O'Brien",
	_city: "Seattle",
	income$: "Dollar sign"
};
alert("Chris lives in " + chris._city);

JSON property names, on the other hand, may contain any Unicode characters, provided proper escaping of control characters†. For example, the object defined in by the { } literal below is valid JSON, yet cannot be used to create an object in JavaScript.

// throws an Error
var impossible = {
	"411": "yohoho.com",
	"xmlns:x": "Invalid character",
	"Company name": "Spaces",
	"": "Zero-length string"
};

Because JSON property names can contain spaces and JavaSript’s cannot, JSON property names must be encapsulated in quotations. In JavaScript, encapsulating property names is optional. Quotations also make an empty zero- or variable-length property name possible in JSON.

Reserved Words

Furthermore, JavaScript (like all well-adopted languages) has reserved words. These are words that have special meaning and cannot be used as property names. Below is a table of reserved words. Words without asterisks are reserved keywords and words with asterisks are words reserved for future use.These can be found in ECMA-262 under heading 7.5.

abstract* boolean* break byte* case catch char* class* const* continue

debugger* default delete do double* class* const* continue debugger* default

delete do double* else enum* export* extends* final* finally float*

for function goto* if implements* import* in instanceof int* interface*

long* native* new package* private* protected* public* return short* static*

switch super* synchronized* this throw throws* transient* try typeof volatile*

Ordered Steps for Transforming JSON for JavaScript

To guarantee that JSON data can be consumed by JavaScript, like so:

var obj = eval("(" + jsonString + ")");

…the operations below need to be performed on a string of JSON data. This will ensure that the JSON data is JavaScript-compliant.

1. Remove or replace any spaces in property names.
2. Remove or replace the first character in the name of a property if it is not an underscore, letter, or dollar sign.
3. Remove or replace any non-alphanumeric character, save for underscores and dollar signs.
4. Replace property names that are reserved words in JavaScript.
5. If a property name is an empty, zero-length string, replace it with a valid JavaScript property name.
6. Optionally, quotations around property names may be removed.

† Quotations and control characters must be escaped. Check out the RFC for details.

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