JSON in JavaScript

JavaScript is a general
purpose programming language that was introduced as the page scripting
language for Netscape Navigator. It
is still widely believed to be a subset of Java, but it is not. It
is a Scheme-like
language with C-like
syntax and soft
objects. JavaScript was standardized in the ECMAScript
Language Specification, Third Edition.

JSON is a subset of the object literal notation
of JavaScript. Since JSON is a subset of JavaScript, it can be used in the language
with no muss or fuss.

var myJSONObject = {"bindings": [
{"ircEvent": "PRIVMSG", "method": "newURI", "regex": "^http://.*"},
{"ircEvent": "PRIVMSG", "method": "deleteURI", "regex": "^delete.*"},
{"ircEvent": "PRIVMSG", "method": "randomURI", "regex": "^random.*"}
]
};

In this example, an object is created containing a single member "bindings", which contains an array containing three objects, each containing "ircEvent", "method", and "regex" members.

Members can be retrieved using dot or subscript operators.

myJSONObject.bindings[0].method    // "newURI"

To convert a JSON text into an object, you can use the eval()
function. eval() invokes the JavaScript compiler. Since JSON
is a proper subset of JavaScript, the compiler will correctly parse the
text and produce an object structure. The text must be wrapped in parens
to avoid tripping on an ambiguity in JavaScript's syntax.

var myObject = eval('(' + myJSONtext + ')');

The

eval

function is very fast. However, it can compile
and execute any JavaScript program, so there can be security issues. The
use of

eval

is indicated when the source is trusted and competent.
It is much safer to use a JSON parser. In web applications over XMLHttpRequest,
communication is permitted only to the same origin that provide that page,
so it is trusted. But it might not be competent. If the server is not
rigorous in its JSON encoding, or if it does not scrupulously validate
all of its inputs, then it could deliver invalid JSON text that could
be carrying dangerous script. The

eval

function would execute
the script, unleashing its malice.

To defend against this, a JSON parser should be used. A JSON parser will
recognize only JSON text, rejecting all scripts. In browsers that provide
native JSON support, JSON parsers are also much faster than

eval

.
It is expected that native JSON support will be included in the next ECMAScript
standard.

var myObject = JSON.parse(myJSONtext, reviver);

The optional

reviver

parameter is a function that will be
called for every key and value at every level of the final result. Each
value will be replaced by the result of the

reviver

function.
This can be used to reform generic objects into instances of pseudoclasses,
or to transform date strings into

Date

objects.

myData = JSON.parse(text, function (key, value) {
var type;
if (value && typeof value === 'object') {
type = value.type;
if (typeof type === 'string' && typeof window[type] === 'function') {
return new (window[type])(value);
}
}
return value;
});

A JSON stringifier goes in the opposite direction, converting JavaScript
data structures into JSON text. JSON does not support cyclic data structures,
so be careful to not give cyclical structures to the JSON stringifier.

var myJSONText = JSON.stringify(myObject, replacer);

If the

stringify

method sees an object that contains a

toJSON

method, it calls that method, and stringifies the value returned. This
allows an object to determine its own JSON representation.

The

stringifier

method can take an optional array of strings.
These strings are used to select the properties that will be included
in the JSON text.

The

stringifier

method can take an optional replacer function.
It will be called after the

toJSON

method (if there is one)
on each of the values in the structure. It will be passed each key and
value as parameters, and

this

will be bound to object holding
the key. The value returned will be stringified.

If you do not provide an array or replacer function, then an optional
replacer function will be provided for you that omits inherited properties.
If you want all inherited properties, you can provide a simple replacer
function:

var myJSONText = JSON.stringify(myObject, function (key, value) {
return value;
});

Values that do not have a representation in JSON (such as functions and

undefined

) are excluded.

Nonfinite numbers are replaced with

null

. To substitute
other values, you could use a replacer function like this:

function replacer(key, value) {
if (typeof value === 'number' && !isFinite(value)) {
return String(value);
}
return value;
}
The open source code of a JSON parser and JSON stringifier
is available. When minified
it is less than 2.5K.