JSON

JSON (/ˈdʒeɪsɒn/ JAY-sawn, /ˈdʒeɪsən/ JAY-sun), or JavaScript Object Notation, is a text-based open standard designed for human-readable data interchange. It is derived from the JavaScript scripting language for representing simple data structures and associative arrays, called objects. Despite its relationship to JavaScript, it is language-independent, with parsers available for many languages.

The JSON format was originally specified by Douglas Crockford, and is described in RFC 4627. The official Internet media type for JSON is application/json. The JSON filename extension is .json.

The JSON format is often used for serializing and transmitting structured data over a network connection. It is used primarily to transmit data between a server and web application, serving as an alternative to XML.

JSON’s basic types are:

• Number (double precision floating-point format in JavaScript, generally depends on implementation)
• String (double-quoted Unicode, with backslash escaping)
• Boolean (true or false)
• Array (an ordered sequence of values, comma-separated and enclosed in square brackets; the values do not need to be of the same type)
• Object (an unordered collection of key:value pairs with the ‘:’ character separating the key and the value, comma-separated and enclosed in curly braces; the keys must be strings and should be distinct from each other)
• null (empty)

Non-significant white space may be added freely around the “structural characters” (i.e. brackets “{ } [ ]”, colons “:” and commas “,”).

The following example shows the JSON representation of an object that describes a person. The object has string fields for first name and last name, a number field for age, an object representing the person’s address and an array of phone number objects.

{
    "firstName": "John",
    "lastName": "Smith",
    "age": 25,
    "address": {
        "streetAddress": "21 2nd Street",
        "city": "New York",
        "state": "NY",
        "postalCode": 10021
    },
    "phoneNumbers": [
        {
            "type": "home",
            "number": "212 555-1234"
        },
        {
            "type": "fax",
            "number": "646 555-4567"
        }
    ]
}

One potential pitfall of the free-form nature of JSON comes from the ability to write numbers as either numeric literals or quoted strings. For example, ZIP Codes in the northeastern U.S. begin with zeroes (for example, 07728 for Freehold, New Jersey). If written with quotes by one programmer but not by another, the leading zero could be dropped when exchanged between systems, when searched for within the same system, or when printed. In addition, postal codes in the U.S. are numbers but other countries use letters as well. This is a type of problem that the use of a JSON Schema (see below) is intended to reduce.

Since JSON is almost a subset of JavaScript, it is possible, but not recommended,^[7] to parse most JSON text into an object by invoking JavaScript’s eval() function. For example, if the above JSON data is contained within a JavaScript string variable contact, one could use it to create the JavaScript object p as follows:

 var p = eval("(" + contact + ")");

The contact variable must be wrapped in parentheses to avoid an ambiguity in JavaScript’s syntax.^[8]

The recommended way, however, is to use a JSON parser. Unless a client absolutely trusts the source of the text, or must parse and accept text that is not strictly JSON-compliant, one should avoid eval(). A correctly implemented JSON parser accepts only valid JSON, preventing potentially malicious code from being executed inadvertently.

 var p = JSON.parse(contact);

Browsers, such as Firefox 4 and Internet Explorer 8, include special features for parsing JSON. As native browser support is more efficient and secure than eval(), native JSON support is included in the recently-released Edition 5 of the ECMAScript standard.^[9]

jQuery library wrap JSON object in function constructor and execute it immediately if JSON.parse is not present. This avoid using eval in the code.

 var p = new Function('return ' + contact ';')();

Despite the widespread belief that JSON is a JavaScript subset, this is not the case. Specifically, JSON allows the Unicode line terminators U+2028 line separator and U+2029 paragraph separator to appear unescaped in quoted strings, while JavaScript does not.^[10] This is a consequence of JSON disallowing only “control characters”. This subtlety is important when generating JSONP.