JSON

The JSON object contains methods for parsing JavaScript Object Notation (JSON) and converting values to JSON. It can't be called or constructed, and aside from its two method properties, it has no interesting functionality of its own.

Description

JavaScript and JSON differences

JSON is a syntax for serializing objects, arrays, numbers, strings, booleans, and null. It is based upon JavaScript syntax but is distinct from it: some JavaScript is not JSON.

Objects and Arrays

Property names must be double-quoted strings; trailing commas are forbidden.

Numbers

Leading zeros are prohibited. A decimal point must be followed by at least one digit. NaN and Infinity are unsupported.

Any JSON text is a valid JavaScript expression...

...But only in JavaScript engines that have implemented the proposal to make all JSON text valid ECMA-262. In engines that haven't implemented the proposal, U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR are allowed in string literals and property keys in JSON; but their use in these features in JavaScript string literals is a SyntaxError.

Consider this example where JSON.parse() parses the string as JSON and Global_Objects/eval executes the string as JavaScript:

let code = '"\u2028\u2029"'
JSON.parse(code)  // evaluates to "\u2028\u2029" in all engines
eval(code)        // throws a SyntaxError in old engines

Other differences include allowing only double-quoted strings and having no provisions for undefined or comments. For those who wish to use a more human-friendly configuration format based on JSON, there is JSON5, used by the Babel compiler, and the more commonly used YAML.

Full JSON grammar

Valid JSON syntax is formally defined by the following grammar, expressed in ABNF, and copied from IETF JSON standard (RFC):

JSON-text = object / array
begin-array     = ws %x5B ws  ; [ left square bracket
begin-object    = ws %x7B ws  ; { left curly bracket
end-array       = ws %x5D ws  ; ] right square bracket
end-object      = ws %x7D ws  ; } right curly bracket
name-separator  = ws %x3A ws  ; : colon
value-separator = ws %x2C ws  ; , comma
ws = *(
     %x20 /              ; Space
     %x09 /              ; Horizontal tab
     %x0A /              ; Line feed or New line
     %x0D                ; Carriage return
     )
value = false / null / true / object / array / number / string
false = %x66.61.6c.73.65   ; false
null  = %x6e.75.6c.6c      ; null
true  = %x74.72.75.65      ; true
object = begin-object [ member *( value-separator member ) ]
         end-object
member = string name-separator value
array = begin-array [ value *( value-separator value ) ] end-array
number = [ minus ] int [ frac ] [ exp ]
decimal-point = %x2E       ; .
digit1-9 = %x31-39         ; 1-9
e = %x65 / %x45            ; e E
exp = e [ minus / plus ] 1*DIGIT
frac = decimal-point 1*DIGIT
int = zero / ( digit1-9 *DIGIT )
minus = %x2D               ; -
plus = %x2B                ; +
zero = %x30                ; 0
string = quotation-mark *char quotation-mark
char = unescaped /
    escape (
        %x22 /          ; "    quotation mark  U+0022
        %x5C /          ; \    reverse solidus U+005C
        %x2F /          ; /    solidus         U+002F
        %x62 /          ; b    backspace       U+0008
        %x66 /          ; f    form feed       U+000C
        %x6E /          ; n    line feed       U+000A
        %x72 /          ; r    carriage return U+000D
        %x74 /          ; t    tab             U+0009
        %x75 4HEXDIG )  ; uXXXX                U+XXXX
escape = %x5C              ; \
quotation-mark = %x22      ; "
unescaped = %x20-21 / %x23-5B / %x5D-10FFFF

HEXDIG = DIGIT / %x41-46 / %x61-66   ; 0-9, A-F, or a-f
       ; HEXDIG equivalent to HEXDIG rule in [RFC5234]
DIGIT = %x30-39            ; 0-9
      ; DIGIT equivalent to DIGIT rule in [RFC5234]

Insignificant whitespace may be present anywhere except within a JSONNumber (numbers must contain no whitespace) or JSONString (where it is interpreted as the corresponding character in the string, or would cause an error). The tab character (U+0009), carriage return (U+000D), line feed (U+000A), and space (U+0020) characters are the only valid whitespace characters.

Static methods

JSON.parse(text[, reviver])

Parse the string text as JSON, optionally transform the produced value and its properties, and return the value. Any violations of the JSON syntax, including those pertaining to the differences between JavaScript and JSON, cause a SyntaxError to be thrown. The reviver option allows for interpreting what the replacer has used to stand in for other datatypes.

JSON.stringify(value[, replacer[, space]])

Return a JSON string corresponding to the specified value, optionally including only certain properties or replacing property values in a user-defined manner. By default, all instances of undefined are replaced with null, and other unsupported native data types are censored. The replacer option allows for specifying other behavior.

Examples

Example JSON

{
  "browsers": {
    "firefox": {
      "name": "Firefox",
      "pref_url": "about:config",
      "releases": {
        "1": {
          "release_date": "2004-11-09",
          "status": "retired",
          "engine": "Gecko",
          "engine_version": "1.7"
        }
      }
    }
  }
}

Specifications

Specification
ECMAScript Language Specification (ECMAScript)
# sec-json-object

Browser compatibility

BCD tables only load in the browser

See also