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JSON

{{JSRef}} 

The JSON namespace object contains static methods for parsing values from and converting values to JavaScript Object Notation ({{Glossary("JSON")}} ).

Description

Unlike most global objects, JSON is not a constructor. You cannot use it with the new operator or invoke the JSON object as a function. All properties and methods of JSON are static (just like the {{jsxref("Math")}}  object).

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 JavaScript: most of JavaScript is not JSON. For example:

Any JSON text is a valid JavaScript expression, but only after the JSON superset revision. Before the revision, U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR are allowed in string literals and property keys in JSON; but the same use in JavaScript string literals is a {{jsxref("SyntaxError")}} .

Other differences include allowing only double-quoted strings and no support for {{jsxref("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.

The same text may represent different values in JavaScript object literals vs. JSON as well. For more information, see Object literal syntax vs. JSON.

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 {{Glossary("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 (U+0009), carriage return (U+000D), line feed (U+000A), and space (U+0020) characters are the only valid whitespace characters.

Static properties

Static methods

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"
        }
      }
    }
  }
}

You can use the {{jsxref("JSON.parse()")}}  method to convert the above JSON string into a JavaScript object:

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

console.log(JSON.parse(jsonText));

Lossless number serialization

JSON can contain number literals of arbitrary precision. However, it is not possible to represent all JSON numbers exactly in JavaScript, because JavaScript uses floating point representation which has a fixed precision. For example, 12345678901234567890 === 12345678901234567000 in JavaScript because they have the same floating point representation. This means there is no JavaScript number that corresponds precisely to the 12345678901234567890 JSON number.

Let’s assume you have a exact representation of some number (either via {{jsxref("BigInt")}}  or a custom library):

const data = {
  // Using a BigInt here to store the exact value,
  // but it can also be a custom high-precision number library,
  // if the number might not be an integer.
  gross_gdp: 12345678901234567890n,
};

You want to serialize it and then parse to the same exact number. There are several difficulties:

There are, in general, two ways to ensure that numbers are losslessly converted to JSON and parsed back: one involves a JSON number, another involves a JSON string. JSON is a communication format, so if you use JSON, you are likely communicating with another system (HTTP request, storing in database, etc.). The best solution to choose depends on the recipient system.

Using JSON strings

If the recipient system does not have same JSON-handling capabilities as JavaScript, and does not support high precision numbers, you may want to serialize the number as a string, and then handle it as a string on the recipient side. This is also the only option in older JavaScript.

To specify how custom data types (including BigInt) should be serialized to JSON, either add a toJSON method to your data type, or use the replacer function of {{jsxref("JSON.stringify()")}} .

// Using toJSON() method
BigInt.prototype.toJSON = function () {
  return this.toString();
};
const str1 = JSON.stringify(data);

// Using JSON.stringify() with replacer
const str2 = JSON.stringify(data, (key, value) => {
  if (key === "gross_gdp") {
    return value.toString();
  }
  return value;
});

In either case, the JSON text will look like {"gross_gdp":"12345678901234567890"}, where the value is a string, not a number. Then, on the recipient side, you can parse the JSON and handle the string.

Using JSON numbers

If the recipient of this message natively supports high precision numbers (such as Python integers), passing numbers as JSON numbers is obviously better, because they can directly parse to the high precision type instead of parsing a string from JSON, and then parsing a number from the string. In JavaScript, you can serialize arbitrary data types to JSON numbers without producing a number value first (resulting in loss of precision) by using {{jsxref("JSON.rawJSON()")}}  to precisely specify what the JSON source text should be.

// Using toJSON() method
BigInt.prototype.toJSON = function () {
  return JSON.rawJSON(this.toString());
};
const str1 = JSON.stringify(data);

// Using JSON.stringify() with replacer
const str2 = JSON.stringify(data, (key, value) => {
  if (key === "gross_gdp") {
    return JSON.rawJSON(value.toString());
  }
  return value;
});

The text passed to JSON.rawJSON is treated as if it is already a piece of JSON, so it won’t be serialized again as a string. Therefore, the JSON text will look like {"gross_gdp":12345678901234567890}, where the value is a number. This JSON can then be parsed by the recipient without any extra processing, provided that the recipient system does not have the same precision limitations as JavaScript.

When parsing JSON containing high-precision numbers in JavaScript, take extra care because when JSON.parse() invokes the reviver function, the value you receive is already parsed (and has lost precision). You can use the context.source parameter of the {{jsxref("JSON.parse()")}}  reviver function to re-parse the number yourself.

const parsedData = JSON.parse(str, (key, value, context) => {
  if (key === "gross_gdp") {
    // Or use the constructor of your custom high-precision number library
    return BigInt(context.source);
  }
  return value;
});
// { gross_gdp: 12345678901234567890n }

Specifications

{{Specifications}} 

Browser compatibility

{{Compat}} 

See also

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