Unlike most global objects,JSON is not a constructor. You cannot use it with thenew operator or invoke theJSON object as a function. All properties and methods ofJSON are static (just like theMath object).

JSON is a syntax for serializing objects, arrays, numbers, strings, booleans, andnull. It is based upon JavaScript syntax, but is distinct from #"#objects_and_arrays">Objects and Arrays

Any JSON text is a valid JavaScript expression, but only after theJSON 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 aSyntaxError.

Other differences include allowing only double-quoted strings and no support forundefined or comments. For those who wish to use a more human-friendly configuration format based on JSON, there isJSON5, used by the Babel compiler, and the more commonly usedYAML.

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

Valid JSON syntax is formally defined by the following grammar, expressed inABNF, and copied fromIETF JSON standard (RFC):

JSON-text = object / arraybegin-array     = ws %x5B ws  ; [ left square bracketbegin-object    = ws %x7B ws  ; { left curly bracketend-array       = ws %x5D ws  ; ] right square bracketend-object      = ws %x7D ws  ; } right curly bracketname-separator  = ws %x3A ws  ; : colonvalue-separator = ws %x2C ws  ; , commaws = *(     %x20 /              ; Space     %x09 /              ; Horizontal tab     %x0A /              ; Line feed or New line     %x0D                ; Carriage return     )value = false / null / true / object / array / number / stringfalse = %x66.61.6c.73.65   ; falsenull  = %x6e.75.6c.6c      ; nulltrue  = %x74.72.75.65      ; trueobject = begin-object [ member *( value-separator member ) ]         end-objectmember = string name-separator valuearray = begin-array [ value *( value-separator value ) ] end-arraynumber = [ minus ] int [ frac ] [ exp ]decimal-point = %x2E       ; .digit1-9 = %x31-39         ; 1-9e = %x65 / %x45            ; e Eexp = e [ minus / plus ] 1*DIGITfrac = decimal-point 1*DIGITint = zero / ( digit1-9 *DIGIT )minus = %x2D               ; -plus = %x2B                ; +zero = %x30                ; 0string = quotation-mark *char quotation-markchar = 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+XXXXescape = %x5C              ; \quotation-mark = %x22      ; "unescaped = %x20-21 / %x23-5B / %x5D-10FFFFHEXDIG = 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]

Insignificantwhitespace may be present anywhere except within aJSONNumber (numbers must contain no whitespace) orJSONString (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.

JSON[Symbol.toStringTag]

The initial value of the[Symbol.toStringTag] property is the string"JSON". This property is used inObject.prototype.toString().

JSON.isRawJSON()

Tests whether a value is an object returned byJSON.rawJSON().

JSON.parse()

Parse a piece of string text as JSON, optionally transforming the produced value and its properties, and return the value.

JSON.rawJSON()

Creates a "raw JSON" object containing a piece of JSON text. When serialized to JSON, the raw JSON object is treated as if it is already a piece of JSON. This text is required to be valid JSON.

JSON.stringify()

Return a JSON string corresponding to the specified value, optionally including only certain properties or replacing property values in a user-defined manner.

{  "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 theJSON.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));

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 the12345678901234567890 JSON number.

Let's assume you have a exact representation of some number (either viaBigInt 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:

• On the serialization side, in order to obtain a number in JSON, you have to pass a number toJSON.stringify, either via thereplacer function or via thetoJSON method. But, in either case, you have already lost precision during number conversion. If you pass a string toJSON.stringify, it will be serialized as a string, not a number.
• On the parsing side, not all numbers can be represented exactly. For example,JSON.parse("12345678901234567890") returns12345678901234568000 because the number is rounded to the nearest representable number. Even if you use areviver function, the number will already be rounded before thereviver function is called.

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 acommunication 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 (includingBigInt) should be serialized to JSON, either add atoJSON method to your data type, or use thereplacer function ofJSON.stringify().

// Using toJSON() methodBigInt.prototype.toJSON = function () {  return this.toString();};const str1 = JSON.stringify(data);// Using JSON.stringify() with replacerconst 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 usingJSON.rawJSON() to precisely specify what the JSON source text should be.

// Using toJSON() methodBigInt.prototype.toJSON = function () {  return JSON.rawJSON(this.toString());};const str1 = JSON.stringify(data);// Using JSON.stringify() with replacerconst str2 = JSON.stringify(data, (key, value) => {  if (key === "gross_gdp") {    return JSON.rawJSON(value.toString());  }  return value;});

The text passed toJSON.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 whenJSON.parse() invokes thereviver function, the value you receive is already parsed (and has lost precision). You can use thecontext.source parameter of theJSON.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 }

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