text

The string to parse as JSON. See theJSON object for a description of JSON syntax.

reviverOptional

If a function, this prescribes how each value originally produced by parsing is transformed before being returned. Non-callable values are ignored. The function is called with the following arguments:

key

The key associated with the value.

value

The value produced by parsing.

contextOptional

A context object that holds state relevant to the current expression being revived. It is a new object for each invocation of the reviver function. It is only passed when reviving primitive values, but not whenvalue is an object or array. It contains the following property:

source

The original JSON string representing this value.

TheObject,Array, string, number, boolean, ornull value corresponding to the given JSONtext.

SyntaxError

Thrown if the string to parse is not valid JSON.

JSON.parse() parses a JSON string according to theJSON grammar, then evaluates the string as if it's a JavaScript expression. The only instance where a piece of JSON text represents a different value from the same JavaScript expression is when dealing with the"__proto__" key — seeObject literal syntax vs. JSON.

If areviver is specified, the value computed by parsing istransformed before being returned. Specifically, the computed value and all its properties (in adepth-first fashion, beginning with the most nested properties and proceeding to the original value itself) are individually run through thereviver.

Thereviver is called with the object containing the property being processed asthis (unless you define thereviver as an arrow function, in which case there's no separatethis binding) and two arguments:key andvalue, representing the property name as a string (even for arrays) and the property value. For primitive values, an additionalcontext parameter is passed, which contains the source text of this value. If thereviver function returnsundefined (or returns no value — for example, if execution falls off the end of the function), the property is deleted from the object. Otherwise, the property is redefined to be the return value. If thereviver only transforms some values and not others, be certain to return all untransformed values as-is — otherwise, they will be deleted from the resulting object.

Similar to thereplacer parameter ofJSON.stringify(), for arrays and objects,reviver will be last called on the root value with an empty string as thekey and the root object as thevalue. For other valid JSON values,reviver works similarly and is called once with an empty string as thekey and the value itself as thevalue.

If you return another value fromreviver, that value will completely replace the originally parsed value. This even applies to the root value. For example:

const transformedObj1 = JSON.parse('[1,5,{"s":1}]', (key, value) =>  typeof value === "object" ? undefined : value,);console.log(transformedObj1); // undefined

There is no way to work around this generically. You cannot specially handle the case wherekey is an empty string, because JSON objects can also contain keys that are empty strings. You need to know very precisely what kind of transformation is needed for each key when implementing the reviver.

Note thatreviver is run after the value is parsed. So, for example, numbers in JSON text will have already been converted to JavaScript numbers, and may lose precision in the process. One way to transfer large numbers without loss of precision is to serialize them as strings, and revive them toBigInts, or other appropriate arbitrary precision formats.

You can also use thecontext.source property to access the original JSON source text representing the value, as shown below:

const bigJSON = '{"gross_gdp": 12345678901234567890}';const bigObj = JSON.parse(bigJSON, (key, value, context) => {  if (key === "gross_gdp") {    // Ignore the value because it has already lost precision    return BigInt(context.source);  }  return value;});

In order for a value to properly round-trip (that is, it gets deserialized to the same original object), the serialization process must preserve the type information. For example, you can use thereplacer parameter ofJSON.stringify() for this purpose:

// Maps are normally serialized as objects with no properties.// We can use the replacer to specify the entries to be serialized.const map = new Map([  [1, "one"],  [2, "two"],  [3, "three"],]);const jsonText = JSON.stringify(map, (key, value) =>  value instanceof Map ? Array.from(value.entries()) : value,);console.log(jsonText);// [[1,"one"],[2,"two"],[3,"three"]]const map2 = JSON.parse(jsonText, (key, value) =>  Array.isArray(value) && value.every(Array.isArray) ? new Map(value) : value,);console.log(map2);// Map { 1 => "one", 2 => "two", 3 => "three" }

Because JSON has no syntax space for annotating type metadata, in order to revive values that are not plain objects, you have to consider one of the following:

• Serialize the entire object to a string and prefix it with a type tag.
• "Guess" based on the structure of the data (for example, an array of two-member arrays)
• If the shape of the payload is fixed, based on the property name (for example, all properties calledregistry holdMap objects).

WhenJSON.parse receives a string that does not conform to the JSON grammar, it throws aSyntaxError.

Arrays and objects cannot havetrailing commas in JSON:

JSON.parse("[1, 2, 3, 4, ]");// SyntaxError: Unexpected token ] in JSON at position 13JSON.parse('{"foo": 1, }');// SyntaxError: Unexpected token } in JSON at position 12

JSON strings must be delimited by double (not single) quotes:

JSON.parse("{'foo': 1}");// SyntaxError: Unexpected token ' in JSON at position 1JSON.parse("'string'");// SyntaxError: Unexpected token ' in JSON at position 0

If you are writing JSON inside a JavaScript string literal, you should either use single quotes to delimit the JavaScript string literal, or escape the double quotes that delimit the JSON string:

JSON.parse('{"foo": 1}'); // OKJSON.parse("{\"foo\": 1}"); // OK

• Polyfill of modernJSON.parse behavior (reviver'scontext parameter) incore-js
• JSON.stringify()