JSON.parse()

text

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

reviver Optional

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.

context Optional

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 when value is an object or array. It contains the following property:

source

The original JSON string representing this value.

The Object, Array, string, number, boolean, or null value corresponding to the given JSON text.

SyntaxError

Thrown if the string to parse is not valid JSON.

JSON.parse() parses a JSON string according to the JSON 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 — see Object literal syntax vs. JSON.

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

The reviver is called with the object containing the property being processed as this (unless you define the reviver as an arrow function, in which case there's no separate this binding) and two arguments: key and value, representing the property name as a string (even for arrays) and the property value. For primitive values, an additional context parameter is passed, which contains the source text of this value. If the reviver function returns undefined (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 the reviver 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 the replacer parameter of JSON.stringify(), for arrays and objects, reviver will be last called on the root value with an empty string as the key and the root object as the value. For other valid JSON values, reviver works similarly and is called once with an empty string as the key and the value itself as the value.

If you return another value from reviver, 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) => {
  return typeof value === "object" ? undefined : value;
});

console.log(transformedObj1); // undefined

There is no way to work around this generically. You cannot specially handle the case where key 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 that reviver 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 to BigInts, or other appropriate arbitrary precision formats.

You can also use the context.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 the replacer parameter of JSON.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) ? 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 called registry hold Map objects).

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

Arrays and objects cannot have trailing commas in JSON:

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

JSON.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 1

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

• Polyfill of modern JSON.parse behavior (reviver's context parameter) in core-js
• JSON.stringify()