JSON.parse()

The JSON.parse() static method parses a JSON string, constructing the JavaScript value or object described by the string. An optional reviver function can be provided to perform a transformation on the resulting object before it is returned.

Syntax

JSON.parse(text)
JSON.parse(text, reviver)

Parameters

text
- : The string to parse as JSON. See the JSON object for a description of JSON syntax.
reviver
- : 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.

Return value

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

Exceptions

SyntaxError
- : Thrown if the string to parse is not valid JSON.

Description

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.

The reviver parameter

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. 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. To transfer large numbers without loss of precision, serialize them as strings, and revive them to BigInts, or other appropriate arbitrary precision formats.

Examples

Using JSON.parse()

JSON.parse("{}"); // {}
JSON.parse("true"); // true
JSON.parse('"foo"'); // "foo"
JSON.parse('[1, 5, "false"]'); // [1, 5, "false"]
JSON.parse("null"); // null

Using the reviver parameter

JSON.parse(
  '{"p": 5}',
  (key, value) =>
    typeof value === "number"
      ? value * 2 // return value * 2 for numbers
      : value, // return everything else unchanged
);
// { p: 10 }

JSON.parse('{"1": 1, "2": 2, "3": {"4": 4, "5": {"6": 6}} -->
}', (key, value) => {
  console.log(key);
  return value;
});
// 1
// 2
// 4
// 6
// 5
// 3
// ""

Using reviver when paired with the replacer of JSON.stringify()

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).

JSON.parse() does not allow trailing commas

// both will throw a SyntaxError
JSON.parse("[1, 2, 3, 4, ]");
JSON.parse('{"foo" : 1, }');

JSON.parse() does not allow single quotes

// will throw a SyntaxError
JSON.parse("{'foo': 1}");