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.
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.
JSON.parse(text) JSON.parse(text, reviver)
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:
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. 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.
JSON.parse("{}"); // {} JSON.parse("true"); // true JSON.parse('"foo"'); // "foo" JSON.parse('[1, 5, "false"]'); // [1, 5, "false"] JSON.parse("null"); // null
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 // ""
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:
registry
hold Map
objects).// both will throw a SyntaxError JSON.parse("[1, 2, 3, 4, ]"); JSON.parse('{"foo" : 1, }');
// will throw a SyntaxError JSON.parse("{'foo': 1}");
Specification |
---|
ECMAScript Language Specification # sec-json.parse |
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Opera | Safari | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | WebView Android | Deno | Node.js | ||
parse |
3 | 12 | 3.5 | 10.5 | 4 | 18 | 4 | 11 | 4 | 1.0 | ≤37 | 1.0 | 0.10.0 |
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse