JSON.stringify()

The JSON.stringify() method converts a JavaScript object or value to a JSON string, optionally replacing values if a replacer function is specified or optionally including only the specified properties if a replacer array is specified.

value

The value to convert to a JSON string.

replacer Optional

A function that alters the behavior of the stringification process, or an array of strings or numbers naming properties of value that should be included in the output. If replacer is null or not provided, all properties of the object are included in the resulting JSON string.

space Optional

A String or Number object that's used to insert white space (including indentation, line break characters, etc.) into the output JSON string for readability purposes.

If this is a Number, it indicates the number of space characters to use as white space for indenting purposes; this number is capped at 10 (if it is greater, the value is just 10). Values less than 1 indicate that no space should be used.

If this is a String, the string (or the first 10 characters of the string, if it's longer than that) is used as white space.

If this parameter is not provided (or is null), no white space is used.

A JSON string representing the given value, or undefined.

• Throws a TypeError ("cyclic object value") exception when a circular reference is found.
• Throws a TypeError ("BigInt value can't be serialized in JSON") when trying to stringify a BigInt value.

JSON.stringify() converts a value to JSON notation representing it:

• If the value has a toJSON() method, it's responsible to define what data will be serialized.
• Boolean, Number, and String objects are converted to the corresponding primitive values during stringification, in accord with the traditional conversion semantics.
• undefined, Functions, and Symbols are not valid JSON values. If any such values are encountered during conversion they are either omitted (when found in an object) or changed to null (when found in an array). JSON.stringify() can return undefined when passing in "pure" values like JSON.stringify(function() {}) or JSON.stringify(undefined).
• All Symbol-keyed properties will be completely ignored, even when using the replacer function.
• The instances of Date implement the toJSON() function by returning a string (the same as date.toISOString()). Thus, they are treated as strings.
• The numbers Infinity and NaN, as well as the value null, are all considered null.
• All the other Object instances (including Map, Set, WeakMap, and WeakSet) will have only their enumerable properties serialized.

The replacer parameter can be either a function or an array.

As a function, it takes two parameters: the key and the value being stringified. The object in which the key was found is provided as the replacer's this parameter.

Initially, the replacer function is called with an empty string as key representing the object being stringified. It is then called for each property on the object or array being stringified.

It should return the value that should be added to the JSON string, as follows:

• If you return a Number, String, Boolean, or null, the stringified version of that value is used as the property's value.
• If you return a Function, Symbol, or undefined, the property is not included in the output.
• If you return any other object, the object is recursively stringified, calling the replacer function on each property.

Example replacer, as a function

function replacer(key, value) {
  // Filtering out properties
  if (typeof value === 'string') {
    return undefined;
  }
  return value;
}

var foo = {foundation: 'Mozilla', model: 'box', week: 45, transport: 'car', month: 7};
JSON.stringify(foo, replacer);
// '{"week":45,"month":7}'

Example replacer, as an array

If replacer is an array, the array's values indicate the names of the properties in the object that should be included in the resulting JSON string.

JSON.stringify(foo, ['week', 'month']);
// '{"week":45,"month":7}', only keep "week" and "month" properties

The space argument may be used to control spacing in the final string.

• If it is a number, successive levels in the stringification will each be indented by this many space characters (up to 10).
• If it is a string, successive levels will be indented by this string (or the first ten characters of it).

JSON.stringify({ a: 2 }, null, ' ');
// '{
//  "a": 2
// }'

Using a tab character mimics standard pretty-print appearance:

JSON.stringify({ uno: 1, dos: 2 }, null, '\t');
// returns the string:
// '{
//     "uno": 1,
//     "dos": 2
// }'

If an object being stringified has a property named toJSON whose value is a function, then the toJSON() method customizes JSON stringification behavior: instead of the object being serialized, the value returned by the toJSON() method when called will be serialized. JSON.stringify() calls toJSON with one parameter:

• if this object is a property value, the property name
• if it is in an array, the index in the array, as a string
• an empty string if JSON.stringify() was directly called on this object

For example:

var obj = {
    data: 'data',

    toJSON (key) {
        if (key)
            return `Now I am a nested object under key '${key}'`;
        else
            return this;
    }
};

JSON.stringify(obj);
// '{"data":"data"}'

JSON.stringify({ obj }); // Shorthand property names (ES2015).
// '{"obj":"Now I am a nested object under key 'obj'"}'

JSON.stringify([ obj ]);
// '["Now I am a nested object under key '0'"]'

Note that since the JSON format doesn't support object references (although an IETF draft exists), a TypeError will be thrown if one attempts to encode an object with circular references.

const circularReference = {};
circularReference.myself = circularReference;

// Serializing circular references throws "TypeError: cyclic object value"
JSON.stringify(circularReference);

To serialize circular references you can use a library that supports them (e.g. cycle.js by Douglas Crockford) or implement a solution by yourself, which will require finding and replacing (or removing) the cyclic references by serializable values.

Historically, JSON was not a completely strict subset of JavaScript. The literal code points U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR could appear literally in string literals and property names in JSON text. But they could not appear literally in similar context in JavaScript text, only using Unicode escapes as \u2028 and \u2029. This recently changed: now both code points may appear literally in strings in JSON and JavaScript both.

Therefore, if compatibility with older JavaScript engines is required, it is perilous to directly substitute the string returned by JSON.stringify into a JavaScript string to be passed to eval or new Function or as part of a JSONP URL, and the following utility can be used:

function jsFriendlyJSONStringify (s) {
    return JSON.stringify(s).
        replace(/\u2028/g, '\\u2028').
        replace(/\u2029/g, '\\u2029');
}

var s = {
    a: String.fromCharCode(0x2028),
    b: String.fromCharCode(0x2029)
};
try {
    eval('(' + JSON.stringify(s) + ')');
} catch (e) {
    console.log(e); // "SyntaxError: unterminated string literal"
}

// No need for a catch
eval('(' + jsFriendlyJSONStringify(s) + ')');

// console.log in Firefox unescapes the Unicode if
//   logged to console, so we use alert
alert(jsFriendlyJSONStringify(s)); // {"a":"\u2028","b":"\u2029"}

var a = JSON.stringify({ foo: "bar", baz: "quux" })
//'{"foo":"bar","baz":"quux"}'
var b = JSON.stringify({ baz: "quux", foo: "bar" })
//'{"baz":"quux","foo":"bar"}'
console.log(a !== b) // true

// some memoization functions use JSON.stringify to serialize arguments,
// which may cause a cache miss when encountering the same object like above

In a case where you want to store an object created by your user and allowing it to be restored even after the browser has been closed, the following example is a model for the applicability of JSON.stringify():

// Creating an example of JSON
var session = {
  'screens': [],
  'state': true
};
session.screens.push({ 'name': 'screenA', 'width': 450, 'height': 250 });
session.screens.push({ 'name': 'screenB', 'width': 650, 'height': 350 });
session.screens.push({ 'name': 'screenC', 'width': 750, 'height': 120 });
session.screens.push({ 'name': 'screenD', 'width': 250, 'height': 60 });
session.screens.push({ 'name': 'screenE', 'width': 390, 'height': 120 });
session.screens.push({ 'name': 'screenF', 'width': 1240, 'height': 650 });

// Converting the JSON string with JSON.stringify()
// then saving with localStorage in the name of session
localStorage.setItem('session', JSON.stringify(session));

// Example of how to transform the String generated through
// JSON.stringify() and saved in localStorage in JSON object again
var restoredSession = JSON.parse(localStorage.getItem('session'));

// Now restoredSession variable contains the object that was saved
// in localStorage
console.log(restoredSession);

Engines implementing the well-formed JSON.stringify specification will stringify lone surrogates, any code point from U+D800 to U+DFFF, using Unicode escape sequences rather than literally. Before this change JSON.stringify would output lone surrogates if the input contained any lone surrogates; such strings could not be encoded in valid UTF-8 or UTF-16:

JSON.stringify("\uD800"); // '"�"'

But with this change JSON.stringify represents lone surrogates using JSON escape sequences that can be encoded in valid UTF-8 or UTF-16:

JSON.stringify("\uD800"); // '"\\ud800"'

This change should be backwards-compatible as long as you pass the result of JSON.stringify to APIs such as JSON.parse that will accept any valid JSON text, because they will treat Unicode escapes of lone surrogates as identical to the lone surrogates themselves. Only if you are directly interpreting the result of JSON.stringify do you need to carefully handle JSON.stringify's two possible encodings of these code points.

• Polyfill of modern JSON.stringify behavior (symbol and well-formed unicode) in core-js
• JSON.parse()