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A super light and fast circular JSON parser.


Version published
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increased by5.73%

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Package description

What is flatted?

The flatted npm package is a library for encoding and decoding JavaScript objects into a flat JSON format. It is particularly useful for handling circular references and complex data structures that JSON.stringify cannot handle by default.

What are flatted's main functionalities?

Encoding objects with circular references

Flatted allows you to encode objects that have circular references, which would throw an error with JSON.stringify. For example, if you have an object 'circularObj' that references itself, you can use flatted.stringify(circularObj) to get a string representation of the object.

{"encoded": "flatted.stringify(circularObj)"}

Decoding flatted strings back to objects

After encoding an object with circular references using flatted, you can decode the resulting string back into an object with flatted.parse(flattedString). This will restore the circular references and give you back the original object structure.

{"decoded": "flatted.parse(flattedString)"}

Other packages similar to flatted




Downloads Coverage Status Build Status License: ISC WebReflection status

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A super light (0.5K) and fast circular JSON parser, directly from the creator of CircularJSON.

Available also for PHP.

Available also for Python.

Announcement 📣

There is a standard approach to recursion and more data-types than what JSON allows, and it's part of the Structured Clone polyfill.

Beside acting as a polyfill, its @ungap/structured-clone/json export provides both stringify and parse, and it's been tested for being faster than flatted, but its produced output is also smaller than flatted in general.

The @ungap/structured-clone module is, in short, a drop in replacement for flatted, but it's not compatible with flatted specialized syntax.

However, if recursion, as well as more data-types, are what you are after, or interesting for your projects/use cases, consider switching to this new module whenever you can 👍

npm i flatted

Usable via CDN or as regular module.

// ESM
import {parse, stringify, toJSON, fromJSON} from 'flatted';

// CJS
const {parse, stringify, toJSON, fromJSON} = require('flatted');

const a = [{}];
a[0].a = a;

stringify(a); // [["1","0"],{"a":"0"}]

toJSON and fromJSON

If you'd like to implicitly survive JSON serialization, these two helpers helps:

import {toJSON, fromJSON} from 'flatted';

class RecursiveMap extends Map {
  static fromJSON(any) {
    return new this(fromJSON(any));
  toJSON() {
    return toJSON([...this.entries()]);

const recursive = new RecursiveMap;
const same = {};
same.same = same;
recursive.set('same', same);

const asString = JSON.stringify(recursive);
const asMap = RecursiveMap.fromJSON(JSON.parse(asString));
asMap.get('same') === asMap.get('same').same;
// true

Flatted VS JSON

As it is for every other specialized format capable of serializing and deserializing circular data, you should never JSON.parse(Flatted.stringify(data)), and you should never Flatted.parse(JSON.stringify(data)).

The only way this could work is to Flatted.parse(Flatted.stringify(data)), as it is also for CircularJSON or any other, otherwise there's no granted data integrity.

Also please note this project serializes and deserializes only data compatible with JSON, so that sockets, or anything else with internal classes different from those allowed by JSON standard, won't be serialized and unserialized as expected.

New in V1: Exact same JSON API

  • Added a reviver parameter to .parse(string, reviver) and revive your own objects.
  • Added a replacer and a space parameter to .stringify(object, replacer, space) for feature parity with JSON signature.


All ECMAScript engines compatible with Map, Set, Object.keys, and Array.prototype.reduce will work, even if polyfilled.

How does it work ?

While stringifying, all Objects, including Arrays, and strings, are flattened out and replaced as unique index. *

Once parsed, all indexes will be replaced through the flattened collection.

* represented as string to avoid conflicts with numbers

// logic example
var a = [{one: 1}, {two: '2'}];
a[0].a = a;
// a is the main object, will be at index '0'
// {one: 1} is the second object, index '1'
// {two: '2'} the third, in '2', and it has a string
// which will be found at index '3'

// [["1","2"],{"one":1,"a":"0"},{"two":"3"},"2"]
// a[one,two]    {one: 1, a}    {two: '2'}  '2'



Last updated on 21 Feb 2024

Did you know?

Socket installs a GitHub app to automatically flag issues on every pull request and report the health of your dependencies. Find out what is inside your node modules and prevent malicious activity before you update the dependencies.


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