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6.7.0 • Public • Published

BSON parser

BSON is short for "Binary JSON," and is the binary-encoded serialization of JSON-like documents. You can learn more about it in the specification.

Table of Contents

Bugs / Feature Requests

Think you've found a bug? Want to see a new feature in bson? Please open a case in our issue management tool, JIRA:

  1. Create an account and login:
  2. Navigate to the NODE project:
  3. Click Create Issue - Please provide as much information as possible about the issue and how to reproduce it.

Bug reports in JIRA for the NODE driver project are public.


To build a new version perform the following operations:

npm install
npm run build

Node.js or Bundling Usage

When using a bundler or Node.js you can import bson using the package name:

import { BSON, EJSON, ObjectId } from 'bson';
// or:
// const { BSON, EJSON, ObjectId } = require('bson');

const bytes = BSON.serialize({ _id: new ObjectId() });
const doc = BSON.deserialize(bytes);
// {"_id":{"$oid":"..."}}

Browser Usage

If you are working directly in the browser without a bundler please use the .mjs bundle like so:

<script type="module">
  import { BSON, EJSON, ObjectId } from './lib/bson.mjs';

  const bytes = BSON.serialize({ _id: new ObjectId() });
  const doc = BSON.deserialize(bytes);
  // {"_id":{"$oid":"..."}}


npm install bson

MongoDB Node.js Driver Version Compatibility

Only the following version combinations with the MongoDB Node.js Driver are considered stable.

bson@1.x bson@4.x bson@5.x bson@6.x
mongodb@6.x N/A N/A N/A
mongodb@5.x N/A N/A N/A
mongodb@4.x N/A N/A N/A
mongodb@3.x N/A N/A N/A



API documentation


EJSON.parse(text, [options])

Param Type Default Description
text string
[options] object Optional settings
[options.relaxed] boolean true Attempt to return native JS types where possible, rather than BSON types (if true)

Parse an Extended JSON string, constructing the JavaScript value or object described by that string.


const { EJSON } = require('bson');
const text = '{ "int32": { "$numberInt": "10" } }';

// prints { int32: { [String: '10'] _bsontype: 'Int32', value: '10' } }
console.log(EJSON.parse(text, { relaxed: false }));

// prints { int32: 10 }

EJSON.stringify(value, [replacer], [space], [options])

Param Type Default Description
value object The value to convert to extended JSON
[replacer] function | array A function that alters the behavior of the stringification process, or an array of String and Number objects that serve as a whitelist for selecting/filtering the properties of the value object to be included in the JSON string. If this value is null or not provided, all properties of the object are included in the resulting JSON string
[space] string | number A String or Number object that's used to insert white space into the output JSON string for readability purposes.
[options] object Optional settings
[options.relaxed] boolean true Enabled Extended JSON's relaxed mode
[options.legacy] boolean true Output in Extended JSON v1

Converts a BSON document to an Extended JSON string, optionally replacing values if a replacer function is specified or optionally including only the specified properties if a replacer array is specified.


const { EJSON } = require('bson');
const Int32 = require('mongodb').Int32;
const doc = { int32: new Int32(10) };

// prints '{"int32":{"$numberInt":"10"}}'
console.log(EJSON.stringify(doc, { relaxed: false }));

// prints '{"int32":10}'

EJSON.serialize(bson, [options])

Param Type Description
bson object The object to serialize
[options] object Optional settings passed to the stringify function

Serializes an object to an Extended JSON string, and reparse it as a JavaScript object.

EJSON.deserialize(ejson, [options])

Param Type Description
ejson object The Extended JSON object to deserialize
[options] object Optional settings passed to the parse method

Deserializes an Extended JSON object into a plain JavaScript object with native/BSON types

Error Handling

It is our recommendation to use BSONError.isBSONError() checks on errors and to avoid relying on parsing error.message and strings in your code. We guarantee BSONError.isBSONError() checks will pass according to semver guidelines, but errors may be sub-classed or their messages may change at any time, even patch releases, as we see fit to increase the helpfulness of the errors.

Any new errors we add to the driver will directly extend an existing error class and no existing error will be moved to a different parent class outside of a major release. This means BSONError.isBSONError() will always be able to accurately capture the errors that our BSON library throws.

Hypothetical example: A collection in our Db has an issue with UTF-8 data:

let documentCount = 0;
const cursor = collection.find({}, { utf8Validation: true });
try {
  for await (const doc of cursor) documentCount += 1;
} catch (error) {
  if (BSONError.isBSONError(error)) {
    console.log(`Found the troublemaker UTF-8!: ${documentCount} ${error.message}`);
    return documentCount;
  throw error;

React Native

BSON vendors the required polyfills for TextEncoder, TextDecoder, atob, btoa imported from React Native and therefore doesn't expect users to polyfill these. One additional polyfill, crypto.getRandomValues is recommended and can be installed with the following command:

npm install --save react-native-get-random-values

The following snippet should be placed at the top of the entrypoint (by default this is the root index.js file) for React Native projects using the BSON library. These lines must be placed for any code that imports BSON.

// Required Polyfills For ReactNative
import 'react-native-get-random-values';

Finally, import the BSON library like so:

import { BSON, EJSON } from 'bson';

This will cause React Native to import the node_modules/bson/lib/bson.rn.cjs bundle (see the "react-native" setting we have in the "exports" section of our package.json.)

Technical Note about React Native module import

The "exports" definition in our package.json will result in BSON's CommonJS bundle being imported in a React Native project instead of the ES module bundle. Importing the CommonJS bundle is necessary because BSON's ES module bundle of BSON uses top-level await, which is not supported syntax in React Native's runtime hermes.


Why does undefined get converted to null?

The undefined BSON type has been deprecated for many years, so this library has dropped support for it. Use the ignoreUndefined option (for example, from the driver ) to instead remove undefined keys.

How do I add custom serialization logic?

This library looks for toBSON() functions on every path, and calls the toBSON() function to get the value to serialize.

const BSON = require('bson');

class CustomSerialize {
  toBSON() {
    return 42;

const obj = { answer: new CustomSerialize() };
// "{ answer: 42 }"



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