Documentation of EJSON, Meteor's JSON extension.

EJSON is an extension of JSON to support more types. It supports all JSON-safe types, as well as:

  • Date (JavaScript Date)
  • Binary (JavaScript Uint8Array or the result of EJSON.newBinary)
  • Special numbers (JavaScript NaN, Infinity, and -Infinity)
  • Regular expressions (JavaScript RegExp)
  • User-defined types (see EJSON.addType. For example, Mongo.ObjectID is implemented this way.)

All EJSON serializations are also valid JSON. For example an object with a date and a binary buffer would be serialized in EJSON as:

  "d": { "$date": 1358205756553 },
  "b": { "$binary": "c3VyZS4=" }

Meteor supports all built-in EJSON data types in publishers, method arguments and results, Mongo databases, and Session variables.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 412)

Parse a string into an EJSON value. Throws an error if the string is not valid EJSON.


str String

A string to parse into an EJSON value.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 394)

Serialize a value to a string. For EJSON values, the serialization fully represents the value. For non-EJSON values, serializes the same way as JSON.stringify.


val EJSON-able Object

A value to stringify.

options.indent Boolean, Integer, or String

Indents objects and arrays for easy readability. When true, indents by 2 spaces; when an integer, indents by that number of spaces; and when a string, uses the string as the indentation pattern.

options.canonical Boolean

When true, stringifies keys in an object in sorted order.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 371)

Deserialize an EJSON value from its plain JSON representation.


val JSON-compatible Object

A value to deserialize into EJSON.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 296)

Serialize an EJSON-compatible value into its plain JSON representation.


val EJSON-able Object

A value to serialize to plain JSON.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 443)

Return true if a and b are equal to each other. Return false otherwise. Uses the equals method on a if present, otherwise performs a deep comparison.


a EJSON-able Object
b EJSON-able Object


keyOrderSensitive Boolean

Compare in key sensitive order, if supported by the JavaScript implementation. For example, {a: 1, b: 2} is equal to {b: 2, a: 1} only when keyOrderSensitive is false. The default is false.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 557)

Return a deep copy of val.


val EJSON-able Object

A value to copy.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 621)

Allocate a new buffer of binary data that EJSON can serialize.


size Number

The number of bytes of binary data to allocate.

Buffers of binary data are represented by Uint8Array instances on JavaScript platforms that support them. On implementations of JavaScript that do not support Uint8Array, binary data buffers are represented by standard arrays containing numbers ranging from 0 to 255, and the $Uint8ArrayPolyfill key set to true.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 425)

Returns true if x is a buffer of binary data, as returned from EJSON.newBinary.


x Object

The variable to check.

import { EJSON } from 'meteor/ejson'
(ejson/ejson.js, line 90)

Add a custom datatype to EJSON.


name String

A tag for your custom type; must be unique among custom data types defined in your project, and must match the result of your type's typeName method.

factory Function

A function that deserializes a JSON-compatible value into an instance of your type. This should match the serialization performed by your type's toJSONValue method.

The factory function passed to the EJSON.addType method should create an instance of our custom type and initialize it with values from an object passed as the first argument of the factory function. Here is an example:

class Distance {
  constructor(value, unit) {
    this.value = value;
    this.unit = unit;

  // Convert our type to JSON.
  toJSONValue() {
    return {
      value: this.value,
      unit: this.unit

  // Unique type name.
  typeName() {
    return 'Distance';

EJSON.addType('Distance', function fromJSONValue(json) {
  return new Distance(json.value, json.unit);

EJSON.stringify(new Distance(10, 'm'));
// Returns '{"$type":"Distance","$value":{"value":10,"unit":"m"}}'

When you add a type to EJSON, Meteor will be able to use that type in:

  • publishing objects of your type if you pass them to publish handlers.
  • allowing your type in the return values or arguments to methods.
  • storing your type client-side in Minimongo.
  • allowing your type in Session variables.

Instances of your type must implement typeName and toJSONValue methods, and may implement clone and equals methods if the default implementations are not sufficient.

Return the tag used to identify this type. This must match the tag used to register this type with EJSON.addType.

Serialize this instance into a JSON-compatible value.

For example, the toJSONValue method for Mongo.ObjectID could be:

function () {
  return this.toHexString();

Return a value r such that this.equals(r) is true, and modifications to r do not affect this and vice versa.

If your type does not have a clone method, EJSON.clone will use toJSONValue and the factory instead.

Return true if other has a value equal to this; false otherwise.


other Object

Another object to compare this to.

The equals method should define an equivalence relation. It should have the following properties:

  • Reflexivity - for any instance a: a.equals(a) must be true.
  • Symmetry - for any two instances a and b: a.equals(b) if and only if b.equals(a).
  • Transitivity - for any three instances a, b, and c: a.equals(b) and b.equals(c) implies a.equals(c).

If your type does not have an equals method, EJSON.equals will compare the result of calling toJSONValue instead.

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