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Tiny framework for building models with the event sourcing pattern (events and snapshots) that works in Node.js and the browser. Now written in Typescript and published in ESM and CJS.

Unlike Active Record where entity state is persisted on a one-model-per row database format, event sourcing stores all the changes (events) to the entity, rather than just its current state. The current state is derived by loading all events, or a latest snapshot plus subsequent events, and replaying them against the entity.

One large benefit of event sourcing: your data is your audit trail. Zero discrepancies.

For example usage, see the examples and tests.

Sourced makes no assumptions about how you store your events and snapshots. The library is small and tight with only the required functionality to define entities and their logic, enqueue and emit events, and track event state to later be persisted. To actually persist, use one of the following libraries or implement your own:

Partially Implemented (only get, commit - no getAll, commitAll - ok for many use cases - PRs welcome!)


const Entity = require('sourced').Entity;

class Market extends Entity {
  constructor(snapshot, events) {
    this.orders = [];
    this.price = 0;

    this.rehydrate(snapshot, events)

  init(param) { =;
    this.digest('init', param);
    this.emit('initialized', param, this);

  createOrder(param) {
    var total = 0;
    this.orders.forEach(function (order) {
      total += order.price;
    this.price = total / this.orders.length;
    this.digest('createOrder', param);
    this.emit('done', param, this);




{Function} Entity


{Function} EntityError


{Function} Entity

Kind: global class Requires: module:eventemitter3, module:debug, module:lodash.clonedeep License: MIT

new Entity([snapshot], [events])

Creates an event-sourced Entity.

Param Type Description
[snapshot] Object A previously saved snapshot of an entity.
[events] Array An array of events to apply on instantiation.


Wrapper around the EventEmitter.emit method that adds a condition so events are not fired during replay.

Kind: instance method of Entity


Add events to the queue of events to emit. If called during replay, this method does nothing.

Kind: instance method of Entity

entity.digest(method, data)

Digest a command with given data.This is called whenever you want to record a command into the events for the entity. If called during replay, this method does nothing.

Kind: instance method of Entity

Param Type Description
method String the name of the method/command you want to digest.
data Object the data that should be passed to the replay.


Merge a snapshot onto the entity.

For every property passed in the snapshot, the value is deep-cloned and then merged into the instance through mergeProperty. See mergeProperty for details.

Kind: instance method of Entity See: Entity.prototype.mergeProperty

Param Type Description
snapshot Object snapshot object.

entity.mergeProperty(name, value)

Merge a property onto the instance.

Given a name and a value, mergeProperty checks first attempt to find the property in the mergeProperties map using the constructor name as key. If it is found and it is a function, the function is called. If it is NOT found we check if the property is an object. If so, we merge. If not, we simply assign the passed value to the instance.

Kind: instance method of Entity See

  • mergeProperties
  • Entity.mergeProperty
Param Type Description
name String the name of the property being merged.
value Object the value of the property being merged.


Replay an array of events onto the instance.

The goal here is to allow application of events without emitting, enqueueing nor digesting the replayed events. This is done by setting this.replaying to true which emit, enqueue and digest check for.

If the method in the event being replayed exists in the instance, we call the mathod with the data in the event and set the version of the instance to the version of the event. If the method is not found, we attempt to parse the constructor to give a more descriptive error.

Kind: instance method of Entity

Param Type Description
events Array an array of events to be replayed.

entity.state() ⇒ Object

Return a clone of current state of the entity instance without event sourcing properties. Does not modify anything.

Kind: instance method of Entity

entity.snapshot() ⇒ Object

Create a snapshot of the current state of the entity instance.

Here the instance's snapshotVersion property is set to the current version, then the instance is deep-cloned and the clone is trimmed of the internal sourced attributes using trimSnapshot and returned.

Kind: instance method of Entity


Remove the internal sourced properties from the passed snapshot.

Snapshots are to contain only entity data properties. This trims all other properties from the snapshot.

Kind: instance method of Entity See: Entity.prototype.snapshot

Param Type Description
snapshot Object the snapshot to be trimmed.

Entity.digestMethod(type, fn)

Helper function to automatically create a method that calls digest on the param provided. Use it to add methods that automatically call digest.

Kind: static method of Entity

Param Type Description
type Object the entity class to which the method will be added.
fn function the actual function to be added.


Entity.digestMethod(Car, function clearSettings (param) {

    const self = this;

    this.settings.get(, config) => {

      config.sources.forEach((source) => {




    return this.settings;


Entity.mergeProperty(type, name, fn)

Convenience function to store references to functions that should be run when mergin a particular property.

Kind: static method of Entity See: mergeProperties

Param Type Description
type Object the entity class to which the property->fn belongs to.
name String the name of the property that holds the fn.
fn function the function to execute when merging the property.


function Wheel (status) {
   this.status = status;

 Wheel.prototype.go = function () {
   this.status = 'going';

 function Car () { = null;
   this.wheel = new Wheel(); // for instantiating our default wheel, when we first 'new' up a Car

   Entity.apply(this, arguments);

 util.inherits(Car, Entity);

 Entity.mergeProperty(Car, 'interests', function (obj) {
   this.wheel = new Wheel(); // for instantiating our wheel from saved values in a database

eventsToEmit : Array


Kind: global variable Todo

  • [ ] discuss the use of this so it can be documented better.

newEvents : Array


Kind: global variable Todo

  • [ ] discuss the use of this so it can be documented better.

replaying : Boolean

Boolean to prevent emit, enqueue and digest from running during replay.

Kind: global variable

snapshotVersion : Number

Holds the version of the latest snapshot for the entity.

Kind: global variable

timestamp : Number

Holds the event's timestamp in the entity.

Kind: global variable

version : Number

Holds the current version of the entity.

Kind: global variable


mergeProperties holds a map of entity types to properties.

Kind: global variable See

  • Entity.mergeProperty
  • Entity.prototype.mergeProperty


{Function} EntityError

Kind: global class

new EntityError(msg, [constr])

Extending native Error.

Param Type Default Description
msg String The error message.
[constr] Object this The constructor or instance.

In the browser

Sourced works just the same in the browser as in Node.js, though you'll want to use it with a browser based repo, such as sourced-repo-svelte-local-storage-store.

You can see an example event sourced SvelteKit application here: CloudNativeEntrepreneur/sveltekit-eventsourced-funnel

Upgrading from V2 to v3

To upgrade from v2, to v3, update your imports of SourcedEntity to Entity.


import { SourcedEntity as Entity } from 'sourced'

class YourAggregate extends Entity {


import { Entity } from 'sourced'

class YourAggregate extends Entity {
// ...

If you are using classical prototypical inheritance, then you were using the Entity export - this was actually a proxy constructor that allowed the classical syntax to work with ES6 classes. This is now named EntityProxy.


const Entity = require('sourced').Entity


const Entity = require('sourced').EntityProxy

See examples directory for an examply of using the EntityProxy - run it with npm ci && npm run build && node examples/auth/example.js


sourced v4

  • Typescript rewrite - thanks to work on this at by @alexander.zeitler and @stephanknull, pulled in by @patrickleet
  • BREAKING CHANGE: now publishes ESM and CJS modules at /dist/esm and /dist/cjs

sourced v3

  • Browser support by using eventemitter3.EventEmitter instead of Node events.EventEmitter by @patrickleet
  • BREAKING CHANGE: Entity is now EntityProxy. SourcedEntity is now Entity

sourced v2

  • updated to ES6 syntax by @patrickleet
  • SourcedEntity an ES6 implementation of Entity added.
  • Entity is now just a proxy to the new SourcedEntity ES6 class.
  • Deprecation notice: SourcedEntity will become Entity in the next major version, and Entity, which is now a proxy, left for backwards compatibility

sourced v1

  • original lib by @mateodelnorte - battle tested in financial systems that tranact billions of dollars.

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  • mateodelnorte
  • patrickleet