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soul v1.0.0

A simple yet extensible mutable model library with attribute comparisons and change events.


NPM version

Soul.js is a simple yet extensible mutable model library for JavaScript. Soul.js compares attributes when setting and informs of changes via the observer pattern (a.k.a publish/subscribe) by emitting a change event with the changes. Relying on the tiny Egal.js for attribute comparisons permits comparing value objects (like Dates) along with primitives out of the box. Soul.js is most suitable for front-end user interfaces where a few observable models among otherwise functional rendering go long a way.

Soul.js's simple design also lends itself to easy subclassing and mixins. One mixin, Parseable, is included for letting you define functions for parsing individual attributes. See below for how to parse individual attributes before setting


npm install soul

Soul.js follows semantic versioning, so feel free to depend on its major version with something like >= 1.0.0 < 2 (a.k.a ^1.0.0).


The simplest way to use Soul.js is just to require soul and create a few model instances:

var Soul = require("soul")
var john = new Soul({name: "John", age: 42})

The set attributes are available as plain properties on the Soul instance: // => "John"
john.age // => 42

To then be notified of when john changes, listen to its change event:

john.on("change", function(old) {
  if ("age" in old) console.log("John aged to " + old.age)

Then, set John's attributes with Soul.prototype.set:

john.set({age: 43})

The given attributes are then compared to their current values (the given age of 43 is compared to 42 set prior, for example) and change gets triggered if any were different. The change event gets triggered only once per Soul.prototype.set call. This way you can handle multiple attribute changes together and trigger a UI rerender only once, for example.

The change event is triggered with two arguments. The first being the old attribute values for only those attributes that changed (permitting you to identify which attributes changed). The second is the attribute object that was passed to Soul.prototype.set. This could help identify when to overwrite attributes and when not to:

john.on("change", function(old, set) {
  // Set the updatedAt attribute only if it wasn't explicitly given.
  if (set.updatedAt == null) this.updatedAt = new Date

// The following call sets john.updatedAt to `new Date` in the change handler.
john.set({age: 43})

// The updatedAt attribute gets set to new Date(2015, 5, 18) and isn't
// overwritten in the `change` handler as `set.updatedAt` will be present.
john.set({age: 44, updatedAt: new Date(2015, 5, 18)})

As Soul.js relies on Egal.js to compare attribute values, Soul.js correctly identifies changes, or lack of, in Dates and other value objects.

For symmetry, there's also Soul.prototype.get, but that's really just identical to accessing the property on the object directly:

var john = new Soul({name: "John"})
john.get("name") // => "John" // => "John"

Subclassing Soul

Subclassing Soul has a few advantages over using it directly.

  • You can identify object types with the instanceof operator.
  • You can add methods to all object instances.
  • You can pre-bind event handlers.

As Soul is a regular JavaScript class (constructor and prototype), you can subclass it either with the ECMAScript 5 syntax or with the ECMAScript 6 syntax.

Here's how ECMAScript 5-compatible subclassing would look like:

var Soul = require("soul")

function Person(attrs) {, attrs)

Person.prototype = Object.create(Object.prototype, {
  constructor: {value: Person, configurable: true, writeable: true}

And ECMAScript 6 subclassing:

var Soul = require("soul")

class Person extends Soul {}

Regardless of which way you've subclassed Soul, you can then create and identify your Person instances with instanceof:

var john = new Person({name: "John", age: 42})
john instanceof Person // => true

With a subclass, you can also add methods to all Person instances at once:

Person.prototype.isSemicentennial = function() {
  return this.age >= 50

var john = new Person({name: "John", age: 42})
john.isSemicentennial() // => false

var mary = new Person({name: "Mary", age: 69})
mary.isSemicentennial() // => true

Prototype Event Handlers

As Soul.js relies on Concert.js for event functionality (i.e the change event), Soul.js supports setting event handlers on the prototype (class) once. This makes Soul.js unique among other model libraries in that you don't have to set the same event listeners every time in the class constructor during runtime.

For example, if you want all Person instances to have a updatedAt attribute tracking their last change time, you could bind the change event once:

Person.prototype.on("change", function(old) {
  this.updatedAt = new Date

Next time a new Person is created, that change handler will already be present. It won't interfere with new handlers set after instantiating either. You get the best of both worlds.

var john = new Person({name: "John", age: 42})
john.set({age: 43})
john.updatedAt // => Current time

Note that change is not triggered when creating a Soul/Person instance. If you want updatedAt to be initialized, set it in the constructor function (Person in the above subclassing example).

Parsing Attributes

Soul.js comes with a simple mixin that enables parsing of individual attributes.

Mixing in Parseable consists of passing Soul to it and getting a new class back:

var Soul = require("soul")
var Parseable = require("soul/parseable")
var ParseableSoul = Parseable(Soul)

Then subclass from ParseableSoul instead of Soul. Next, define parsing functions for individual attributes by prefixing them with parse (incl. space). To parse the name attribute, define a parse name function taking in a value and returning a new parsed value.

Using the ECMAScript 6 syntax makes it rather succinct:

var Soul = require("soul")
var Parseable = require("soul/parseable")

class Person extends Parseable(Soul) {
  "parse name"(name) {
    return name[0].toUpperCase() + name.slice(1)

Next time you construct a new Person instance or call Person.prototype.set, name gets passed to your parsing function and only then compared it the existing attribute value:

var john = new Person({name: "john"}) // => "John"
john.set({name: "john"}) // => No "change" event as "john" parses to "John" // => "John"
john.set({name: "johnny"}) // => "change" event // => "Johnny"


Soul.js is released under a Lesser GNU Affero General Public License, which in summary means:

  • You can use this program for no cost.
  • You can use this program for both personal and commercial reasons.
  • You do not have to share your own program's code which uses this program.
  • You have to share modifications (e.g. bug-fixes) you've made to this program.

For more convoluted language, see the LICENSE file.


Andri Möll typed this and the code.
Monday Calendar supported the engineering work.

If you find Soul.js needs improving, please don't hesitate to type to me now at or create an issue online.

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