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A powerful, pragmatic implementation of the observer pattern for javascript objects and arrays.


A powerful, pragmatic implementation of the observer pattern for javascript objects and arrays.


var observe = require('observe')
var object = {a:1, b:{}, c:[]}
var observer = observe(object)
observer.on('change', function(change) {
   if([0] === 'a') {
      console.log("My 'a' property changed to: "+observer.subject.a + '.')
   } else if([0] === 'b' &&[1] === 'x') {
      console.log("FINALLY someone sets my b.x property!")
   } else if([0] === 'c' && change.type === 'added') {
      var s = change.count>1 ? 's' : '' // plural 
      console.log("My c property got "+change.count+" new value"+s+""+observer.subject.c.slice(change.index, change.index+change.count) +'.')
   } else if([0] === 'c' && change.type === 'removed') {
      var s = change.count>1 ? 's' : '' // plural 
      console.log("Someone took "+change.removed+" from c!")
   } else {
      console.log("Well i just don't know *what's* going on with "'.') +".")
observer.set('a', 2)             // prints "My 'a' property changed to: 2." 
observer.set('b.x', 'hi')        // prints "FINALLY someone sets my b.x property!" 
observer.get('c').push(3, 4)     // prints "My c property got 2 new values: 3,4." 
observer.get('c').splice(0,1)    // prints "Someone took 3 from c!" 
observer.set('b.y', 'ho')        // prints "Well i just don't know *what's* going on with b.y." 
observer.get('c').append([5,6,7])// prints "My c property got 3 new values: 5,6,7." 

Why not use Object.observe?

Object.observe is fantastic - if you live in the future! Attempts at polyfilling this simply aren't practical, because they have to use polling. Since this won't be practically usable until ECMAScript 7 is widespread, this module will do what you need almost as elegantly. Also, Object.observe doesn't have the data feature (see below).


npm install observe

or download the built package generatedBuilds/observe.umd.js from the repository and include it in a <script> tag.


var Future = require('observe')  // node-js and webpack 
define(['observe'], function(observe) { ... } // amd 
observe; // defines proto globally 
       // if you really  want to shun module-based design 

var observer = observe(obj) - returns an observer object, which is an instance of node.js's EventEmitter. The obj argument can be any Object or Array. Note that even though this uses node.js's EventEmitter, it still works in the browser.

observer.subject - the object being observed (same as the obj passed in)

observer.get(property) - Returns a new observer for a property within the observer's subject. Any changes done to the returned observer will trigger events on the calling observer, but you can also set up an event listener on the returned observer.

  • property - The propety to get an observer for, in dot notation (see below).

observer.set(property, value) - Sets a value on the observer's subject and emits a "set" change event.

  • property - The propety to set, in dot notation (see below).
  • value - The value to set on that property.

Standard Array Mutator Methods

All the standard Array mutator methods are supported by observe:

observer.splice(...) - Can emit a "removed" change event, then an "added" change event (only emits both if values are both added and removed).

observer.push(...) - Also emits an `"added" change event.

observer.pop() - Emits a "removed" change event.

observer.shift() - Emits a "removed" change event.

observer.unshift(...) - Emits an "added" change event.

observer.sort(...) - Emits a "set" change event.

observer.reverse() - Emits a "set" change event.

Other methods

observer.append(...) - Slightly optimized shorthand for observer.splice(observer.subject.length,0,...). - Returns a new observer that will include the passed value as the data property in change events caused by that observer. This can be useful if you need to ignore a change event in certain handlers but not other, or if you want some way to know when an action caused by the change has been completed, or if you just need to pass some additional information. For example:

var observer = observer({})
var ignoreA = {}
observer.on('change', function(change) {
   if( !== ignoreA) {
var ignoreB = {}
observer.on('change', function(change) { // somewhere else... 
   if( !== ignoreB) {
})'a', 1)  // prints "B", but not "A" 

observer.union(collapse) - Returns an observer object for which any property added via set, push, splice, or append joins an internal observee together with this observee, so that the internal observee and the containing observee will both send 'change' events appropriately.

  • collapse - (default: false) If true, any property added will be set to the subject of the value added (so that value won't be an observee anymore). Note: only use collapse:true if the observees you're unioning isn't actually an object that inherits from an observee - any instance methods on the observee that come from child classes won't be accessible anymore.


var x = observe({a:5})
var b = observe({})
x.subject.a === 5    ;; true
b.union(true).set('x', x)
b.subject.x.a === 5            ;; true
b.subject.x.subject.a === 5    ;; false

Dot notation

The properties passed into get and set are passed in dot notation. Array indexes can also be passed with dot notation. That means that passing "a.b.c" refers to observer.subject.a.b.c and "a.3.x" refers to observer.subject.a[3].x.

Change Event

The change event comes through whenever an observer is used to set, push, splice, or append on its subject. Listen to the change event using observer.on or any of the other standard EventEmitter methods. The event has the following properties:

  • type - Either "set" (for value changes), "added" (for values added to an array), or "removed" (for values removed from an array).
  • property - An array where each element of the array is one part of the path of the property being changed. For example for "a.b.c", the property parameter will hold ['a','b','c'].
  • index - The array index at which values were added or removed. Only exists for "added" and "removed" events.
  • count - The number of elements added. Only exists for "added" events.
  • removed - The list of values removed from an array. Only exists for type="removed" events.
  • data - The value set with the data() method (see above).

Inheriting from observe

So while observe is usually used as a function, it's actually an object constructor. You can inherit from observe just like you might inherit from any other object in javascript. Instances created from a constructor who's parent is observe will have all the methods above. For example:

var SpecialObserver = function() {
    observe.apply(this,arguments) // superclass constructor call 
SpecialObserver.prototype = observe({})
SpecialObserver.prototype.specialMethod = function() {
   console.log("Hi! My prototype says I'm special! My x is "+this.subject.x)
var s = new SpecialObserver({x:1})
s.set('x', 2)
s.specialMethod() // prints ""Hi! My prototype says I'm special! My x is 2" 

Or if you're using proto:

var SpecialObserver2 = proto(observe, function() {
   this.specialMethod = function() {
      console.log("Hi! My prototype says I'm special! My x is "+this.subject.x)
var s2 = SpecialObserver2({x:1})
s.specialMethod() // same thing as above 

Note that if you do this, observer.union(true) will not add objects in such a way that preserves the special methods. You'll need to use observer.union(false) (or simply leave out the argument) and deal with the double subject in there.


  • 1.3.6 - Fixing bug where ObserveeChild was throwing an exception after being removed from its parent
  • 1.3.5 - Fixing bug where ObserveeChild change wasn't getting triggered for arrays after elements have been shifted via a splice
  • 1.3.4 - Fixing bug introduced by 1.3.3
  • 1.3.3 - Fixing bug where ObserveeChild objects pointing to an array element stopped getting the correct events after an element was inserted in front of it
  • 1.3.2 - Fixing bug where an exception was thrown if an ObserveeChild was removed, and then a sibling ObserveeChild was set
  • 1.3.1 - Fixing bug where an exception was thrown if an ObserveeChild was at least a grandchild property of a property that got removed
  • 1.3.0 - Allowing ChildObservees to get direct changes to their subject, as long as they're not a unioned observee
  • 1.2.1 - Adding support for sort
  • 1.2.0 - Adding support for pop, unshift, shift, and reverse
  • 1.1.3 - fixing issues with the demo code in the readme and fixed a bug where push wasn't able to push multiple items -
  • 1.1.2 - fixing bug where the change event data/id wasn't coming through for child observees
  • 1.1.1 - fixing bug where change event wasn't properly being called for a pull or splice call on a child observee
  • 1.1.0
    • Fixing bug: id wasn't working right when chained after a get
    • Changing method name, and event property name, from 'id' to 'data'. The method 'id' and change event property 'id' can still be accessed, tho that name is deprecated now.
  • 1.0.3 - Fixing bug: inner unioned objects weren't getting their change event called when a set was done on their container
  • 1.0.2 - Fixing bug: splice not returning removed values for ObserveeChild
  • 1.0.0 - Initial commit - code transferred from private project.

How to Contribute!

Anything helps:

  • Creating issues (aka tickets/bugs/etc). Please feel free to use issues to report bugs, request features, and discuss changes
  • Updating the documentation: ie this readme file. Be bold! Help create amazing documentation!
  • Submitting pull requests.

How to submit pull requests:

  1. Please create an issue and get my input before spending too much time creating a feature. Work with me to ensure your feature or addition is optimal and fits with the purpose of the project.
  2. Fork the repository
  3. clone your forked repo onto your machine and run npm install at its root
  4. If you're gonna work on multiple separate things, its best to create a separate branch for each of them
  5. edit!
  6. If it's a code change, please add to the unit tests (at test/observer.tests.js) to verify that your change
  7. When you're done, run the unit tests and ensure they all pass
  8. Commit and push your changes
  9. Submit a pull request:


Released under the MIT license: