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Lead Maintainer: Philip Roberts

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Part of the Ampersand.js toolkit for building clientside applications.

An observable, extensible state object with derived watchable properties.

Ampersand-state serves as a base object for ampersand-model but is useful any time you want to track complex state.

ampersand-model extends ampersand-state to include assumptions that you'd want if you're using models to model data from a REST API. But by itself ampersand-state is useful for anytime you want something to model state, that fires events for changes and lets you define and listen to derived properties.

For further explanation see the learn ampersand-state guide.


npm install ampersand-state --save

API Reference

extend AmpersandState.extend({ })

To create a State class of your own, you extend AmpersandState and provide instance properties and options for your class. Typically, this is when you'll define the properties (props, session, and derived) of your state class, and any instance methods to be attached to instances of your class.

extend correctly sets up the prototype chain, so that subclasses created with extend can be further extended as many times as you like.

Definitions like props, session, derived, etc. will be merged with superclass definitions.

var Person = AmpersandState.extend({
    props: {
        firstName: 'string',
        lastName: 'string'
    session: {
        signedIn: ['boolean', true, false],
    derived: {
        fullName: {
            deps: ['firstName', 'lastName'],
            fn: function () {
                return this.firstName + ' ' + this.lastName;

AmpersandState.extend does more than just copy attributes from one prototype to another. As such, it is incompatible with CoffeeScript's class-based extend. TypeScript users may have similar issues.

For instance, this will not work (since it never actually calls AmpersandState.extend):

// don't do this!
class Foo extends AmpersandView
         @special = options.special

constructor/initialize new AmpersandState([attrs], [options])

When creating an instance of a state object, you can pass in the initial values of the attributes which will be set on the state. Unless extraProperties is set to allow, you will need to have defined these attributes in props or session.

If you have defined an initialize function for your subclass of State, it will be invoked at creation time.

var me = new Person({
    firstName: 'Phil',
    lastName: 'Roberts'
me.firstName //=> Phil

Available options:

  • [parse] {Boolean} - whether to call the class's parse function with the initial attributes. Defaults to false.
  • [parent] {AmpersandState} - pass a reference to a state's parent to store on the state.

idAttribute state.idAttribute

The attribute that should be used as the unique id of the state. getId uses this to determine the id for use when constructing a model's url for saving to the server.

Defaults to 'id'.

var Person = AmpersandModel.extend({
    idAttribute: 'personId',
    urlRoot: '/people',
    props: {
        personId: 'number',
        name: 'string'
var me = new Person({ personId: 123 });
console.log(me.url()) //=> "/people/123"

getId state.getId()

Gets the state's ID, per idAttribute configuration. Should always be how ID is determined by other code.

namespaceAttribute state.namespaceAttribute

The property name that should be used as a namespace. Namespaces are completely optional, but exist in case you need to make an additional distinction between states, that may be of the same type, with potentially conflicting IDs but are in fact different.

Defaults to 'namespace'.

getNamespace state.getNamespace()

Get namespace of state per namespaceAttribute configuration. Should always be how namespace is determined by other code.


The property name that should be used to specify what type of state this is. This is optional, but specifying a state type types provides a standard, yet configurable way to determine what type of state it is.

Defaults to 'modelType'.

getType state.getType()

Get type of state per typeAttribute configuration. Should always be how type is determined by other code.

extraProperties AmpersandState.extend({ extraProperties: 'allow' })

Determines how properties that aren't defined in props, session or derived are handled. May be set to 'allow', 'ignore', or 'reject'.

Defaults to 'ignore'.

var StateA = AmpersandState.extend({
    extraProperties: 'allow',
var stateA = new StateA({ foo: 'bar' }); === 'bar' //=> true
var StateB = AmpersandState.extend({
    extraProperties: 'ignore',
var stateB = new StateB({ foo: 'bar' }); === undefined //=> true
var stateC = AmpersandState.extend({
    extraProperties: 'reject'
var stateC = new StateC({ foo: 'bar' })
//=> TypeError('No foo property defined on this state and extraProperties not set to "ignore" or "allow".');

collection state.collection

A reference to the collection a state is in, if in a collection.

This is used for building the default url property, etc.

Which is why you can do this:

// some ampersand-rest-collection instance
// with a `url` property
widgets.url //=> '/api/widgets'
// get a widget from our collection
var badWidget = widgets.get('47');
// Without a `collection` reference, this
// widget wouldn't know what URL to build
// when calling destroy
badWidget.destroy(); // does a DELETE /api/widgets/47

cid state.cid

A special property of states, the cid (or client id) is a unique identifier automatically assigned to all states when they are first created. Client IDs are handy when the state has not been saved to the server, and so does not yet have its true id, but still needs a unique id (for rendering in the UI, and so on).

var userA = new User();
console.log(userA.cid) //=> "state-1"
var userB = new User();
console.log(userB.cid) //=> "state-2"

isNew state.isNew()

Has this state been saved to the server yet? If the state does not yet have an id (using getId()), it is considered to be new.

escape state.escape()

Similar to get, but returns the HTML-escaped version of a state's attribute. If you're interpolating data from the state into HTML, use escape when retrieving attributes to help prevent XSS attacks.

var hacker = new PersonModel({
    name: "<script>alert('xss')</script>"
document.body.innerHTML = hacker.escape('name');

isValid state.isValid()

Check if the state is currently valid. It does this by calling the state's validate method (if you've provided one).

dataTypes AmpersandState.extend({ datatypes: myCustomTypes })

ampersand-state defines several built-in datatypes:

  • string
  • number
  • boolean
  • array
  • object
  • date
  • state
  • any

Of these, object, array, and any allow for a lot of extra flexibility.

However, sometimes it's useful to define your own custom datatypes. Doing so allows you to use them in the props below, along with all their features (like required, default, etc).

Setting type is required. A typeError will be thrown if it's missing or has not been chosen (either from default types or your custom ones).

To define a type, you generally will provide an object with 4 member functions (though only 2 are usually necessary) get, set, default, and compare.

set : function(newVal){} {type : type, val : newVal} Called on every set. Should return an object with two members: val and type. If the type value does not equal the name of the dataType you defined, a TypeError should be thrown.
compare : function(currentVal, newVal, attributeName){} boolean Called on every set. Should return true if oldVal and newVal are equal. Non-equal values will eventually trigger change events, unless the state's set (not the dataTypes's!) is called with the option {silent : true}.
onChange : function (value, previousValue, attributeName){} Called after the value changes. Useful for automatically setting up or tearing down listeners on properties.
get : function(val){} val Overrides the default getter of this type. Useful if you want to make defensive copies. For example, the date dataType returns a clone of the internally saved date to keep the internal state consistent.
default : function(){} val Returns the default value for this type.

For example, let's say your application uses a special type of date: JulianDate. You'd like to define this as a type in state, but don't want to just use any or object as the type.

To define it:

// Julian Date is a 'class' defined elsewhere:
// it has an 'equals' method and takes `{julianDays : number}` as a constructor
var Person = AmpersandState.extend({
   dataTypes : {
        julianDate : {
           // set called every time someone tried to set a property of this datatype
           set : function(newVal){
               if(newVal instanceof JulianDate){
                   return {
                       val : newVal,
                       type : 'julianDate'
                   // try to parse it from passed in value:
                   var newDate = new JulianDate(newVal);
                   return {
                       val : newDate,
                       type : 'julianDate'
                   // return the value with what we think its type is
                   return {
                       val : newVal,
                       type : typeof newVal
           compare : function(currentVal, newVal, attributeName){
               return currentVal.equals(newVal);
   props : {
       bornOn : 'julianDate',
       retiresOn : {
           type : 'julianDate',
           required : 'true',
           default : function(){
                  // assuming an 'add' function on julian date which returns a new JulianDate
                  return this.bornOn.add('60','years');
var person = new Person({ bornOn : new JulianDate({julianDays : 1000}); }
// this will also work and will build a new JulianDate
var person = new Person({bornOn : {julianDays : 1000}});
// will construct a new julian date for us
// and will also trigger a change event
person.bornOn = {julianDays : 1001};
// but this will not trigger a change event since the equals method would return true
person.bornOn = {julianDays : 1001};

props AmpersandState.extend({ props: { name: 'string' } })

The props object describes the observable properties of your state class.

Always pass props to extend! Never set it on an instance, as it won't define new properties.

Properties can be defined in three different ways:

  1. As a string with the expected dataType
  • One of string, number, boolean, array, object, date, or any. (Example: name: 'string'.)
  • Can also be set to the name of a custom dataTypes, if the class defines any.
  1. An array of [dataType, required, default]
  2. An object { type: 'string', required: true, default: '' , values: [], allowNull: false, setOnce: false }
  • default: the value that the property will be set to if it is undefined (either by not being set during initialization, or by being explicit set to undefined).
  • If required is true:
    • If the property has a default, it will start with that value, and revert to it after a call to unset(propertyName)
    • Otherwise, calls to unset(propertyName) throw an error
  • If values array is passed, then you'll be able to change the property to one of those values only.
  • If setOnce is true, then you'll be able to set property only once.
    • If the property has a default, and you don't set the value initially, the property will be permanently set to the default value.
    • Otherwise, if you don't set the value initially, it can be set later, but only once.
  • If test function is passed, then a negative validation test will be executed every time this property is about to be set.
    • If the validation passes, the function must return false to tell State to go ahead and set the value.
    • Otherwise, it should return a string with the error message describing the validation failure. (In this case, State will throw a TypeError with "Property '<property>' failed validation with error: <errorMessage>".)

Trying to set a property to an invalid type will throw an error.

See get and set for more information about getting and setting properties.

var Person = AmpersandState.extend({
    props: {
        name: 'string',
        age: 'number',
        paying: ['boolean', true, false], // required attribute, defaulted to false
        type: {
            type: 'string',
            values: ['regular-hero', 'super-hero', 'mega-hero']
        numberOfChildren: {
            type: 'number',
            test: function(value){
                if (value < 0) {
                    return "Must be a positive number";
                return false;

reserved prop, session names

The following should not be used as prop names for any state object. This of course includes things based on state such as ampersand-model and ampersand-view.

If you're consuming an API you don't control, you can rename keys by overwriting parse and serialize methods.

bind, changedAttributes, cid, clear, collection, constructor, createEmitter, escape, extraProperties, get, getAttributes, getId, getNamespace, getType, hasChanged, idAttribute, initialize, isNew, isValid, listenTo, listenToAndRun, listenToOnce, namespaceAttribute, off, on, once, parent, parse, previous, previousAttributes, serialize, set, stopListening, toJSON, toggle, trigger, typeAttribute, unbind, unset, url

defaulting to objects/arrays

You will get an error if you try to set the default of any property as either an object or array. This is because those two dataTypes are mutable and passed by reference. (Thus, if you did set a property's default to ['a','b'], it would return the same array on every new instantiation of the state.)

Instead, if you want a property to default to an array or an object, just set default to a function, like this:

    props: {
        checkpoints: {
            type: 'array',
            default: function () { return []; }

NOTE: Both array and object have this behavior built-in: they default to empty versions of themselves. You would only need to do this if you wanted to default to an array/object that wasn't empty.

session AmpersandState.extend({ session: { name: 'string' } })

Session properties are defined and work in exactly the same way as props, but generally only exist for the lifetime of the page. They would not typically be persisted to the server, and are not returned by calls to toJSON() or serialize().

var Person = AmpersandState.extend({
    props: {
        name: 'string',
    session: {
        isLoggedIn: 'boolean'

derived AmpersandState.extend({ derived: { derivedProperties }})

Derived properties (also known as computed properties) are properties of the state object that depend on other properties to determine their value. They may depend on properties defined in props, session, or even derived—as well as the same from state props or children.

Best demonstrated with an example:

var Address = AmpersandState.extend({
  props: {
    'street': 'string',
    'city': 'string',
    'region': 'string',
    'postcode': 'string'
var Person = AmpersandState.extend({
    props: {
        firstName: 'string',
        lastName: 'string',
        address: 'state'
    derived: {
        fullName: {
            deps: ['firstName', 'lastName'],
            fn: function () {
                return this.firstName + ' ' + this.lastName;
        mailingAddress: {
            deps: ['address.street', '', 'address.region', 'address.postcode'],
            fn: function () {
                var self = this;
                return ['street','city','region','postcode'].map(function (prop) {
                    var val = self.address[prop];
                    if (!val) return val;
                    return (prop === 'street' || prop === 'city') ? val + ',' : val;
                }).filter(function (val) {
                    return !!val;
                }).join(' ');
var person = new Person({
    firstName: 'Phil',
    lastName: 'Roberts',
    address: new Address({
        street: '123 Main St',
        city: 'Anyplace',
        region: 'BC',
        postcode: 'V6A 2S5'
console.log(person.fullName) //=> "Phil Roberts"
console.log(person.mailingAddress) //=> "123 Main St, Anyplace, BC V6A 2S5"
person.firstName = 'Bob';
person.address.street = '321 St. Charles Pl'
console.log(person.fullName) //=> "Bob Roberts"
console.log(person.mailingAddress) //=> "321 St. Charles Pl, Anyplace, BC V6A 2S5"

See working example at RequireBin

Each derived property is defined as an object with the following properties:

  • deps {Array} - An array of property names which the derived property depends on.
  • fn {Function} - A function which returns the value of the computed property. It is called in the context of the current object, so that this is set correctly.
  • cache {Boolean} - Whether to cache the property. Uncached properties are computed every time they are accessed. Useful if it depends on the current time for example. Defaults to true.

Derived properties are retrieved and fire change events just like any other property. However, they cannot be set directly. Caching ensures that the fn function is only run when any of the dependencies change, and change events are only fired if the result of calling fn() has actually changed.

children AmpersandState.extend({ children: { profile: Profile } })

Define child state objects to attach to the object. Attributes passed to the constructor or to set() will be proxied to the children/collection. Children's change events are proxied to the parent.

var AmpersandState = require('ampersand-state');
var Hat = AmpersandState.extend({
    props: {
        color: 'string'
var Person = AmpersandState.extend({
    props: {
        name: 'string'
    children: {
        hat: Hat
var me = new Person({ name: 'Phil', hat: { color: 'red' } });
me.on('all', function (eventName) {
    console.log('Got event: ', eventName);
console.log(me.hat) //=> Hat{color: 'red'}
me.set({ hat: { color: 'green' } });
//-> "Got event: change:hat.color"
//-> "Got event: change"
console.log(me.hat) //=> Hat{color: 'green'}

NOTE: If you want to be able to swap out and get a change event from a child model, don't use children. Instead, define a prop in props of type state.

children and collections are not just a property of the parent; they're part of the parent. When you create the parent, an instance of any children or collections will be instantiated as part of instantiating the parent, whether they have any data or not.

Calling .set() on the parent with a nested object will automatically set() them on children and collections, too. This is super handy for APIs like this one that return nested JSON structures.

Also, there will be no change events triggered if you replace a child with something else after you've instantiated the parent, because it's not a true property in the props sense. If you need a prop that stores a state instance, define it as such—don't use children.

The distinction is important! Without it, the following would be problematic:

var Person = State.extend({
    props: {
        child: {
            type: 'state'
var person = new Person()
// throws type error because `{}` isn't a state object
person.child = {};
// should this work? What should happen if the `child` prop isn't defined yet?
person.set({child: {name: 'mary'}});

So, while having children in addition to props of type state may feel redundant, they both exist to help disambiguate how they're meant to be used.

collections AmpersandState.extend({ collections: { widgets: Widgets } })

Define child collection objects to attach to the object. Attributes passed to the constructor or to set() will be proxied to the collections.

NOTE: Currently, events don't automatically proxy from collections to parent. This is for efficiency reasons. But there are ongoing discussions about how to best handle this.

var State = require('ampersand-state');
var Collection = require('ampersand-collection');
var Widget = State.extend({
    props: {
        name: 'string',
        funLevel: 'number'
var WidgetCollection = Collection.extend({
    model: Widget
var Person = State.extend({
    props: {
        name: 'string'
    collections: {
        widgets: WidgetCollection
var me = new Person({
    name: 'Henrik',
    widgets: [
        { name: 'rc car', funLevel: 8 },
        { name: 'skis', funLevel: 11 }
console.log(me.widgets.length); //=> 2
console.log(me.widgets instanceof WidgetCollection); //=> true


parse is called when the state is initialized, allowing the attributes to be modified, remapped, renamed, etc., before they are actually applied to the state.

In ampersand-state, parse is only called when the state is initialized, and only if { parse: true } is passed to the constructor's options.

var Person = AmpersandState.extend({
    props: {
        id: 'number',
        name: 'string'
    parse: function (attrs) { = attrs.personID; //remap an oddly named attribute
        delete attrs.personID;
        return attrs;
var me = new Person({ personID: 123, name: 'Phil' },{ parse: true});
console.log( //=> 123
console.log(me.personID) //=> undefined

(parse is arguably more useful in ampersand-model, where data typically comes from the server.)

serialize state.serialize([options])

Serialize the state object into a plain object, ready for sending to the server (typically called via toJSON).

By default, this returns only properties defined in props, omitting properties in session and derived. To also serialize session or derived attributes, you can pass in a options object. The options object should match that accepted by .getAttributes(...).

This method will also serialize any children or collections by calling their serialize methods.

get state.get(attribute); state[attribute]; state.firstName

Get the current value of an attribute from the state object. Attributes can be accessed directly, or a call to the Backbone style get.

// these are all equivalent

Get will retrieve props, session, or derived properties all in the same way.

set state.set(attributes, [options]); state.firstName = 'Henrik';

Sets an attribute, or multiple attributes, on the state object. If any of the state object's attributes change, it will trigger a "change" event.

Change events for specific attributes are also triggered, which you can listen for as well. For example: "change:firstName" and "change:content". If the update affects any derived properties, their values will be updated and fire "change" events as well.

Attributes can be set directly, or via a call to the backbone style set (useful if you wish to update multiple attributes at once):

person.set({firstName: 'Phil', lastName: 'Roberts'});
person.set('firstName', 'Phil');
person.firstName = 'Phil';

Possible options (when using state.set()):

  • silent {Boolean} - prevents triggering of any change events as a result of the set operation.
  • unset {Boolean} - unset the attributes keyed in the attributes object instead of setting them.

NOTE: When passing an object as the attributes argument, only that object's own enumerable properties (i.e. those that can be accessed with Object.keys(object)) are read and set. This behaviour is new as of v5.0.0. (Prior version relied on to access an object's properties, both owned by that object and those inherited through the prototypal chain.)

unset state.unset(attribute|attributes[], [options])

Clear the named attribute, or an array of named attributes, from the state object. Fires a "change" event and a "change:attributeName" event unless silent is passed as an option.

If the attribute being unset is required and has a default value as defined in either props or session, it will be set to that value, otherwise it will be undefined.

// unset a single attribute
// unset multiple attributes
person.unset(['firstName', 'lastName'])

clear state.clear([options])

Clear all the attributes from the state object, by calling the unset function for each attribute, with the options provided.


toggle state.toggle('a')

Shortcut to toggle boolean properties, or to cycle through array of specified property's values. (See the values option and example, below.)

When you reach the last available value from given array, toggle will go back to the beginning and use first one.

Fires "change" events, as you would expect from set().

var Person = AmpersandState.extend({
    props: {
        active: 'boolean',
        color: {
            type: 'string',
            values: ['red', 'green', 'blue']
var me = new Person({ active: true, color: 'green' });
console.log( //=> false
console.log(me.color) //=> 'blue'
console.log(me.color) //=> 'red'

previousAttributes state.previousAttributes()

Return a copy of the object's previous attributes (the state before the last "change" event). Useful for getting a diff between versions of a state, or getting back to a valid state after an error occurs.

hasChanged state.hasChanged([attribute])

Determine if the state has been modified since the last "change" event. If an attribute name is passed, determine if that one attribute has changed.

NOTE: This will only be true if checked inside a handler while the various change events are firing. Once the change events are done, this will always return false. This has nothing to do with determining whether a property has changed since the last time it was saved to the server.

changedAttributes state.changedAttributes([objectToDiff])

Return an object containing all the attributes that have changed, or false if there are no changed attributes. Useful for determining what parts of a view need to be updated and/or what attributes need to be persisted to the server. Unset attributes will be set to undefined. You can also pass an attributes object to diff against the state, determining if there would be a change.

NOTE: When passing an attributes object to diff against, only changes to properties defined on the model will be detected. This means that changes to children or collections will not be returned as changes by this method.

NOTE: This will only return values if checked inside a handler for "change" events (i.e. while the events are firing). Once the change events are done, this will always return an empty object. This has nothing to do with determining which properties have been changed since the last time it was saved to the server.

toJSON state.toJSON()

Return a shallow copy of the state's attributes for JSON stringification. This can be used for persistence, serialization, or augmentation, before being sent to the server.

(The name of this method is a bit confusing, as it doesn't actually return a JSON string—but I'm afraid that it's the way that the JavaScript API for JSON.stringify works.)

Calls serialize to determine which values to return in the object. Will be called implicitly by JSON.stringify.

var me = new Person({ firstName: 'Phil', lastName: 'Roberts' });
me.toJSON() //=> { firstName: 'Phil', lastName: 'Roberts' }
//JSON.stringify implicitly calls toJSON:
JSON.stringify(me) //=> "{\"firstName\":\"Phil\",\"lastName\":\"Roberts\"}"

getAttributes state.getAttributes([options, raw])

Returns a shallow copy of the state's attributes while only including the types (props, session, derived) specified by the options parameter. The desired keys should be set to true on options (props, session, derived) if attributes of that type should be returned by getAttributes.

The second parameter, raw, is a boolean that specifies whether returned values should be the raw value or should instead use the getter associated with its dataType. If you are using getAttributes to pass data to a template, most of the time you will not want to use the raw parameter, since you will want to take advantage of any built-in and custom dataTypes on your state instance.

var Person = AmpersandState.extend({
  props: {
      firstName: 'string',
      lastName: 'string'
  session: {
    lastSeen: 'date',
    active: 'boolean'
  derived: {
    fullName: {
      deps: ['firstName', 'lastName'],
      fn: function () {
        return this.firstName + ' ' + this.lastName;
var me = new Person({ firstName: 'Luke', lastName: 'Karrys', active: true, lastSeen: 1428430444479 });
me.getAttributes({derived: true}) //=> { fullName: 'Luke Karrys' }
me.getAttributes({session: true}) //=> { active: true, lastSeen: Tue Apr 07 2015 11:14:04 GMT-0700 (MST) }
me.getAttributes({session: true}, true) //=> { active: true, lastSeen: 1428430444479 }
  props: true,
  session: true,
  derived: true
}) //=> { firstName: 'Luke', lastName: 'Karrys', active: true, lastSeen: Tue Apr 07 2015 11:14:04 GMT-0700 (MST), fullName: 'Luke Karrys' }






  • 5.0.2 - use lodash/xzy pkg requires (@samhashemi)