Immutable collections for JavaScript

Immutable data cannot be changed once created, leading to much simpler application development, no defensive copying, and enabling advanced memoization and change detection techniques with simple logic. Persistent data presents a mutative API which does not update the data in-place, but instead always yields new updated data.

Immutable.js provides many Persistent Immutable data structures including: List, Stack, Map, OrderedMap, Set, OrderedSet and Record.

These data structures are highly efficient on modern JavaScript VMs by using structural sharing via hash maps tries and vector tries as popularized by Clojure and Scala, minimizing the need to copy or cache data.

Immutable.js also provides a lazy Seq, allowing efficient chaining of collection methods like map and filter without creating intermediate representations. Create some Seq with Range and Repeat.

Want to hear more? Watch the presentation about Immutable.js:


Getting started

Install immutable using npm.

npm install immutable

Then require it into any module.

const { Map } = require('immutable')
const map1 = Map({ a: 1, b: 2, c: 3 })
const map2 = map1.set('b', 50)
map1.get('b') + " vs. " + map2.get('b') // 2 vs. 50


Immutable.js has no dependencies, which makes it predictable to include in a Browser.

It's highly recommended to use a module bundler like webpack,
rollup, or
browserify. The immutable npm module works
without any additional consideration. All examples throughout the documentation
will assume use of this kind of tool.

Alternatively, Immutable.js may be directly included as a script tag. Download
or link to a CDN such as CDNJS
or jsDelivr.

Use a script tag to directly add Immutable to the global scope:

<script src="immutable.min.js"></script>
  var map1 = Immutable.Map({a:1, b:2, c:3});
  var map2 = map1.set('b', 50);
  map1.get('b'); // 2
  map2.get('b'); // 50

Or use an AMD-style loader (such as RequireJS):

require(['./immutable.min.js'], function (Immutable) {
  var map1 = Immutable.Map({a:1, b:2, c:3});
  var map2 = map1.set('b', 50);
  map1.get('b'); // 2
  map2.get('b'); // 50

Flow & TypeScript

Use these Immutable collections and sequences as you would use native
collections in your Flowtype or TypeScript programs while still taking
advantage of type generics, error detection, and auto-complete in your IDE.

Installing immutable via npm brings with it type definitions for Flow (v0.55.0 or higher)
and TypeScript (v2.1.0 or higher), so you shouldn't need to do anything at all!

Using TypeScript with Immutable.js v4

Immutable.js type definitions embrace ES2015. While Immutable.js itself supports
legacy browsers and environments, its type definitions require TypeScript's 2015
lib. Include either "target": "es2015" or "lib": "es2015" in your
tsconfig.json, or provide --target es2015 or --lib es2015 to the
tsc command.

const { Map } = require("immutable");
const map1 = Map({ a: 1, b: 2, c: 3 });
const map2 = map1.set('b', 50);
map1.get('b') + " vs. " + map2.get('b') // 2 vs. 50

Using TypeScript with Immutable.js v3 and earlier:

Previous versions of Immutable.js include a reference file which you can include
via relative path to the type definitions at the top of your file.

///<reference path='./node_modules/immutable/dist/immutable.d.ts'/>
import Immutable from require('immutable');
var map1: Immutable.Map<string, number>;
map1 = Immutable.Map({a:1, b:2, c:3});
var map2 = map1.set('b', 50);
map1.get('b'); // 2
map2.get('b'); // 50

The case for Immutability

Much of what makes application development difficult is tracking mutation and
maintaining state. Developing with immutable data encourages you to think
differently about how data flows through your application.

Subscribing to data events throughout your application creates a huge overhead of
book-keeping which can hurt performance, sometimes dramatically, and creates
opportunities for areas of your application to get out of sync with each other
due to easy to make programmer error. Since immutable data never changes,
subscribing to changes throughout the model is a dead-end and new data can only
ever be passed from above.

This model of data flow aligns well with the architecture of React
and especially well with an application designed using the ideas of Flux.

When data is passed from above rather than being subscribed to, and you're only
interested in doing work when something has changed, you can use equality.

Immutable collections should be treated as values rather than objects. While
objects represent some thing which could change over time, a value represents
the state of that thing at a particular instance of time. This principle is most
important to understanding the appropriate use of immutable data. In order to
treat Immutable.js collections as values, it's important to use the function or .equals() method to determine value equality
instead of the === operator which determines object reference identity.

const { Map } = require('immutable')
const map1 = Map({ a: 1, b: 2, c: 3 })
const map2 = Map({ a: 1, b: 2, c: 3 })
map1.equals(map2) // true
map1 === map2 // false

Note: As a performance optimization Immutable.js attempts to return the existing
collection when an operation would result in an identical collection, allowing
for using === reference equality to determine if something definitely has not
changed. This can be extremely useful when used within a memoization function
which would prefer to re-run the function if a deeper equality check could
potentially be more costly. The === equality check is also used internally by and .equals() as a performance optimization.

const { Map } = require('immutable')
const map1 = Map({ a: 1, b: 2, c: 3 })
const map2 = map1.set('b', 2) // Set to same value
map1 === map2 // true

If an object is immutable, it can be "copied" simply by making another reference
to it instead of copying the entire object. Because a reference is much smaller
than the object itself, this results in memory savings and a potential boost in
execution speed for programs which rely on copies (such as an undo-stack).

const { Map } = require('immutable')
const map = Map({ a: 1, b: 2, c: 3 })
const mapCopy = map; // Look, "copies" are free!

JavaScript-first API

While Immutable.js is inspired by Clojure, Scala, Haskell and other functional
programming environments, it's designed to bring these powerful concepts to
JavaScript, and therefore has an Object-Oriented API that closely mirrors that
of ES2015 Array, Map, and Set.

The difference for the immutable collections is that methods which would mutate
the collection, like push, set, unshift or splice, instead return a new
immutable collection. Methods which return new arrays, like slice or concat,
instead return new immutable collections.

const { List } = require('immutable')
const list1 = List([ 1, 2 ]);
const list2 = list1.push(3, 4, 5);
const list3 = list2.unshift(0);
const list4 = list1.concat(list2, list3);
assert.equal(list1.size, 2);
assert.equal(list2.size, 5);
assert.equal(list3.size, 6);
assert.equal(list4.size, 13);
assert.equal(list4.get(0), 1);

Almost all of the methods on Array will be found in similar form on
Immutable.List, those of Map found on Immutable.Map, and those of Set
found on Immutable.Set, including collection operations like forEach()
and map().

const { Map } = require('immutable')
const alpha = Map({ a: 1, b: 2, c: 3, d: 4 });, k) => k.toUpperCase()).join();
// 'A,B,C,D'

Convert from raw JavaScript objects and arrays.

Designed to inter-operate with your existing JavaScript, Immutable.js
accepts plain JavaScript Arrays and Objects anywhere a method expects a

const { Map, List } = require('immutable')
const map1 = Map({ a: 1, b: 2, c: 3, d: 4 })
const map2 = Map({ c: 10, a: 20, t: 30 })
const obj = { d: 100, o: 200, g: 300 }
const map3 = map1.merge(map2, obj);
// Map { a: 20, b: 2, c: 10, d: 100, t: 30, o: 200, g: 300 }
const list1 = List([ 1, 2, 3 ])
const list2 = List([ 4, 5, 6 ])
const array = [ 7, 8, 9 ]
const list3 = list1.concat(list2, array)
// List [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

This is possible because Immutable.js can treat any JavaScript Array or Object
as a Collection. You can take advantage of this in order to get sophisticated
collection methods on JavaScript Objects, which otherwise have a very sparse
native API. Because Seq evaluates lazily and does not cache intermediate
results, these operations can be extremely efficient.

const { Seq } = require('immutable')
const myObject = { a: 1, b: 2, c: 3 }
Seq(myObject).map(x => x * x).toObject();
// { a: 1, b: 4, c: 9 }

Keep in mind, when using JS objects to construct Immutable Maps, that
JavaScript Object properties are always strings, even if written in a quote-less
shorthand, while Immutable Maps accept keys of any type.

const { fromJS } = require('immutable')

const obj = { 1: "one" }
console.log(Object.keys(obj)) // [ "1" ]
console.log(obj["1"], obj[1]) // "one", "one"

const map = fromJS(obj)
console.log(map.get("1"), map.get(1)) // "one", undefined

Property access for JavaScript Objects first converts the key to a string, but
since Immutable Map keys can be of any type the argument to get() is
not altered.

Converts back to raw JavaScript objects.

All Immutable.js Collections can be converted to plain JavaScript Arrays and
Objects shallowly with toArray() and toObject() or deeply with toJS().
All Immutable Collections also implement toJSON() allowing them to be passed
to JSON.stringify directly. They also respect the custom toJSON() methods of
nested objects.

const { Map, List } = require('immutable')
const deep = Map({ a: 1, b: 2, c: List([ 3, 4, 5 ]) })
console.log(deep.toObject()) // { a: 1, b: 2, c: List [ 3, 4, 5 ] }
console.log(deep.toArray()) // [ 1, 2, List [ 3, 4, 5 ] ]
console.log(deep.toJS()) // { a: 1, b: 2, c: [ 3, 4, 5 ] }
JSON.stringify(deep) // '{"a":1,"b":2,"c":[3,4,5]}'

Embraces ES2015

Immutable.js supports all JavaScript environments, including legacy
browsers (even IE8). However it also takes advantage of features added to
JavaScript in ES2015, the latest standard version of JavaScript, including
Iterators, Arrow Functions, Classes, and Modules. It's inspired
by the native Map and Set collections added to ES2015.

All examples in the Documentation are presented in ES2015. To run in all
browsers, they need to be translated to ES5.

// ES2015
const mapped = => x * x);
// ES5
var mapped = (x) { return x * x; });

All Immutable.js collections are Iterable, which allows them to be
used anywhere an Iterable is expected, such as when spreading into an Array.

const { List } = require('immutable')
const aList = List([ 1, 2, 3 ])
const anArray = [ 0, ...aList, 4, 5 ] // [ 0, 1, 2, 3, 4, 5 ]

Note: A Collection is always iterated in the same order, however that order may
not always be well defined, as is the case for the Map and Set.

Nested Structures

The collections in Immutable.js are intended to be nested, allowing for deep
trees of data, similar to JSON.

const { fromJS } = require('immutable')
const nested = fromJS({ a: { b: { c: [ 3, 4, 5 ] } } })
// Map { a: Map { b: Map { c: List [ 3, 4, 5 ] } } }

A few power-tools allow for reading and operating on nested data. The
most useful are mergeDeep, getIn, setIn, and updateIn, found on List,
Map and OrderedMap.

const { fromJS } = require('immutable')
const nested = fromJS({ a: { b: { c: [ 3, 4, 5 ] } } })

const nested2 = nested.mergeDeep({ a: { b: { d: 6 } } })
// Map { a: Map { b: Map { c: List [ 3, 4, 5 ], d: 6 } } }

console.log(nested2.getIn([ 'a', 'b', 'd' ])) // 6

const nested3 = nested2.updateIn([ 'a', 'b', 'd' ], value => value + 1)
// Map { a: Map { b: Map { c: List [ 3, 4, 5 ], d: 7 } } }

const nested4 = nested3.updateIn([ 'a', 'b', 'c' ], list => list.push(6))
// Map { a: Map { b: Map { c: List [ 3, 4, 5, 6 ], d: 7 } } }

Equality treats Collections as Values

Immutable.js collections are treated as pure data values. Two immutable
collections are considered value equal (via .equals() or is()) if they
represent the same collection of values. This differs from JavaScript's typical
reference equal (via === or ==) for Objects and Arrays which only
determines if two variables represent references to the same object instance.

Consider the example below where two identical Map instances are not
reference equal but are value equal.

// First consider:
const obj1 = { a: 1, b: 2, c: 3 }
const obj2 = { a: 1, b: 2, c: 3 }
obj1 !== obj2 // two different instances are always not equal with ===

const { Map, is } = require('immutable')
const map1 = Map({ a: 1, b: 2, c: 3 })
const map2 = Map({ a: 1, b: 2, c: 3 })
map1 !== map2 // two different instances are not reference-equal
map1.equals(map2) // but are value-equal if they have the same values
is(map1, map2) // alternatively can use the is() function

Value equality allows Immutable.js collections to be used as keys in Maps or
values in Sets, and retrieved with different but equivalent collections:

const { Map, Set } = require('immutable')
const map1 = Map({ a: 1, b: 2, c: 3 })
const map2 = Map({ a: 1, b: 2, c: 3 })
const set = Set().add(map1)
set.has(map2) // true because these are value-equal

Note: is() uses the same measure of equality as for scalar
strings and numbers, but uses value equality for Immutable collections,
determining if both are immutable and all keys and values are equal
using the same measure of equality.

Performance tradeoffs

While value equality is useful in many circumstances, it has different
performance characteristics than reference equality. Understanding these
tradeoffs may help you decide which to use in each case, especially when used
to memoize some operation.

When comparing two collections, value equality may require considering every
item in each collection, on an O(N) time complexity. For large collections of
values, this could become a costly operation. Though if the two are not equal
and hardly similar, the inequality is determined very quickly. In contrast, when
comparing two collections with reference equality, only the initial references
to memory need to be compared which is not based on the size of the collections,
which has an O(1) time complexity. Checking reference equality is always very
fast, however just because two collections are not reference-equal does not rule
out the possibility that they may be value-equal.

Return self on no-op optimization

When possible, Immutable.js avoids creating new objects for updates where no
change in value occurred, to allow for efficient reference equality checking
to quickly determine if no change occurred.

const { Map } = require('immutable')
const originalMap = Map({ a: 1, b: 2, c: 3 })
const updatedMap = originalMap.set('b', 2)
updatedMap === originalMap // No-op .set() returned the original reference.

However updates which do result in a change will return a new reference. Each
of these operations occur independently, so two similar updates will not return
the same reference:

const { Map } = require('immutable')
const originalMap = Map({ a: 1, b: 2, c: 3 })
const updatedMap = originalMap.set('b', 1000)
// New instance, leaving the original immutable.
updatedMap !== originalMap
const anotherUpdatedMap = originalMap.set('b', 1000)
// Despite both the results of the same operation, each created a new reference.
anotherUpdatedMap !== updatedMap
// However the two are value equal.

Batching Mutations

If a tree falls in the woods, does it make a sound?

If a pure function mutates some local data in order to produce an immutable
return value, is that ok?

— Rich Hickey, Clojure

Applying a mutation to create a new immutable object results in some overhead,
which can add up to a minor performance penalty. If you need to apply a series
of mutations locally before returning, Immutable.js gives you the ability to
create a temporary mutable (transient) copy of a collection and apply a batch of
mutations in a performant manner by using withMutations. In fact, this is
exactly how Immutable.js applies complex mutations itself.

As an example, building list2 results in the creation of 1, not 3, new
immutable Lists.

const { List } = require('immutable')
const list1 = List([ 1, 2, 3 ]);
const list2 = list1.withMutations(function (list) {
assert.equal(list1.size, 3);
assert.equal(list2.size, 6);

Note: Immutable.js also provides asMutable and asImmutable, but only
encourages their use when withMutations will not suffice. Use caution to not
return a mutable copy, which could result in undesired behavior.

Important!: Only a select few methods can be used in withMutations including
set, push and pop. These methods can be applied directly against a
persistent data-structure where other methods like map, filter, sort,
and splice will always return new immutable data-structures and never mutate
a mutable collection.

Lazy Seq

Seq describes a lazy operation, allowing them to efficiently chain
use of all the higher-order collection methods (such as map and filter)
by not creating intermediate collections.

Seq is immutable — Once a Seq is created, it cannot be
changed, appended to, rearranged or otherwise modified. Instead, any mutative
method called on a Seq will return a new Seq.

Seq is lazySeq does as little work as necessary to respond to any
method call. Values are often created during iteration, including implicit
iteration when reducing or converting to a concrete data structure such as
a List or JavaScript Array.

For example, the following performs no work, because the resulting
Seq's values are never iterated:

const { Seq } = require('immutable')
const oddSquares = Seq([ 1, 2, 3, 4, 5, 6, 7, 8 ])
  .filter(x => x % 2 !== 0)
  .map(x => x * x)

Once the Seq is used, it performs only the work necessary. In this
example, no intermediate arrays are ever created, filter is called three
times, and map is only called once:

oddSquares.get(1); // 9

Any collection can be converted to a lazy Seq with Seq().

const { Map, Seq } = require('immutable')
const map = Map({ a: 1, b: 2, c: 3 })
const lazySeq = Seq(map)

Seq allows for the efficient chaining of operations, allowing for the
expression of logic that can otherwise be very tedious:

  .map(key => key.toUpperCase())
// Seq { A: 1, B: 1, C: 1 }

As well as expressing logic that would otherwise seem memory or time
limited, for example Range is a special kind of Lazy sequence.

const { Range } = require('immutable')
Range(1, Infinity)
  .map(n => -n)
  .filter(n => n % 2 === 0)
  .reduce((r, n) => r * n, 1)
// 1006008