Immutable data cannot be changed once created, leading to much simpler application development and enabling techniques from functional programming such as lazy evaluation. This provides a lazy Sequence, allowing efficient chaining of sequence methods like map and filter without creating intermediate represenations.

immutable-data implements a sparse Vector, Map, OrderedMap, Set and Range by using lazy sequences and hash maps tries. They achieve acceptable performance by using structural sharing and minimizing the need to copy data.

Install immutable-data using npm

npm install immutable-data

Then require it into any module.

var Immutable = require('immutable-data');
var map = Immutable.Map({a:1, b:2, c:3});

To use immutable-data from a browser, try Browserify.

To use immutable-data from a TypeScript program.

import Immutable = require('./node_modules/immutable-data/dist/Immutable');
var map: Immutable.Map<string, number>;
map = Immutable.Map({a:1, b:2, c:3});

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.

When data is passed from above rather than being subscribed to, and you're only interested in doing work when something has changed, you use equality. Immutable data allows for using === equality to determine if something has changed.

var map1 = Immutable.Map({a:1, b:2, c:3});
var map2 = map1.set('b', 2);
assert(map1 === map2);

If an object is immutable, it can be copied simply by making a copy of a 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.

var map1 = Immutable.Map({a:1, b:2, c:3});
var map2 = map1.clone();
assert(map1 === map2);

immutable-data provides a lazy Sequence, which is the base class for all of its collections. This allows for efficient chaining of sequence operations like map and filter as well as allowing for defining logic that is otherwise very difficult to express.

var map1 = Immutable.Map({a:1, b:1, c:1});
var map2 = map1.flip().map(key => key.toUpperCase()).flip().toMap();
console.log(map2); // Map { A: 1, B: 1, C: 1 }

immutable-data provides equality which treats immutable data structures as pure data, performing a deep equality check if necessary.

var map1 = Immutable.Map({a:1, b:1, c:1});
var map2 = Immutable.Map({a:1, b:1, c:1});
assert(map1 !== map2);
assert(Immutable.is(map1, map2) === true);

Immutable.is uses the same measure of equality as Object.is however adds:

• if both are immutable sequences and all keys and values are equal using the same measure of equality.

While immutable-data is inspired by Clojure, 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 Array, Map, and Set.

The only difference is that every method that would mutate the collection instead returns a new collection.

var vect1 = Immutable.Vector(1,2);
var vect2 = vect1.push(3,4,5);
var vect3 = vect2.slice(1, -1).toVector();
var vect4 = vect1.concat(vect2, vect3, vect4);
assert(vect1.length === 2);
assert(vect2.length === 5);
assert(vect3.length === 3);
assert(vect4.length === 10);

Almost all of the methods on Array will be found in similar form on Immutable.Vector, those of Map found on Immutable.Map, and those of Set found on Immutable.Set, including sequence operations.

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

There is a performance penalty paid every time you create a new immutable object via applying a mutation. If you need to apply a series of mutations immutable-data gives you the ability to create a temporary mutable copy of a collection and applying mutations in a highly performant manner by using withMutations. In fact, this is exactly how immutable-data applies complex mutations itself.

As an example, this results in 1, not 3, new immutable objects.

var vect1 = Immutable.Vector();
var vect2 = vect1.withMutations(function (vect) {
  vect.push(1).push(2).push(3);
});
assert(vect1.length === 0);
assert(vect2.length === 3);

Reference: Immutable.d.ts

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