Immutable Records, Vectors, Maps and Sets for ECMAScript

Immutability and referential transparency has many known benefits and ability for optimization. Several modern JavaScript libraries take advantage of this, and many more functional compile-to-JS languages.

This is based upon the Value Types proposal (Typed Objects / Explainer).

All these types provide value equality for both == and ===.

Record

Records are a new value type that represents the value type analogy of an immutable object.

const xy = #{ x: 1, y: 2 }; // frozen value type 
const xyz = #{ ...xy, z: 3 }; // functional extension 

Immutable Vector

ImmutableVector is a new value type that represents the value type analogy of an immutable array, without holes. It cannot be sparse.

const xy = #[ x, y ]; // frozen value type 
const xyz = #[ ...xy, z ]; // functional extension 

Immutable Map

ImmutableMap is an immutable version of Map. Any mutable operation returns a new ImmutableMap instead of mutating the existing reference.

const a = ImmutableMap([['x', 1], ['y', 2]]);
const b = a.set('y', 3);
a.get('y'); // 2 
b.get('y'); // 3 

Immutable Set

ImmutableSet is an immutable version of Set. Any mutable operation returns a new ImmutableSet instead of mutating the existing reference.

const a = ImmutableSet([1, 2]);
const b = a.add(3);
a.size; // 2 
b.size; // 3 

Status of this Proposal

It is my intention to propose this as a Stage 0 proposal to ECMAScript 7.

Known Issues

See why this matters.