Skip to content
Switch branches/tags

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time


This library is deprecated.

Everything here is documented and works, but immutable vectors are much more useful combined with other immutable data structures (particularly maps and sets).

Before I could get around to implementing maps and sets, Lee Byron at Facebook did it first with the much more feature-complete, BSD licensed immutable-js.

I plan to use and contribute to immutable-js in the future, and recommend you do the same!

Original description

An efficient immutable vector in pure JavaScript. Not ready for real-world use yet, but feedback appreciated. Announced here.


In Node, or a frontend project using Browserify:

npm install git+

var ImmutableVector = require('immutable-vector');

// Create an immutable vector
var v1 = new ImmutableVector(1, 2, 3);
console.log(v1.toArray()); // [ 1, 2, 3 ]

// "Change" it by pushing 4 to the end
var v2 = v1.push(4);
console.log(v2.toArray()); // [ 1, 2, 3, 4 ]

// The original is still unchanged:
console.log(v1.toArray()); // [ 1, 2, 3 ]

If you're using AMD or the a-bunch-of-script-tags approach, it doesn't support that yet but I can if there's interest. Or you can; patches welcome.


Unless otherwise noted, all operations are O(log32 n). Because log32 is a very slowly growing function (the log base 32 of two billion is ~6.2), this can be thought of as "effectively O(1)".

For more on how it works, see this excellent blog post.

v = new ImmutableVector(...)

Creates a new immutable vector with the arguments as values.

v = ImmutableVector.from(arrayLike)

Creates a new immutable vector from the array-like object, which can be any object that contains a .length property and has numeric indices from 0 to length-1. Similar to ES6 Array.from but does not support the optional additional arguments.


Number of items in the vector.


Analog to array[index]. Returns the value at the index given, if 0 <= index < vector.length, else undefined.

v.set(index, val)

Returns a new vector that contains val at index. For behavior similar to array[index] = val, use vector = vector.set(index, val).

Note: Unlike with Arrays, sparse vectors are not supported. The index must already exist within the vector. To append, use push.


Returns a new vector with val appended. For behavior similar to array.push(val), use vector = vector.push(val).


Returns the last element in the vector, or undefined if the vector is empty. Equivalent to vector.get(vector.length - 1).


Returns a new vector with the last element removed. For behavior similar to x = array.pop(), use:

x = vector.peek();
vector = vector.pop();

v.slice(begin, [end])

Like array.slice. Returns a new vector that contains all the elements starting from index begin, up to but not including index end. If end is omitted, copies up to the end of the vector.

Note: Negative indices are not supported.

Note: This is O(1), but there's a catch: it works by creating a view into the original vector. This prevents objects that are in the original vector, but not in the slice, from being garbage collected as long as references to the slice, or modified versions of it, remain.


Returns true if the two vectors are equal.

If any elements are instances of ImmutableVector, descends into those nested vectors to check for value equality.

Does not attempt to descend into any other collection types.

Worst case O(n), where n is the total number of elements in the vector itself and any nested vectors.


Returns a plain, mutable Array with the same elements as the vector.

v.forEach(fun, [thisArg])

Like array.forEach. Calls fun once for each element in the vector, with this set to thisArg (or undefined), with three arguments: value, index, and a reference to the whole vector., [thisArg])

Like Returns a new vector containing the result of calling fun on each element of v. If provided, thisArg is bound to this within the provided function.

v.filter(fun, [thisArg])

Like array.filter. Returns a new vector containing only the elements for which fun(elementValue, elementIndex) returns a truthy value. If provided, thisArg is bound to this within the calls to fun.

v.reduce(fun, [initial])

Like array.reduce. Calls fun repeatedly with four arguments: accumulator, currentValue, index, and v itself. Accumulator is initialized to the value of initial, if provided, or v.get(0) otherwise (in which case the zeroth element is then skipped).

If the vector is empty and a value for initial is not provided, this method throws a TypeError.

v.indexOf(element, [fromIndex])

Returns the first index at which element occurs in the vector, or -1 if element does not occur in the vector. Starts the search at fromIndex if provided, else index 0.


Returns an iterator compatible with the ES6 Iterator protocol.

Note: This API design doesn't match how you make an ES6 Iterator from an Array. Sadly, that behavior appears impossible to define for custom objects in a backwards-compatible way.

Tests and benchmarks

Tests use Mocha. npm test to run them via Node. To run them in a browser, install testling globally (sudo npm install -g testling), run testling -u and open the URL it prints out.

Benchmarks use Matcha. npm run benchmark.


Feedback welcome, especially on the API. Is reusing Array method names like push and pop a good idea, or confusing in light of the different semantics?

Patches welcome, though I am picky about commit messages and code style looking like my code style.

Also, if you live in/will be visiting the NYC area, are friendly, and want to pair-program on this on a weekend, get in touch. :)


Immutable array for JavaScript supporting fast "modification" via structural sharing







No releases published


No packages published