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· 6 min read
Arvid Nicolaas

Immutable collections and tools for TypeScript

A new collection library for TypeScript? Aren't Array, Set, and Map good enough? What about immutable.js? Well, I understand you have a lot of questions my friend. Let me start by describing what Rimbu is all about.

Immutability: Create safer code

When you start using TypeScript, you are taught it is better to use const where possible instead of let or var. Why is that? Well, mainly because, once you assign a value with const, you cannot change its value. If you want a new value from that value, you will need to write a new const and give it a new name:

const a = 5;
a += 1; // Compiler error!
const b = a + 1; // OK

However, this principle breaks down when we use referenced values like object or Array. When we assign such a value to a const, we cannot make it null. But we can change its contents:

const arr = [1, 2, 3];
arr.length = 0; // this is fine
// => []

You see that we can certainly change the value even though we use const. This breaks the whole story about not being able to change const values!

This is where immutable collections and objects come in to save the day. An immutable object, by definition, cannot be changed. Once it's created, its values will always be the same, no matter what you do to it. Combined with const, we have back our sacred principle of not being able to change assigned values.

But, I hear you say, what's the use? How can we add values to immutable collections? Well, it simply the same as the story of adding 1 to a const number:

import { List } from '@rimbu/collections';

const list1 = List.of(1, 2, 3);
const list2 = list1.append(4);

// => List(1, 2, 3, 4);
// => List(1, 2, 3);

In this example you can clearly see that list2 is a new list with other values than list1. Even though we added a value to list1, its contents did not change. This is exactly the benefit of immutability, you never have to worry about values having changed while you were not looking.

Smart and strong type safety

Rimbu goes to great lenghts to ensure that the compiler will 'understand' what you are trying to do, but still prevent you from making obvious mistakes. For example, imagine the following:

import { List } from '@rimbu/collection';
const list1 = List.of(1, 2, 3);
// type of list1: List.NonEmpty<number>
const list2 = list1.append('a'); // error: string is not assignable to number
const list3 = list1.extendType<number | string>().append('a');
// type of list3: List.NonEmpty<number | string>

The compiler will rightly prevent you from appending a string to a list of numbers. However, you can still do that without needing to cast using the extendType method. Furthermore, you see that the lists get type List.NonEmpty instead of List. All Rimbu collections have this type information to indicate whether the compiler can know at compile time that the collection has at least one value. This allows us to write code that by definition rejects empty collections, and saves us from having to write checks for empty collections and thinking about how to handle such situations. In particular, it saves us from writing boring tests.

These are just small examples from a plethora of built-in smartness that the Rimbu collections possess.

Performance: Rimbu collections designed for performance

Because immutable collections are often described as 'copying' data, they are often assumed to be slow. However, there is a principle called 'persistence', which allows them to actually be really fast. Persistance actually means memory sharing. Rimbu immutable collections will share memory as much as possible as long as no changes need to be made. The collections only duplicates parts of shared data that are modified. The rest of the data remains shared. This is because the collections keep their data in block-like structures, comparable to blocks on hard-drives. If some collection uses 50 blocks of data, and one value is changed, chances are that only 2 or 3 blocks need to be copied and changed, while the rest stays the same and shared. In some cases, persistent collections can be faster than mutable collections. For example, reversing a List of N elements in Rimbu has complexity O(logM(N)) where M is usually 32, so it is really fast for large collections as well. For a mutable array, the complexity is O(N), and therefore much slower. A similar story goes for memory complexity, however I will probably cover this topic some other time.


Rimbu is mostly about giving the programmer new powerful tools to write safe and effective programs that are simple. For example, it avoids using advanced functional programming concepts like Monads, because they make code harder to understand and read.

This is why you will not find an Option or Maybe type (which are monads). Still, Rimbu provides nearly the same power in all methods that can fail:

import { List, err } from '@rimbu/core';

const list = List.of(1, 2, 3);

const v1 = list.getAtIndex(1);
// type of v1: number | undefined

const v2 = list.getAtIndex(1, 'not found');
// type of v2: number | 'not found'

const v3 = list.getAtIndex(1, -1); // return -1 if not found
// type of v3: number

const v4 = list.getAtIndex(1, () => 'lazy failure');
// type of v4: number | 'lazy failure'

// err is a function that throws an error when invoked
const v5 = list.getAtIndex(1, err);
// type of v5: number

In this way it is not needed to write things like list.getOption(..).map(..).getOrElse(..) or if (list.get(1) === undefined) throw Error()

But still, what about immmutable.js?

Of course we already have a very nice JavaScript immutable collection library for years, called immutable.js. To be honost, I've never used it myself in projects, but I have read their documentation. The first thing that strikes me, in comparison to Rimbu, is that immutable.js is focused firstly on JavaScript and added types later on. Secondly, it has a few basic collections, like Maps and Sets, but Rimbu has many more (up to Graphs). Finally, it has Records to define immutable objects. However, with TypeScript, we can use the compiler to supply deep readonly types that prevent modification of plain objects. In this way this is a very lightweight and easier to use approach.

What's to come?

In the coming time, I want to write much more documentation, and I hope to receive feedback on the current state of the library from other users. If there are requests for other types of collections, I would be willing to implement them and add them to the library.