Functional Reactive State for JavaScript & TypeScript

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State made simple → Effects made easy

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Derivables are an Observable-like state container with superpowers. Think MobX distilled to a potent essence, served with two heaped tablespoons of extra performance, a garnish of declarative effects management, and a healthy side-salad of immutability.

With React With Immutable Debugging Examples

Quick start

There are two types of Derivable:
  • Atoms

Atoms are simple mutable references to immutable values. They represent the ground truth from which all else is derived.
```js import { atom } from "derivable";
const name = atom("Richard");
name.get(); // => 'Richard'
name.get(); // => 'William' ```
  • Derivations

Derivations are declarative transformations of values held in atoms. You can create them with the derive function.
```js import { derive } from "derivable";
const cyber = word =>
  .join(" ");
const cyberName = derive(() => cyber(name.get()));
cyberName.get(); // 'W I L L I A M'
cyberName.get(); // 'S A R A H' ```
Unlike atoms, derivations cannot be modified in-place with a .set method. Their values change only when one or more of the values that they depend upon change. Here is an example with two dependencies.
```js const transformer = atom(cyber);
const transformedName = derive(() => transformer.get()(name.get()));
transformedName.get(); // => 'S A R A H'
const reverse = string =>
transformedName.get(); // => 'haraS'
transformedName.get(); // => 'naibaF' ```
derive takes a function of zero arguments which should dereference one or more Derivables to compute the new derived value. DerivableJS then sneakily monitors who is dereferencing who to infer the parent-child relationships.


Declarative state management is nice in and of itself, but the real benefits come from how it enables us to more effectively manage side effects. DerivableJS has a really nice story on this front: changes in atoms or derivations can be monitored by things called Reactors, which do not themselves have any kind of 'current value', but are more like independent agents which exist solely for executing side effects.
Let's have a look at a tiny example app which greets the user:
import { atom, derive, transact } from "derivable";

// global application state
const name = atom("World"); // the name of the user
const countryCode = atom("en"); // for i18n

// static constants don't need to be wrapped
const greetings = {
  en: "Hello",
  de: "Hallo",
  es: "Hola",
  cn: "您好",
  fr: "Bonjour"

// derive a greeting message based on the user's name and country.
const greeting = derive(() => greetings[countryCode.get()]);
const message = derive(() => `${greeting.get()}, ${name.get()}!`);

// set up a Reactor to print the message every time it changes, as long as
// we know how to greet people in the current country.
message.react(msg => console.log(msg), { when: greeting });
// $> Hello, World!

// $> Hallo, World!
// $> Hallo, Dagmar!

// we can avoid unwanted intermediate reactions by using transactions
transact(() => {
// $> Bonjour, Étienne!

// if we set the country code to a country whose greeting we don't know,
// `greeting` becomes undefined, so the `message` reactor won't run
// In fact, the value of `message` won't even be recomputed.
// ... crickets chirping

The structure of this example can be depicted as the following DAG:

Key differences with MobX

  • Smaller API surface area.

There are far fewer kinds of thing in DerivableJS, and therefore fewer things to learn and fewer surprising exceptions and spooky corners. This reduces noise and enhances one's ability to grok the concepts and wield the tools on offer. It also shrinks the set of tools on offer, but maybe that's not a bad thing:
> It seems that perfection is attained not when there is nothing more to add, > but when there is nothing more to remove.
- Antoie de Saint Exupéry
  • No transparent dereferencing and assignment.

It is always necessary to call .get on derivables to find out what's inside, and you always have to call .set on atoms to change what's inside. This provides a consistent semantic and visual distinction between ordinary values and derivable values.
  • No observable map and array types.

So you probably have to use something extra like Immutable, icepick or pure javascript immutable arrays to deal with collections. Not great if you're just out to get shit done fast, but the benefits of immutable collections become more and more valuable as projects mature and grow in scope.
  • More subtle control over reactors

DerivableJS has a tidy and flexible declarative system for defining when reactors should start and stop. This is rather nice to use for managing many kinds of side effects.
  • Speed

DerivableJS is finely tuned, and propagates change significantly faster than MobX. \[link to benchmark-results.html forthcoming\]

API / Documentation

Over Here


DerivableJS is fairly mature, and has been used enough in production by various people to be considered a solid beta-quality piece of kit.

With React

The fantastic project react-derivable lets you use derivables in your render method, providing seamless interop with component-local state and props.

With Immutable

DerivableJS works spiffingly with Immutable, which is practically required if your app deals with medium-to-large collections.


Due to inversion of control, the stack traces you get when your derivations throw errors can be totally unhelpful. There is a nice way to solve this problem for dev time. See setDebugMode for more info.


See here


I heartily welcome questions, feature requests, bug reports, and general suggestions/criticism on the github issue tracker. I also welcome bugfixes via pull request (please read before sumbmitting).

Inspiration <3


Copyright 2015 David Sheldrick <>

Licensed under the Apache License, Version 2.0 (the "License");
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