derivable
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Functional Reactive State for JavaScript & TypeScript
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<h1 align="center">DerivableJS</h1>
<h3 align="center">State made simple → Effects made easy</h3>
## [](https://www.npmjs.com/package/derivable) [](https://travis-ci.org/ds300/derivablejs) [](https://coveralls.io/github/ds300/derivablejs?branch=new-algo) [](https://gitter.im/ds300/derivablejs?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [](http://futurice.com/blog/sponsoring-free-time-open-source-activities?utm_source=github&utm_medium=spice&utm_campaign=derivablejs) [](http://github.com)
Derivables are an Observable-like state container with superpowers. Think
[MobX](https://github.com/mobxjs/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.
<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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* [Quick start](#quick-start)
* [Reactors](#reactors)
* [Key differences with MobX](#key-differences-with-mobx)
* [API / Documentation](#api--documentation)
* [Usage](#usage)
* [With React](#with-react)
* [With Immutable](#with-immutable)
* [Debugging](#debugging)
* [Examples](#examples)
* [Contributing](#contributing)
* [Inspiration <3](#inspiration-3)
* [License](#license)
<!-- END doctoc generated TOC please keep comment here to allow auto update -->
## 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.set("William");
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 =>
word
.toUpperCase()
.split("")
.join(" ");
const cyberName = derive(() => cyber(name.get()));
cyberName.get(); // 'W I L L I A M'
name.set("Sarah");
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 =>
string
.split("")
.reverse()
.join("");
transformer.set(reverse);
transformedName.get(); // => 'haraS'
name.set("Fabian");
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.
## Reactors
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:
```js
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!
countryCode.set("de");
// $> Hallo, World!
name.set("Dagmar");
// $> Hallo, Dagmar!
// we can avoid unwanted intermediate reactions by using transactions
transact(() => {
countryCode.set("fr");
name.set("Étienne");
});
// $> 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.
countryCode.set("dk");
// ... crickets chirping
```
The structure of this example can be depicted as the following DAG:
<img src="https://ds300.github.com/derivablejs/img/example.svg" align="center" width="89%"/>
## 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](https://github.com/aearly/icepick) or
[pure javascript immutable arrays](https://vincent.billey.me/pure-javascript-immutable-array/)
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](https://ds300.github.io/derivablejs)
## Usage
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](https://github.com/andreypopp/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](https://github.com/facebook/immutable), which is practically
required if your app deals with medium-to-large collections.
### Debugging
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](https://ds300.github.com/derivablejs/#derivable-setDebugMode) for
more info.
### Examples
[See here](https://github.com/ds300/derivablejs/tree/master/examples)
## Contributing
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 CONTRIBUTING.md before sumbmitting).
## Inspiration <3
* [Are we there yet?](https://www.infoq.com/presentations/Are-We-There-Yet-Rich-Hickey)
* The [re-frame README](https://github.com/Day8/re-frame)
* [ratom.cljs](https://github.com/reagent-project/reagent/blob/master/src/reagent/ratom.cljs)
* [Turning the database inside out](https://www.youtube.com/watch?v=fU9hR3kiOK0)
* [Simple Made Easy](https://www.infoq.com/presentations/Simple-Made-Easy)
## License
```
Copyright 2015 David Sheldrick <djsheldrick.com>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
```