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https://github.com/fynyky/reactor.js

Simple reactive programming without a framework
https://github.com/fynyky/reactor.js

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Simple reactive programming without a framework

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Reactor.js

==========



Reactor.js is a simple library for [reactive programming](http://en.wikipedia.org/wiki/Reactive_programming). It provides:

- `Reactor` objects that store reactive variables

- `Observer` functions that automatically track the reactive variables they use and retrigger when any of these variables are updated



Here's a quick example of what Reactor.js does:

```javascript

const reactor = new Reactor()

reactor.foo = 'bar'

const observer = new Observer(() => {

  console.log('foo is ', reactor.foo)

})

observer() // prints "foo is bar"

reactor.foo = 'moo' // prints "foo is moo"

```

- `Reactor` objects work like normal objects that you can set and get properties on

- `Observer` functions work like normal functions that you can define and call

- When an `Observer` reads a `Reactor`, it registers itself as a dependent

- When a `Reactor` is updated, it automatically retriggers the dependent `Observer` functions



Reactor.js is designed to be unobtrusive and unopinionated:

- No special syntax to learn. Everything is just plain JavaScript

- There is no need to manually declare listeners or bindings. Reactor.js automatically keeps track of all that for you

- It imposes no particular structure on your code. Any variable can be easily replaced with a reactive one without changing the rest of your codebase



Installation

------------



Reactor.js is [available on npm](https://npmjs.org/package/reactorjs). Install it by running:

```

$ npm install reactorjs

```



Import it using:

```javascript

import { Reactor, Observer, hide, batch, shuck } from 'reactorjs'

```



It is also available directly from [unpkg](unpkg.com). You can import it in JavaScript using:

```javascript

import { Reactor, Observer, hide, batch, shuck } from 'https://unpkg.com/reactorjs'

```



Reactors

--------



A `Reactor` is an object wrapper that automatically tracks `Observer` functions that read its properties and notifies the observers when those properties are updated.



You create a new reactor by calling its constructor:

```javascript

const reactor = new Reactor()

```



You can also wrap an existing object with a reactor by passing it to the constructor. Changes to the reactor are passed through to the underlying object:

```javascript

const reactor = new Reactor({

  foo: "bar"

})

```



Reactors behave mostly like plain JavaScript objects:

```javascript

const reactor = new Reactor({

  foo: "bar"

})

// You can get and set properties as usual

reactor.foo // "bar"

reactor.cow = "moo"

// defineProperty works normally as well

Object.defineProperty(reactor, "milk", {

  get() { return "chocolate" }

})

reactor.milk // "chocolate"

// delete works too

delete reactor.foo

reactor.foo // undefined

```



The key difference of `Reactor` objects is that they track when one of their properties is read by an `Observer` function and will notify that observer when the property is updated:



```javascript

const reactor = new Reactor({ foo: "bar" })



new Observer(() => {

  console.log("foo is ", reactor.foo)

})() // prints "foo is bar"



reactor.foo = "moo" // prints "foo is moo"



Object.defineProperty(reactor, "foo", {

  get() { return "meow" }

}) // prints "foo is meow"



delete reactor.foo // prints "foo is undefined"

```



Tracking is property-specific, so observers will not trigger if a different property is updated:

```javascript

const reactor = new Reactor({

  foo: "bar",

  moo: "mar"

})



new Observer(() => {

  console.log("foo tracker is now", reactor.foo)

})() // prints "foo tracker is now bar"



new Observer(() => {

  console.log("moo tracker is now", reactor.moo)

})() // prints "moo tracker is now mar"



reactor.foo = "bar2" // prints "foo tracker is now bar2"

reactor.moo = "mar2" // prints "moo tracker is now mar2"

reactor.goo = "goop" // does not trigger any observers

```



If reading a reactor's property returns an object, that object is recursively wrapped in a reactor before being returned. This allows observers to track dependencies in nested objects easily:

```javascript

const reactor = new Reactor({

  outer: {

    inner: "cake"

  }

})



new Observer(() => {

  console.log("inner value is ", reactor.outer.inner)

})() // prints "inner value is cake"

```



Reactors are implemented using [Proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) objects. This means reactors created from scratch typecheck as Reactors, but reactors created from an existing object typecheck as the original object:



```javascript

const baseReactor = new Reactor()

baseReactor instanceof Reactor // true

const mapReactor = new Reactor(new Map())

mapReactor instanceof Reactor // false

mapReactor instanceof Map // true

```



This also has implications for native objects or objects that use private properties. Since proxies can't access native or private properties, some methods will fail. To work around this, we provide the `shuck` function which returns a reactor's internal object:



```javascript

// Native object example

const mapReactor = new Reactor(new Map())

Map.prototype.keys.apply(mapReactor) // throws an error

Map.prototype.keys.apply(shuck(mapReactor)) // works fine

```



Observers

---------



An `Observer` is like a normal function that you can define and call. When an `Observer` reads from a `Reactor`, it automatically tracks that dependency, and when that reactor's property is updated, it automatically triggers the observer again.



`Observer` functions are created by passing a function to its constructor:

```javascript

const observer = new Observer(() => {

  console.log("hello world")

})

observer() // prints "hello world" and starts the observer

```



For brevity, observers can also be created and instantly executed like this:

```javascript

new Observer(() => {

  console.log("hello world")

})() // prints "hello world" and starts the observer

```



When an `Observer` reads a `Reactor` property, it gets saved as a dependent. When that property is updated, it notifies the observer which reruns its function. This happens automatically without any need to manually declare dependencies:

```javascript

const reactor = new Reactor()

new Observer(() => {

  console.log("reactor.foo is ", reactor.foo)

})() // prints "reactor.foo is undefined"



reactor.foo = "bar" // prints "reactor.foo is bar"

```



An observer's dependencies are dynamically determined. Only the dependencies actually read in the last execution of an observer can trigger it again. This means that reactor reads that are only conditionally used will not trigger the observer unnecessarily:

```javascript

const reactor = new Reactor({

  a: true,

  b: "bee",

  c: "cee"

})

new Observer(() => {

  if (reactor.a) {

    console.log("reactor.b is ", reactor.b)

  } else {

    console.log("reactor.c is ", reactor.c)

  }

})() // prints "reactor.b is bee"



reactor.b = "boop" // prints "reactor.b is boop"

reactor.c = "cat" // does not trigger the observer



reactor.a = false // prints "reactor.c is cat"

reactor.b = "blue" // does not trigger the observer

reactor.c = "cheese" // prints "reactor.c is cheese"

```



An observer's results are themselves observable via either the `value` property or by triggering the observer via `observer()` and using the return value. This allows you to chain observers together:

```javascript

const reactor = new Reactor({ foo: 'bar' })

const capitalizer = new Observer(() => {

  return reactor.foo.toUpperCase()

})()

const printer = new Observer(() => {

  console.log(capitalizer.value)

})() // prints 'BAR'

reactor.foo = 'baz' // prints 'BAZ'

```



This also works:

```javascript

const reactor = new Reactor({ foo: 'bar' })

const capitalizer = new Observer(() => {

  return reactor.foo.toUpperCase()

}) // Did not start the observer here

const printer = new Observer(() => {

  // Manually calls capitalizer like a function which activates it

  // As well as accesses its return value as a dependency

  console.log(capitalizer())

})() // starts printer which starts capitalizer

reactor.foo = 'baz' // prints 'BAZ'

```



You can stop an observer by calling `stop()` on the returned observer object. This clears any existing dependencies and prevents triggering. You can restart the observer by calling `start()`. Starting is idempotent, so calling `start()` on an already running observer will have no effect:

```javascript

const reactor = new Reactor()

const observer = new Observer(() => {

  console.log(reactor.foo)

})() // prints "undefined"



reactor.foo = "bar" // prints "bar"



observer.stop()



reactor.foo = "cheese" // does not trigger the observer



observer.start() // prints "cheese"

observer.start() // No effect

observer.start() // No effect

observer.start() // No effect



reactor.foo = "moo" // prints "moo"

```



For convenience, you can call an observer to execute like a normal function. This works regardless of whether the observer is stopped. Doing so starts the observer up again:



```javascript

const reactor = new Reactor({ foo: "hello" })

const observer = new Observer(() => {

  console.log(reactor.foo)

})() // prints "hello"

reactor.foo = "hi" // prints "hi"

observer() // prints "hi" again



observer.stop()

reactor.foo = "hola" // does not trigger the observer since it's stopped

observer() // prints "hola"

```



Like normal functions, observers can expect and be called with arguments. They remember the arguments from the last time they were called and reuse them when automatically triggered:



```javascript

const parameterizedObserver = new Observer((arg1, arg2) => {

  console.log(reactor.foo + arg1 + arg2)

})

parameterizedObserver('beep', 'bop') // prints bazbeepbop

reactor.foo = 'bla' // prints blabeepbop

```



Observers can also use and remember the last `this` context. Note that just like normal functions, for the `this` context to be bound to the holding object, it needs to be defined with the traditional `function` keyword instead of ES6 arrow functions:



```javascript

const holdingObject = {

  name: 'Mario',

  greet: new Observer(function () { // Need to use `function`

    console.log("Hello " + reactor.foo + " itsa me " + this.name)

  })

}

holdingObject.greet() // prints "Hello bla itsa me Mario"

reactor.foo = 'bonk' // prints "Hello bonk itsa me Mario"

holdingObject.name = 'Luigi' // prints nothing since holdingObject is not a Reactor

```



If you ever need to access the raw function the observer is wrapping, you do so by using `shuck`:



```javascript

const myFunction = () => {}

const observer = new Observer(myFunction)

myFunction === shuck(observer) // true

```



### Hide



Sometimes you might want to read from a reactor without becoming dependent on it. A common case for this is when using array modification methods. These often also read from the array in order to do the modification:

```javascript

const taskList = new Reactor(["a", "b", "c", "d"])



// Creating the following observer will cause an infinite loop

// because it both reads from and modifies the length property of taskList

// As a result, it triggers itself in the middle of execution

// This loop is detected and creates an exception

new Observer(() => {

  // Even though we only want to modify the array

  // pop() also reads the length property of the array

  console.log(taskList.pop())

})()

```



In these cases, you can use "hide" to shield a block of code from creating dependencies. It takes a function and any reactor properties read inside that function will not be set as dependencies. `hide` also passes through the return value of its function for syntactic simplicity:

```javascript

const taskList = new Reactor(["a", "b", "c", "d"])



new Observer(() => {

  console.log(

    // Because we wrap the pop() call in a hide block

    // it does not create a dependency on the length property

    // unlike our previous example

    hide(() => taskList.pop())

  )

})() // prints "d"



taskList.push("e") // does not trigger the observer

```



Note that only the reads inside the hide block are shielded from creating dependencies. The rest of the observe block still creates dependencies as normal.



### Batching

One problem with automatic watchers is that you might end up with multiple repeated triggering when you're updating a lot of information all at once. The following code shows an example where you want to update multiple properties, but each property update prematurely triggers the observer since you are not done updating yet:



```javascript

const person = new Reactor({

  firstName: "Anakin",

  lastName: "Skywalker",

  faction: "Jedi",

  rank: "Knight"

})



// This observer tracks multiple properties

// and so will be triggered when any of the properties get updated

const observer = new Observer(() => {

  console.log(

    "I am " +

    person.firstName +

    " " +

    person.lastName +

    ", " +

    person.faction +

    " " +

    person.rank

  )

})() // prints "I am Anakin Skywalker, Jedi Knight"



// The following updates will each trigger the observer even though we only

// want to trigger the observer once all the updates are complete

person.firstName = "Darth" // prints "I am Darth Skywalker, Jedi Knight"

person.lastName = "Vader" // prints "I am Darth Vader, Jedi Knight"

person.faction = "Sith" // prints "I am Darth Vader, Sith Knight"

person.rank = "Lord" // prints "I am Darth Vader, Sith Lord"

```



The `batch` function is provided to allow you to batch multiple updates together and only trigger the appropriate observers once at the end of the batch block. So the last part of the previous example can be turned into:

```javascript

// batch postpones any observer triggers that originate from inside it

// Triggers are deduplicated so any observer is triggered at most once

batch(() => {

  // None of the following updates will trigger the observer yet

  person.firstName = "Darth"

  person.lastName = "Vader"

  person.faction = "Sith"

  person.rank = "Lord"

}) // prints "I am Darth Vader, Sith Lord"

```



This is useful when you are making multiple data updates and want to avoid showing an "incomplete" view of the data to observers.



Note that only the observer triggering is postponed until the end. The actual reactor properties are updated in place as expected. This means that you can have other logic with read-what-you-write semantics within the observer block working just fine.



Summary

-------

```javascript

import { Reactor, Observer, hide, batch, shuck } from 'reactorjs'



const reactor = new Reactor({ foo: 'bar' })

const observer = new Observer(() => {

  const result = 'reactor.foo is ' + reactor.foo // Sets a dependency on foo

  console.log(result)

  return result

})

observer() // prints 'reactor.foo is bar' and starts the observer

reactor.foo = 'baz' // prints 'reactor.foo is baz'



observer.stop()

reactor.foo = 'qux' // prints nothing since observer is stopped



observer.start() // prints 'reactor.foo is baz'

observer.start() // prints nothing since observer is already started

observer() // prints 'reactor.foo is baz' even if it is already running



// Observer return values are themselves observable

const trailingObserver = new Observer(() => {

  const result = 'Did you hear: ' + observer.value

  console.log(result)

})

trailingObserver() // prints 'Did you hear: reactor.foo is baz'

reactor.foo = 'blorp' // prints 'reactor.foo is blorp' from observer

                      // also prints 'Did you hear: reactor.foo is blorp' from trailingObserver



// Observers can be given parameters and remember these parameters when triggered

const parameterizedObserver = new Observer((arg1, arg2) => {

  console.log(reactor.foo + arg1 + arg2)

})

parameterizedObserver('beep', 'bop') // prints bazbeepbop

reactor.foo = 'bla' // prints blabeepbop



// Observers can also access and remember the last `this` context

const holdingObject = {

  name: 'Mario',

  greet: new Observer(function () { // Need to use traditional functions instead of arrow functions

    console.log("Hello " + reactor.foo + " itsa me " + this.name)

  })

}

holdingObject.greet() // prints "Hello bla itsa me Mario"

reactor.foo = 'bonk' // prints "Hello bonk itsa me Mario"

holdingObject.name = 'Luigi' // prints nothing since holdingObject is not a Reactor



// hide allows you to avoid particular dependencies in an observer

// This is useful especially when using Array methods that both read and write

reactor.ticker = 1

reactor.names = ["Alice", "Bob", "Charles", "David"]

const partialObserver = new Observer(() => {

  if (reactor.ticker) {

    // hide passes through the return value of its block

    const next = hide(() => reactor.names.pop())

    console.log("next ", next)

  }

})

partialObserver() // prints "next David"

reactor.ticker = 2 // prints "next Charles"

reactor.names.push("Elsie") // Will not trigger the observer



// batch postpones any observer triggers until it is complete

// This allows grouping updates together

const person = new Reactor({

  firstName: 'Clark',

  lastName: 'Kent'

})

new Observer(() => {

  console.log('Look its ' + person.firstName + ' ' + person.lastName)

})() // prints 'Look its Clark Kent'

batch(() => {

  // None of the following updates will trigger the observer yet

  person.firstName = "Bruce"

  person.lastName = "Wayne"

}) // prints 'Look its Bruce Wayne'



// shuck removes the Reactor layer and returns the base object

// This is necessary for some native objects which don't work with proxies

const mapReactor = new Reactor(new Map())

Map.prototype.keys.call(mapReactor) // throws an Error

Map.prototype.keys.call(shuck(mapReactor)) // works fine

```



Development & Testing

---------------------

Tests are stored in `test.js` to be run using Mocha.



Run `npm install` to install the dev dependencies.



To run the tests, run `npm test`.