```
________________________________
___ |/ /_ __ \_ ___/__ __/
__ /|_/ /_ / / /____ \__ /
_ / / / / /_/ /____/ /_ /
/_/ /_/ \____/______/ /_/
```
# Monadic streams for reactive programming
[](https://greenkeeper.io/)
[](https://travis-ci.org/cujojs/most)
[](https://gitter.im/cujojs/most?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
## Starting a new project?
Strongly consider starting with [`@most/core`](https://github.com/mostjs/core). It is the foundation of the upcoming most 2.0, has [improved documentation](http://mostcore.rtfd.io/), new features, better tree-shaking build characteristics, and simpler APIs. Updating from `@most/core` to most 2.0 will be non-breaking and straightforward.
## Using most 1.x already on an existing project?
You can keep using most 1.x, and update to either [`@most/core`](https://github.com/mostjs/core) or most 2.0 when you're ready. See the [upgrade guide](https://mostcore.readthedocs.io/en/latest/upgrading-guide.html) for more information.
## What is it?
Most.js is a toolkit for reactive programming. It helps you compose asynchronous operations on streams of values and events, e.g. WebSocket messages, DOM events, etc, and on time-varying values, e.g. the "current value" of an <input>, without many of the hazards of side effects and mutable shared state.
It features an ultra-high performance, low overhead architecture, APIs for easily creating event streams from existing sources, like DOM events, and a small but powerful set of operations for merging, filtering, transforming, and reducing event streams and time-varying values.
## Learn more
* [API docs](docs/api.md)
* [Recipes](https://github.com/cujojs/most/wiki/Recipes)
* [Examples](https://github.com/mostjs/examples)
* [Get it](#get-it)
* [Core & Community Packages](PACKAGES.md)
## Simple example
Here's a simple program that displays the result of adding two inputs. The result is reactive and updates whenever *either* input changes.
First, the HTML fragment for the inputs and a place to display the live result:
```html
```
Using most.js to make it reactive:
```es6
import { fromEvent, combine } from 'most'
const xInput = document.querySelector('input.x')
const yInput = document.querySelector('input.y')
const resultNode = document.querySelector('.result')
const add = (x, y) => x + y
const toNumber = e => Number(e.target.value)
const renderResult = result => {
resultNode.textContent = result
}
export const main = () => {
// x represents the current value of xInput
const x = fromEvent('input', xInput).map(toNumber)
// y represents the current value of yInput
const y = fromEvent('input', yInput).map(toNumber)
// result is the live current value of adding x and y
const result = combine(add, x, y)
// Observe the result value by rendering it to the resultNode
result.observe(renderResult)
}
```
## More examples
You can find the example above and others in the [Examples repo](https://github.com/mostjs/examples).
## Get it
### Requirements
Most requires ES6 `Promise`. You can use your favorite polyfill, such as [creed](https://github.com/briancavalier/creed), [when](https://github.com/cujojs/when/blob/master/docs/es6-promise-shim.md), [bluebird](http://bluebirdjs.com/docs/getting-started.html), [es6-promise](https://github.com/jakearchibald/es6-promise), etc. Using a polyfill can be especially beneficial on platforms that don't yet have good unhandled rejection reporting capabilities.
### Install
As a module:
```
npm install --save most
```
```es6
// ES6
import { /* functions */ } from 'most'
// or
import * as most from 'most'
```
```js
// ES5
var most = require('most')
```
As `window.most`:
```
bower install --save most
```
```html
```
As a library via cdn :
```html
```
```html
```
### Typescript support
Most.js works with typescript out of the box as it provides [local typings](https://github.com/cujojs/most/blob/master/type-definitions/most.d.ts) that will be read when you import Most.js in your code. You do not need to manually link an external `d.ts` file in your tsconfig.
Most.js has a dependency on native Promises so a type definition for Promise must be available in your setup:
- If your tsconfig is targeting ES6, you do not need to do anything as typescript will include a definition for Promise by default.
- If your tsconfig is targeting ES5, you need to provide your own Promise definition. For instance [es6-shim.d.ts](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/types/es6-shim/es6-shim.d.ts)
## Interoperability
Most.js streams are [compatible with Promises/A+ and ES6 Promises](promises). They also implement [Fantasy Land](https://github.com/fantasyland/fantasy-land) and [Static Land](https://github.com/rpominov/static-land) `Semigroup`, `Monoid`, `Functor`, `Apply`, `Applicative`, `Chain` and `Monad`.
## Reactive Programming
[Reactive programming](https://github.com/cujojs/most/wiki/Concepts) is an important concept that provides a lot of advantages: it naturally handles asynchrony and provides a model for dealing with complex data and time flow while also lessening the need to resort to shared mutable state. It has many applications: interactive UIs and animation, client-server communication, robotics, IoT, sensor networks, etc.
## Why most.js for Reactive Programming?
### High performance
A primary focus of most.js is performance. The [perf test results](test/perf) indicate that it is achieving its goals in this area. Our hope is that by publishing those numbers, and showing what is possible, other libs will improve as well.
### Modular architecture
Most.js is highly modularized. It's internal Stream/Source/Sink architecture and APIs are simple, concise, and well defined. Combinators are implemented entirely in terms of that API, and don't need to use any private details. This makes it easy to implement new combinators externally (ie in contrib repos, for example) while also guaranteeing they can still be high performance.
### Simplicity
Aside from making combinators less "obviously correct", complexity can also lead to performace and maintainability issues. We felt a simple implementation would lead to a more stable and performant lib overall.
### Integration
Most.js integrates with language features, such as promises, iterators, generators, and *asynchronous* generators.
#### Promises
Promises are a natural compliment to asynchronous reactive streams. The relationship between synchronous "sequence" and "value" is clear, and the asynchronous analogue needs to be clear, too. By taking the notion of a sequence and a value and lifting them into the asynchronous world, it seems clear that reducing an asynchronous sequence should produce a promise. Hence, most.js uses promises when a single value is the natural synchronous analogue.
Most.js interoperates seamlessly with ES6 and Promises/A+ promises. For example, reducing a stream returns a promise for the final result:
```es6
import { from } from 'most'
// After 1 second, logs 10
from([1, 2, 3, 4])
.delay(1000)
.reduce((result, y) => result + y, 0)
.then(result => console.log(result))
```
You can also create a stream from a promise:
```es6
import { fromPromise } from 'most'
// Logs "hello"
fromPromise(Promise.resolve('hello'))
.observe(message => console.log(message))
```
#### Generators
Conceptually, [generators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*) allow you to write a function that acts like an iterable sequence. Generators support the standard ES6 [Iterator interface](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/The_Iterator_protocol), so they can be iterated over using ES6 standard `for of` or the iterator's `next()` API.
Most.js interoperates with ES6 generators and iterators. For example, you can create an event stream from any ES6 iterable:
```es6
import { from } from 'most'
function* allTheIntegers() {
let i=0
while(true) {
yield i++
}
}
// Log the first 100 integers
from(allTheIntegers())
.take(100)
.observe(x => console.log(x))
```
#### Asynchronous Generators
You can also create an event stream from an *asynchronous generator*, a generator that yields promises:
```es6
import { generate } from 'most'
function* allTheIntegers(interval) {
let i=0
while(true) {
yield delayPromise(interval, i++)
}
}
const delayPromise = (ms, value) =>
new Promise(resolve => setTimeout(() => resolve(value), ms))
// Log the first 100 integers, at 1 second intervals
generate(allTheIntegers, 1000)
.take(100)
.observe(x => console.log(x))
```