Monadic streams

Monadic streams for reactive programming

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.

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:

    <input class="x"> + <input class="y"> = <span class="result"></span>

Using most.js to make it reactive:

var most = require('most');
var xInput = document.querySelector('input.x');
var yInput = document.querySelector('input.y');
var resultNode = document.querySelector('.result');
exports.main = function() {
    // x represents the current value of xInput 
    var x = most.fromEvent('input', xInput).map(toNumber);
    // y represents the current value of yInput 
    var y = most.fromEvent('input', yInput).map(toNumber);
    // result is the live current value of adding x and y 
    var result = most.combine(add, x, y);
    // Observe the result value by rendering it to the resultNode 
function add(xy) {
    return x + y;
function toNumber(e) {
    return Number(;
function renderResult(result) {
    resultNode.textContent = result;

To run the example above and others using RaveJS: clone the repo into a web servable dir, cd examples/<name> && bower install, and load index.html in your browser.

npm install --save most or bower install --save most


var most = require('most');

Most.js streams are compatible with Promises/A+ and ES6 Promises. They also implement Fantasy Land Monoid, Functor, Applicative, and Monad.

Promises are another elegant and powerful data structure for composing asynchronous operations. Promises and reactive streams are clearly related in that they provide tools for managing asynchrony. However, they each have their strengths.

Promises deal with single, asynchronous, immutable values and provide operations for transforming them, and provide asynchronous error handling and flow control. Event streams represent sequences of asynchronous values or values that vary over time. They provide a similar, but typically broader, set of operations.

Most.js interoperates seamlessly with ES6 and Promises/A+ promises. In fact, it even uses promises internally. For example, reducing a stream returns a promise for the final result:

// After 4 seconds, logs 10 
most.from([1, 2, 3, 4])
    .reduce(function(resulty) {
        return result + y;
    }, 0)
    .then(function(result) {

You can also create a stream from a promise:

// Logs "hello" 
    .observe(function(message) {

Conceptually, generators allow you to write a function that acts like an iterable sequence. Generators support the standard ES6 Iterator interface, 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:

function* allTheIntegers() {
    let i=0;
    while(true) {
        yield i++;
// Log the first 100 integers 
    .observe(=> console.log(x));

You can also create an event stream from an asynchronous generator, a generator that yields promises:

function* allTheIntegers(interval) {
    let i=0;
    while(true) {
        yield delayPromise(interval, i++);
function delayPromise(msvalue) {
    return new Promise(resolve => setTimeout(() => resolve(value), ms));
// Log the first 100 integers, at 1 second intervals 
most.generate(allTheIntegers, 1000)
    .observe(=> console.log(x));