more of an aspiration than a claim

Cycle.js FAQ

Some Cycle.js FAQ notes I threw together and will try rearrange that might help people using Cycle.js. Some of these are notes copied from the Cycle.js Gitter channel.

Why does Cycle.js code often have $ symbols in variables

The $ convention is used to signify a variable that is an Observable.

Another not so standard idea is to import Observable as another name like $, eg

import { Observable as $ } from "rx";

Debugging Cycle.js

  • How to debug RxJS code
  • Adding a debugger; line to your code can be useful to break into your browser debug mode.
  • Logging RxJS (see below)

Logging for RxJS

var foo$ = bar$
  .do(x => console.log('foo$ emits ' + x));

or a prototype from @milankinen’s gist

Rx.Observable.prototype.log = function(prefix) {
  return "production" !== "development" ? this : this
    .doOnCompleted(() => console.log(prefix, "<completed>"))    // eslint-disable-line
    .do(x => console.log(prefix, x))                            // eslint-disable-line

Cycle.js Templating System

Cycle.js recommends using HyperScript Helpers, which I also prefer. There is also plain HyperScript and the React JSX support.

HyperScript and HyperScript Helpers

Hyperscript plain and Hyperscript helper conversion from HTML http://html-to-hyperscript.paqmind.com/


Redux concept in RxJS:

function store(action$) {
	return makeReducerStream(action$)
		.scan((state, reducer) => reducer(state), initialState)

What is Transposition in Cycle.js

@staltz Transposition is: a child of a vdom element can be an Observable<VElement> and at the end it will be flattened as one “video”.

Combinators in this article are the same concept.

Isolate, Children and Events

A really cool example by @laszlokorte on how events and isolation work with surrounding discussion on the TSERS Gitter channel.

function Foo() {
  return {
    DOM: O.just(h("button.foo", "..."))

function Bar(sources) {
  const click$ = DOM.select(".foo").events("click")
  const click2$ = DOM.select(".bar").events("click")

  const foo = isolate(Foo, "f1")(sources)
  const vdom$ = foo.DOM.map((foo) => div('.bar',[
  return {
    DOM: vdom$

click2$ will see the event that happens when the user clicks button.foo

Input Forms in Cycle.js

Here are some example repos with ideas on how to make input forms:

How go listen for keypress events on the whole app

Normally you listen to global keypresses with a driver as @TylorS explains here:

Cycle.run(main, {
  keypress$: () => Rx.Observable.fromEvent(document, 'keypress')

The reason DOM.select() doesn’t select from document, is because it’s external to your main(). Drivers are the place where you listen to what is external to your main().