2.2.0 • Public • Published


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An implementation of an event


var Event = require("geval")
var document = require("global/document")
var clicks = Event(function (broadcast) {
    document.addEventListener("click", function (ev) {
var removeListener = clicks(function listener(ev) {
    console.log('click happened', ev)
// later you can call `removeListener()` to stop listening to events

What about dominictarr/observable ?

Both geval and observable having a similar interface.

  • thing(function (ev) { ... }) listens for new values.

The main difference is that geval is an Event. For discrete events it doesn't make sense to call thing() to get the current state. Events do not have a notion of current state.

So the "click" event doesn't have a .get() method because clicks do not have a notion of current state that makes sense

However you should not make an Event of the windows current width & height. You should make an observable instead which internally listens on the "resize" event and sets the correct new width & height.


EventEmitter's are complex. They are multiplexed events by default

Event is the simpler version of an EventEmitter

The main differences are:

  • just one event.
  • no implicit string based events
  • forces explicit interfaces with named properties that are Event's
  • no inheritance, you don't have to inherit from Event like you have to inherit from EventEmitter.
  • Event interface only has public listening functionality, this gives a clear seperation between broadcast and listen

Instead of something like

var EventEmitter = require('events').EventEmitter
var stream = new EventEmitter()
stream.on('data', onData)
stream.on('end', onEnd)
stream.on('close', onClose)

you can do:

var Event = require('geval')
var stream = {
  ondata: Event(function () { ... }),
  onend: Event(function () { ... }),
  onclose: Event(function () { ... })

Here the benefits are:

  • stream is an object of your shape and choice, you can call the properties whatever you want. the [[Prototype]] can be whatever you want.
  • stream has three well named properties that can be inspected statically or at run time which means the consumer knows exactly what type of events are available.
  • A consumer of stream could pass the ondata event to another object or module without also passing all other events along.
  • the ondata event is a concrete value. This allows for calling higher order functions on the value and enables various types of reactive programming techniques.
  • there are no special "error" semantics. There is no magic integration with domain or "uncaughtException".
  • there is no public emit() function on the stream interface It's impossible for the consumer to emit events that it should not be emitting, you know that all events that come out of ondata are coming from the actual stream implementation.


var removeListener = ev(function listener(value) {})

var Event = require("geval")
var ev = Event(...)
var removeListener = ev(function listener(value) {
  /* do something with the event value */
// call `removeListener()` when you are done with the `ev`.

A concrete ev is a function which you can pass a listener to. The listener you pass to ev will be called with an value each time an event occurs.

When calling ev with a listener it will return a removeListener function. You can call removeListener to remove your listener function from the event. After you call it your listener function will not be called with any future values coming from the event.

var ev = Event(function broadcaster(broadcast) {})

var Event = require("geval")
var ev = Event(function broadcaster(broadcast) {
  /* call broadcast with a value */

Event takes a broadcasting function and returns an event function.

The broadcasting function takes one argument, the broadcast function. The broadcaster can call broadcast each time it wants to make an event occur. Each time you call broadcast with a value, all listeners that are registered with ev will be invoked with the value


npm install geval


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