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A powerful, flexible and highly customizable event system for JavaScript. Supports all browsers and Node.js.
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There are many event systems for JavaScript, but most of them are simplistic and lack power features which are useful in more complex environments. The goal of flex-events is to be highly configurable and support:

Table of Contents
  1. Simple Usage
  2. Configuration
  3. Setup Method
  4. Event Methods
  5. Event Invocation Object
  6. Advanced Examples
  7. Implementation Notes

Simple Usage

This section provides the obligatory very simple example. Its usage will likely feel familiar. Skip to the Configuration or Advanced Examples sections if you're more interested in the advanced features.



flex-events can be installed using npm:

npm install flex-events

Then included using require:

var flexEvents = require('flex-events');
In a Browser:
<script src='[path to file]/flex-events.js'></script>


Setting up events on an object is as simple as calling setup

function ClassA () {


Call the attach method with an event name and a callback.

var a = new ClassA();
a.attach('testEvent', function (e, helloArg, worldArg) {
    console.log(helloArg + ' ' + worldArg);

The first argument sent to the callback is an Event Invocation object. If more than one argument is provided to the invoke method (below), they will be provided to the listener callbacks in the same order.


a.invoke('testEvent', "hello", "world");

When invoked, the listener from the previous section outputs: hello world


Sometimes it makes sense to declare an event hierarchy. This is done by sending a parent object to the setup method.

var b = {};
flexEvents.setup(b, a);

In this example we created a new object b and set its event parent to a. Now when we call:

b.invoke('testEvent', 'one', 'two');

the original event listener we setup on object a is still called because the event bubbles up.

Global Listening

The flexEvents object is, itself, an events-enabled object. In fact, it is the root of the event bubble chain, which means we can use it for listening for events globally.

flexEvents.attach('testEvent', function (e, arg1, arg2) {
    console.log('global ' + arg1 + ' ' + arg2);


Configuration is central to the flexibility of flex-events. Events can be configured on both the global, and the object level.

Configure Method

Allows the default global configuration settings to be overridden. If you decide to call this method, it can be called as many times as desired, but calling configure after the first usage of setup will have no effect and will produce a warning.

flexEvents.configure( config )

  • config : An object which overrides some, or all, of the default config values.

Default Configuration

We'll get into some examples later, but here's the default config values and what they mean:

//default config
var _config =
    arbitraryEvents: true,
    arbitraryInvoke: true,
    eventList: null,
    strict: false,
    methods: {
        attach: "attach",
        detach: "detach",
        hasEvent: "hasEvent",
        invoke: "invoke",
        register: false,
        registerList: false,
        deregister: false,
        destroy: false
    errorHandler: null,
    warningHandler: null,
    bubble: true

true {Boolean} If true, event names can be any arbitrary string. If false, all event names must match an item in the eventList. This may be helpful to prevent typos. See the Events List example.


true {Boolean} If true, events can be invoked by simply calling invoke. If false, events can only be invoked using the function returned from register, and, also, the invoke and registerList methods will be disabled. See the Private Invocation example.


null {Array|null} This can be null unless arbitraryEvents == false, in which case this should be an array of strings which represent the event names your system supports. See the Events List example.

Side note: You might wonder why arbitraryEvents is not simply an implied configuration value based on whether eventList is null or an array. There are two reasons it is not. The first is that arbitraryEvents can be helpful when configuring on the object (vs. global) level, because you could have a global event list, but only enforce it on specific objects. The second reason is that I would like to expand the warning system; maybe you want to be alerted if an event name is in the list, but don't want it to fail. The warning system could solve that.


false {Boolean} If true, only event names which have been explicitly registered on an object, or on a child in the object's event bubble chain, can be listened for (attached), detached, or invoked. If false, attaching or invoking an event will automatically register it. See the Strict Mode example.


(see default above) {Object} This setting allows you to decide which methods are publicly appended to the object for which events are being setup on. See Event Methods for a description of each available method, and see Custom Method Names for an example of how to use this configuration setting.


null {Function|null} When certain actions violate configuration requirements, errors will be created. This error is a javascript Error object and will be passed as the first argument to errorHandler if it is not null. If errorHandler is null, the error will be thrown, and will result in a crash if not caught. An error handler should probably be provided if non-default configuration values are used.

Current Error Messages

  • Arbitrary Events are disabled. [eventName] is not on the event list.
  • Events are configured with strict enabled, and [eventName] is not a registered event on this object.

null {Function|null} The same as errorHandler except the error messages are of lower importance, and if no warning handler is provided, the Error object will simply be discarded (it will not be thrown).

Current Warning Messages

  • Event parent has not been setup for events.
  • The configure method cannot be called after setup.

true {Boolean} If false, events will not bubble up.

Setup Method

Initializes an object so that events can be registered/attached/invoked/etc on the object.

flexEvents.setup( obj [, parent [, config ]] )

  • obj {Object} : The object to have events enabled on.
  • parent {Object} [optional] : The object's parent in the event chain.
  • config {Object} [optional] : If desired, some, or all, global configuration settings can be overridden on a per object basis.

Returns an Event Methods object.

Setup should never be called more than once on an object, since this may cause unexpected behavior. If you're unsure whether setup has been called, you can check for obj.__flexEvents.

Event Methods

The object returned by the flexEvents.setup() method contains the methods documented below. Some, or all, of these methods may also be appended directly onto the object (obj argument in setup), depending on the methods configuration item. See the Custom Method Names example.


Attaches an event listener.

.attach( eventName [, options], callback )

  • eventName {String} : The name of the event to listen for.
  • options {Object} [optional] : Allows the event listener to be customized. See the Options Syntax below.
  • callback {Function} : The function to be called when the event is invoked. The first argument will be an Event Invocation object. The following zero or more arguments will correspond to any extra arguments which were provided to the invoke method.
Options Syntax

The options object has three optional keys which each accept a boolean value:

  • once If true, the listener will be removed after the first time it is called.
  • bubbleOnly If true, the callback will only be called if the object being listened on did NOT invoke the event. In other words, it will be called if the event was the result of a bubble.
  • originOnly The opposite of bubbleOnly. If true, the callback will only be called for the object which invoked the event. The example below may help with clarification. Note that if both originOnly and bubbleOnly are true, the callback will never be called.

An unintended, but potentially useful, side-effect of the options object is that, because the options object becomes part of the Event Invocation object, it is accessible from inside the callback. Therefore, it could be used to send arbitrary information to the callback. Just be careful in your choice of key names in case a future version of this software adds meaning to a previously ignored key/value pair.

originOnly example
function listener (e) { console.log(arguments); }

//attach a listener with originOnly
a.attach('test', { originOnly: true }, listener);

a.invoke('test'); //this will cause 'listener' to be called

//setup another object with 'a' as its event parent
var b = {};
flexEvents.setup(b, a);

//invoking the same event on 'b' will not cause 'listener' to be called because originOnly is true


Detaches an event listener.

.detach( eventName [ [, options], callback] )

  • eventName {String} : The name of the event.
  • options {Object} [optional] : Same syntax as described in the attach method. Think of this as an additional search parameter. When provided, not only will the callback have to match, but the options values must match in order to successfully detach a listener.
  • callback {Function} [optional] : If provided, zero or one event listeners are removed depending on if a listener is found matching this callback function. If multiple listeners share the same callback, only the oldest matching listener will be detached. If callback is not provided, all listeners matching eventName will be removed.

When callback is provided, the function returns true if an event listener was removed, otherwise false. When callback is not provided, the function's output is the same as the hasEvent method.


Checks to see if an event is registered on an object. This method may be useful when operating in the strict configuration mode.

.hasEvent( eventName )

Returns true if the event is registered on the object, otherwise false.


Invokes (aka emits/triggers) an event.

.invoke( eventName [, arg1 [, arg2 [, ..., argN]]] )

  • eventName {String} : The name of the event to invoke.
  • arg1-argN [optional] : Additional arguments of any type can be passed to invoke, which will be, in turn, passed to all listener callbacks in the same order.

invoke is disabled when arbitraryInvoke is false.


method is private by default

Registers an event on an object. Calling register directly is typically unnecessary unless strict is set to true, or arbitraryInvoke is false.

.register( eventName )

Returns a function which is a shortcut for invoking this event. See the Private Invocation example.


method is private by default

Registers multiple events at once. This method accepts any combination of array(s) and string(s). It may be of particular value when strict is true.

// example
obj.registerList('eventOne', [ 'eventTwo', 'eventThree' ], 'eventFour', 'eventFive');

This method returns nothing, which is why it is disabled when arbitraryInvoke is false, since there would be no way to invoke the registered events.


method is private by default

Deregisters an event.

.deregister( eventName )

It should be very rare to need this method, and, should you decide to use it, you may find its usage to be counter-intuitive. Calling deregister does not necessarily remove event listeners, nor cause hasEvent to return false, nor prevent attach from being called for this eventName in strict mode. This is possible because of events which bubble up from children. When an event is registered on an object, it becomes implicitly registered on all objects in its parent chain. However, if no children have eventName registered, or when .deregister(eventName) is called on those children, then all listeners will be removed and the event will be fully deregistered as expected.


method is private by default

Essentially reverses the setup process by cleaning up all public methods appended onto the object and removing references which would prevent garbage collection from reclaiming the object. In a large application, it is very important to call destroy when the object is no longer needed.


Event Invocation Object

The first argument to listener callbacks is an EventInvocation object. For the examples below, we'll assume this argument is labeled e. It contains the following members:


e.origin {Object} A reference to the object which invoked the event.


e.obj {Object} A reference to the object being listened on. obj will be different from origin when the listener is called as the result of event bubbling. In other words, you can use if (e.origin === e.obj) to check whether the event originated on the object you attached to, or one of its children.


e.event {String} The name of the event being invoked.


e.options {Object} The options object supplied to the attach method. If no options argument was supplied, e.options will be an empty object {}.


e.stop() Prevents any further event listeners from being called for this event invocation. Similar to event.stopImmediatePropagation in DOM events.

See the Controlling Event Propagation example.


e.stopBubble() Stops the event from bubbling up any further, however, any remaining listeners on e.obj will still be called. Similar to event.stopPropagation in DOM events.


e.pause() Pause is provided to allow asynchronous propagation. After calling pause, event propagation will not continue until resume is called. See the Asynchronous Propagation example.


e.resume() Resume is the counterpart to pause. It can be important to understand that when calling resume synchronously, it behaves as if it is asynchronous, and when calling resume asynchronously, it behaves synchronously. This is hard to explain, so I'll illustrate by example:

Assume this simple script:

var a = {};

a.attach('test', callback); // we'll define two different versions of callback below
a.attach('test', function (e) { console.log("second callback done"); });

callback example 1 (synchronous resume)
function callback (e) {
    e.resume(); // resume called synchronously relative to the listener callback.
    console.log('first callback done'); // this will execute before any more event listeners are called.

/* Output
first callback done
second callback done
callback example 2 (asynchronous resume)
function callback (e) {
    setTimeout(function () {
        // calling resume asynchronously, relative to the listener callback, causes event propagation to resume synchronously.
        console.log('first callback done'); // further event listeners will have executed before reaching this line.
    }, 500 /* ms */);

/* Output
second callback done
first callback done

See also the Asynchronous Propagation example.

Advanced Examples

Each of these examples can be found in the examples directory. They are setup to be run as node.js scripts.

Private Invocation

Sometimes you may want to prevent .invoke('anyRandomEvent') from happening. You could use strict, but a safer way may be to use private invocation with arbitraryInvoke disabled.

flexEvents.configure({ arbitraryInvoke: false }); // disable arbitraryInvoke

function ClassA () {
    var _events = flexEvents.setup(this);

    // register an event so we can invoke it
    var _onTest = _events.register('test');

    this.privateInvoke = function () {
        _onTest("data"); // invoke the test event

var a = new ClassA();

/* a.invoke('test'); <-- this would crash because arbitraryInvoke is false. */

a.attach('test', function (e, arg1) { console.log(arg1); }); // attach event listener



Events List

If you'd like to establish a fixed list of events which your system supports (maybe you want a centralized list, or maybe it's just to prevent typos), you can do this using the arbitraryEvents and eventList settings.

flexEvents.configure({ arbitraryEvents: false, eventList: [ 'EVENT_1', 'EVENT_2' ], errorHandler: console.log });

var a = {};

a.invoke('EVENT_2'); // <-- this will succeed
a.invoke('EVENT_3'); // <-- this will fail because EVENT_3 is not in the eventList


[Error: Arbitrary Events are disabled. EVENT_3 is not on the event list.]

Strict Mode

When strict is enabled, events cannot be attached, detached, or invoked unless they are explicitly registered first.

flexEvents.configure({ strict: true, errorHandler: console.log, methods: { register: "register" } });

var a = {};

a.attach('test', function () { console.log(arguments); }); // <-- this will fail because 'test' has not been registered.

// register the event (note: 'register' is not normally a public method, notice the above configure statement)

a.attach('test', function () { console.log(arguments); }); // <-- this will succeed because 'test' has now been registered.


[Error: Events are configured with strict enabled, and test is not a registered event on this object.]

Custom Method Names

By default, attach, detach, hasEvent, and invoke are publicly appended to each object initialized using setup. However, any method listed under Event Methods can be made automatically public or private, and you can even rename them.

Important: Changing the accessibility or name of a method only affects how and what is publicly appended to the object. It does NOT effect the object returned by setup, which will always have the full set of methods with the default names.

In the example below, we will override some of the defaults by:

  • renaming attach to on
  • making detach a private method
  • making register a public method
flexEvents.configure({ methods: { attach: 'on', detach: false, register: 'register' } });

var a = {};

a.on('test', function (e, arg1) { console.log(arg1); });
a.invoke('test', 'renaming "attach" worked');

// a.detach('test'); // <-- this would crash because we made detach private

a.register('test2'); // <-- this will succeed because we made register public


renaming "attach" worked

Controlling Event Propagation

As long as bubble is enabled, event propagation normally occurs as follows:

  1. For a given eventName, all event listeners on a particular object are called in the order they were added.
  2. The event bubbles to the object's parent and the process repeats until there are no parents remaining in the bubble chain.

The flexEvents object is always the last parent in the bubble chain.

This propagation can be interrupted by calling e.stop or e.stopBubble.

var a = {};
a.attach('zzz', function (e) { console.log('zzz a'); });
a.attach('yyy', function (e) { console.log('yyy a'); });

var b = {};
flexEvents.setup(b, a);

// zzz 3 is going to stop propagation immediately
b.attach('zzz', function (e) {
    console.log('zzz b1');

// yyy 3 is going to stop bubbling
b.attach('yyy', function (e) {
    console.log('yyy b1');

b.attach('zzz', function (e) { console.log('zzz b2'); });
b.attach('yyy', function (e) { console.log('yyy b2'); });

var c = {};
flexEvents.setup(c, b);
c.attach('zzz', function (e) { console.log('zzz c'); });
c.attach('yyy', function (e) { console.log('yyy c'); });



zzz c
zzz b1

yyy c
yyy b1
yyy b2

Asynchronous Propagation

Sometimes an event callback needs to make some asynchronous calls, and perhaps you want to make the decision about whether to stop propagation based on the outcome of those asynchronous calls. This is what pause and resume are designed for.

var fs = require('fs');

var a = {};

a.attach('read', readFile);
a.attach('read', function (e) { console.log('second listener'); });


function readFile (e) {
    console.log('reading file');
    e.pause(); // pause propagation so no further listeners are called until we're done reading the file
    fs.readFile(__dirname + '/AsynchronousPropagation.js', 'utf8', function (err, file) {
        console.log('done reading');
        if (err)
            e.resume(); // resume propagation if there was no error


reading file
done reading
second listener

Output if e.pause(); is commented out

reading file
second listener
done reading

Object Configuration

Sometimes it may be desirable to override global configuration settings on individual objects. This can be accomplished using the third parameter of the setup method.

flexEvents.configure({ arbitraryEvents: false, eventList: [ 'EVENT_1', 'EVENT_2' ], errorHandler: console.log });

var a = {};
a.invoke('test'); // <-- this will fail because arbitraryEvents is false and 'test' is not in the eventList.

var b = {};
flexEvents.setup(b, null, { arbitraryEvents: true }); // override arbitraryEvents for this object only
b.invoke('test'); // <-- this will succeed because arbitraryEvents were re-enabled on this object


[Error: Arbitrary Events are disabled. test is not on the event list.]

Implementation Notes

There are a few last uncommon or non-obvious scenarios which should be addressed.

Passing a non-events-enabled object as the parent argument in setup

Consider this scenario:

var a = { data: 'test' };

var b = {};
flexEvents.setup(b, a); // pass 'a' as the event parent, event though it is not events-enabled

The above example will not cause an error. The bubble parent for b will implicitly become the global flexEvents object. If we later call flexEvents.setup(a), the bubble chain will now work as intended. However, it is very important to recognize that flexEvents holds an explicit reference to a in a "missing parents" collection until flexEvents.setup(a) is called. This will prevent a from being eligible for garbage collection, even if b.destroy() is called.

So, the moral of the story: don't pass in a parent which you don't intend to setup events on.

Configuring public methods on the flexEvents object

As previously discussed here and here, you can decide which event methods you would like to be public, and even what to name them. As you should know by now, the global flexEvents object is, itself, events-enabled, and therefore it should be possible to adjust its public methods as well. However, since flexEvents.setup(flexEvents) is called internally, we can't pass a custom config object, and all of the public methods are already setup before we could ever have a chance to adjust the global configuration.

To work around this, you can override the default methods configuration in the first call to configure. In addition to updating the global configuration, those changes will take effect on the flexEvents object. However, further calls to configure will only update the global config, and not the flexEvents object. This behavior is provided because you might want the configuration on the flexEvents object to be different from the global configuration.

There are two unique behaviors which are notable in this scenario:

  • destroy cannot be made public. methods:{destroy:'destroy'} will affect the global configuration, but will be ignored in regards to the flexEvents object.
  • setup and configure can be renamed (or even removed, although removing setup is probably a bad idea).

In this example, we will rename setup to init and make the invoke method public for all objects except flexEvents:

// rename 'setup' and disable 'invoke' on the flexEvents object
flexEvents.configure({ methods: { setup: 'init', invoke: false } });

// re-enable the 'invoke' method globally
flexEvents.configure({ methods: { invoke: 'invoke' } });

flexEvents.attach('test', function () { console.log('test'); });

var a = {};
flexEvents.init(a); // we renamed 'setup' to 'init', so we have to use that instead

a.invoke('test'); // <-- this will succeed because we re-enabled 'invoke' publicly

//flexEvents.invoke('test'); // <-- this would crash because 'invoke' was disabled in the first call to configure


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