captain-hook

Configurable event emitter behavior for mixing into JavaScript objects/prototypes/classes

An event emitter API clearly defines interaction between separate pieces of code (e.g. main application vs. plugins). Event emitting allows you to keep the functionality of your application general (make it more suitable to be published Open Source), while external (perhaps even proprietary) code makes the application's behavior more specific.

Methods of your objects will be able to emit "events" to external "event handlers". External code can add event handlers via .on() and remove them via .off(), while your own object can call them via ._emit(). The names of these three methods can be explicitly configured via the factory function.

The name "Captain Hook" is a play on the term "Software Hook".

Why invent yet another event emitter?

• Attribute/method names are configurable
• Returns to the event emitter return values from event handlers as an array
• When adding event handlers, a supplied option object allows
  • sorting the handler according to given priority,
  • setting the this context of the handler,
  • setting of a tag/label of the handler.
• Event handlers can only be removed when their tag is known. Prevents interaction between subscribers.
• The storage object for event handlers and their options can be privately scoped if needed. This is to ensure that external plugins cannot remove or inspect each other's event handlers (privacy).
• Flexible use: add the mix-in to prototypes, plain objects, classes or to instances thereof (see below).
• No dependencies.
• Only ~100 lines of code.
• Only ~2.4 kilobytes minified.
• Works in browsers and in Node.js.
• Extensive tests.

The test file describes usage and features.

Development

Run tests:

npm test

Generate README.md with API documentation parsed from jsdoc sources:

node scripts/make_readme.cjs

How to use

The default export of the module is a factory function (see CaptainHook() in the API section).

The following 5 methods are equivalent in their effects.

1. Mix into prototypes

Methods will be shared across all instances.

If you prefer classes:

captain_hook = CaptainHook(); // use defaults

class Dog {
  constructor(name) {
    this.name = name;
  }
  poop() {
    console.log(`I am pooping.`)
    this._emit('poop');
  }
}

Object.assign(Dog.prototype, captain_hook);

luna = new Dog('Luna');
luna.on('poop', function() { console.log(`Cleaning up poop of ${this.name}`); } )
luna.poop();
// -> I am pooping
// -> Cleaning up poop of Luna

elvis = new Dog('Elvis');
elvis.on('poop', function() { console.log("Oh no, another dog pooped!"); })
elvis.poop();
// -> I am pooping
// -> Oh no, another dog pooped!

If you prefer prototype functions:

captain_hook = CaptainHook();

function Dog(name) {
  this.name = name;
}

Object.assign(Dog.prototype, captain_hook);

Dog.prototype.poop = function() {
  console.log(`I am pooping.`)
  this._emit('poop');
}

luna = new Dog('Luna');
luna.on('poop', function() { console.log(`Cleaning up poop of ${this.name}`); } )
luna.poop();
// -> I am pooping
// -> Cleaning up poop of Luna

elvis = new Dog('Elvis');
elvis.on('poop', function() { console.log("Oh no, another dog pooped!"); })
elvis.poop();
// -> I am pooping
// -> Oh no, another dog pooped!

If you prefer to work with plain objects:

captain_hook = CaptainHook();

proto_dog = {};
proto_dog.poop = function() {
  console.log(`I am pooping.`);
  this._emit('poop', this.name);
}

proto_eventful_dog = Object.assign(proto_dog, captain_hook);

// create a new object from a prototype
luna = Object.create(proto_eventful_dog);
luna.name = 'Luna';
luna.on('poop', function() { console.log(`Cleaning up poop of ${this.name}`); });
luna.poop();
// -> I am pooping
// -> Cleaning up poop of Luna

// create a new object from a prototype
elvis = Object.create(proto_eventful_dog);
elvis.name = 'Elvis';
elvis.on('poop', function() { console.log("Oh no, another dog pooped!"); });
elvis.poop();
// -> I am pooping
// -> Oh no, another dog pooped!

2. Mix into instances

Each instance will have a full copy of the attributes and methods.

In the example below, note that we pass the configuration handlers_prop: null. This makes the storage of the event handler functions truly private, preventing information leaks to external code.

class Dog {
  constructor(name) {
    var captain_hook = CaptainHook({handlers_prop: null});
    Object.assign(this, captain_hook);
    this.name = name;
  }
  poop() {
    console.log(`I am pooping.`)
    this._emit('poop');
  }
}

luna = new Dog('Luna');
luna.on('poop', function() { console.log(`Cleaning up poop of ${this.name}`); })
luna.poop();
// -> I am pooping
// -> Cleaning up poop of Luna

elvis = new Dog('Elvis');
elvis.on('poop', function() { console.log("Oh no, another dog pooped!"); })
elvis.poop();
// -> I am pooping
// -> Oh no, another dog pooped!

// Note that there is no way to read or modify the added event handlers via the `luna` or `elvis` instances.

If you prefer to work with plain objects:

dog = {};
dog.poop = function() {
  console.log(`I am pooping.`);
  this._emit('poop', this.name);
}

luna = Object.assign({}, CaptainHook({handlers_prop: null}), dog);
luna.name = 'Luna';
luna.on('poop', function() { console.log(`Cleaning up poop of ${this.name}`); });
luna.poop();
// -> I am pooping
// -> Cleaning up poop of Luna

elvis = Object.assign({}, CaptainHook({handlers_prop: null}), dog);
elvis.on('poop', function() { console.log("Oh no, another dog pooped!"); })
elvis.poop();
// -> I am pooping
// -> Oh no, another dog pooped!

// Note that there is no way to read or modify the added event handlers via the `luna` or `elvis` instances.

API Reference

Functions

CaptainHook([config]) ⇒ EventEmitter

Factory function which returns a plain object which implements event emission behavior.

The default method/property names for adding and removing handlers are on(), off(), and _emit(). The attached event handler functions are stored in a property named _handlers.

All property/method names can be explicitly configured via config to prevent naming conflicts with existing propety names of the target object.

AddEventHandler(eventname, callable, [options])

Associates an event handler function with an event name.

AddOneTimeEventHandler()

Same as AddEventHandler, but sets options.once to true.

RemoveEventHandler(eventname, tag)

Removes an event handler tagged with tag from eventname. If tag or eventname are not registered, this method does nothing.

EmitEvent(eventname, ...args) ⇒ Array

Calls all registered event handler callbacks with the provided arguments.

Note that return values of event handlers are not visible to any other event handler, which provides isolation between event handlers as well as privacy.

To implement content filtering, you need to pass the content by reference (i.e. to filter strings, you need to wrap the string into an object or array).

Typedefs

EventHandler ⇒ *

Callback function consuming an event.

EventEmitter : Object

Object implementing event emitter behavior which can be mixed into other objects, classes, or prototypes. See README for illustrations.

CaptainHook([config]) ⇒ EventEmitter

Factory function which returns a plain object which implements event emission behavior.

The default method/property names for adding and removing handlers are on(), off(), and _emit(). The attached event handler functions are stored in a property named _handlers.

All property/method names can be explicitly configured via config to prevent naming conflicts with existing propety names of the target object.

Kind: global function

Param	Type	Default	Description
[config]	Object	{}	Property names of the returned object.
[config.on_prop]	String	on	Method property name for setting event handlers.
[config.off_prop]	String	off	Method property name for removing event handlers.
[config.emit_prop]	String	_emit	Method property name for calling event handlers.
[config.handlers_prop]	String | null	_handlers	Property name for storage of event handlers. If null, a truly private storage will be used, in which case you need to mix the EventEmitter into every instance, otherwise event handlers would be shared across instances.

Example

let event_emitter = CaptainHook({
  on_prop:       'on',       // public
  off_prop:      'off',      // public
  emit_prop:     '_emit',    // pseudo-private
  handlers_prop: '_handlers' // pseudo-private
});

AddEventHandler(eventname, callable, [options])

Associates an event handler function with an event name.

Kind: global function

Param	Type	Default	Description
eventname	String
callable	EventHandler
[options]	Object	{}
[options.tag]	String		Name tag of handler
[options.priority]	Number	10	Execution order of handlers is sorted by this number. Higher priority will be sorted first.
[options.context]	Object		Value to use for the first argument of Function.call() when calling handler, changing the meaning of this inside handler. By default this in the handler is the object which emitted the event.
[options.once]	Boolean	false	When true then handler runs only once, then is removed.

AddOneTimeEventHandler()

Same as AddEventHandler, but sets options.once to true.

Kind: global function

RemoveEventHandler(eventname, tag)

Removes an event handler tagged with tag from eventname. If tag or eventname are not registered, this method does nothing.

Kind: global function

Param	Type
eventname	String
tag	String

EmitEvent(eventname, ...args) ⇒ Array

Calls all registered event handler callbacks with the provided arguments.

Note that return values of event handlers are not visible to any other event handler, which provides isolation between event handlers as well as privacy.

To implement content filtering, you need to pass the content by reference (i.e. to filter strings, you need to wrap the string into an object or array).

Kind: global function
Returns: Array - - Each return value of handlers concatenated.

Param	Type	Description
eventname	String
...args	*	Arguments to call the handler with.

EventHandler ⇒ *

Callback function consuming an event.

Kind: global typedef
Returns: * - Optionally returned values of attached EventHandlers are concatenated into an Array and passed back to the EmitEvent.

Param	Type	Description
...args	*	Arguments provided by the emitter of the event (EmitEvent).

EventEmitter : Object

Object implementing event emitter behavior which can be mixed into other objects, classes, or prototypes. See README for illustrations.

Kind: global typedef
Properties

Name	Type	Description
on	AddEventHandler	Associates an event handler function with an event name.
once	AddOneTimeEventHandler	Same as AddEventHandler, but sets options.once to true.
off	RemoveEventHandler	Remove an event handler.
_emit	EmitEvent	Call all registered event handlers.

Use cases

There are three distinct use cases for event handlers:

1. Simple callbacks (simple data type arguments, no return value)
2. Content filtering (arguments modified by reference, no return value)
3. Queries (with return value)

All three cases can be covered with the on() method.

To illustrate, we are going to implement a simple Cat:

var Cat = function() {
  var self = this; // be explicit

  // Generate the mix-in object with default property names
  var hook_mixin = CaptainHook();

  // Mix in the generated hook functionality.
  // This makes available to us self.on(), self.off(), self._emit()
  Object.assign(self, hook_mixin);

  self.makeSound = function() {
    var obj = {sound: 'meow'};
    self._emit('makeSound', obj);
    console.log(`I make sound: "${obj.sound}"`);
  };

  self.scratch = function() {
    var allowed = self._emit('scratch').reduce(function(acc, val) {
      return acc && val
    }, true);

    // All event handlers need to return true if this action is to be allowed.
    if (allowed) {
      console.log("Scratch!");
    } else {
      console.log("I am not allowed to scratch, so I won't do it!");
    }
  };

  self.beHungry = function() {
    Promise.all(self._emit('askForFood'))
    .then(function(given_foods) {
      console.log("I am eating", given_foods);
    })
  }
};

Instantiate the application:

var felix = new Cat();

Generic behavior:

felix.makeSound();
// -> I make sound: "meow"

felix.scratch();
// -> Scratch!

Use event handlers in three possible ways:

1. Simple observer (no return value, no content filtering):

felix.on('makeSound', function() {
  console.log("Felix is about to make a sound.")
});

felix.makeSound();

// -> Felix is about to make a sound.
// -> I make sound: "meow"

2. Filter content passed by reference (no return value):

felix.on('makeSound', function(opts) {
  opts.sound += ' hiss';
});

felix.makeSound();
// -> I make sound: "meow hiss"

3. Query responses. Note that event handlers do not have access to the return values of any other event handler. Here, we define two event handlers who vote for different outcomes:

felix.on('scratch', function() {
  return false; // I do not allow scratching.
});

felix.on('scratch', function() {
  return true; // I allow scratching.
});

felix.scratch();
// -> I am not allowed to scratch, so I won't do it!

This is also useful for Promises:

felix.on('askForFood', function() {
  console.log('Felix is asking for food');
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      console.log('I am giving felix food');
      resolve('dryfood');
    }, 1000);
  })
});

felix.on('askForFood', function() {
  console.log('Felix is asking for food');
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      console.log('I am giving felix food');
      resolve('sardines');
    }, 2000);
  })
});

felix.beHungry()
// after 2 seconds -> I am eating ["dryfood", "sardines"]