naomik naomik - 3 months ago 45
Javascript Question

How to use JavaScript EventTarget?

I would like to create a custom event emitter in my client-side programs. I am referencing this (sparse) documentation for EventTarget

My implementation attempt

var Emitter = function Emitter() {
EventTarget.call(this);
};

Emitter.prototype = Object.create(EventTarget.prototype, {
constructor: {
value: Emitter
}
});


My desired usage

var e = new Emitter();

e.addEventListener("hello", function() {
console.log("hello there!");
});

e.dispatchEvent(new Event("hello"));
// "hello there!"


Where it fails

var e = new Emitter();
// TypeError: Illegal constructor


What am I doing wrong?




Update

The following is possible, but it's a hack that depends on a dummy DOMElement

var fake = document.createElement("phony");
fake.addEventListener("hello", function() { console.log("hello there!"); });
fake.dispatchEvent(new Event("hello"));
// "hello there!"


I'd like to know how to do this without having to use the dummy element

Answer

I gave up on this awhile ago, but recently needed it again. Here's what I ended up using.

ES6

class Emitter {
  constructor() {
    var delegate = document.createDocumentFragment();
    [
      'addEventListener',
      'dispatchEvent',
      'removeEventListener'
    ].forEach(f =>
      this[f] = (...xs) => delegate[f](...xs)
    )
  }
}

// sample class to use Emitter
class Example extends Emitter {}

// run it
var e = new Example()
e.addEventListener('something', event => console.log(event))
e.dispatchEvent(new Event('something'))


ES5

function Emitter() {
  var eventTarget = document.createDocumentFragment()

  function delegate(method) {
    this[method] = eventTarget[method].bind(eventTarget)
  }

  [
    "addEventListener",
    "dispatchEvent",
    "removeEventListener"
  ].forEach(delegate, this)
}

// sample class to use it
function Example() {
  Emitter.call(this)
}

// run it
var e = new Example()

e.addEventListener("something", function(event) {
  console.log(event)
})

e.dispatchEvent(new Event("something"))

Yeah!


For those that need to support older versions of ecmascript, here you go

// IE < 9 compatible
function Emitter() {
  var eventTarget = document.createDocumentFragment();

  function addEventListener(type, listener, useCapture, wantsUntrusted) {
    return eventTarget.addEventListener(type, listener, useCapture, wantsUntrusted);
  }

  function dispatchEvent(event) {
    return eventTarget.dispatchEvent(event);
  }

  function removeEventListener(type, listener, useCapture) {
    return eventTarget.removeEventListener(type, listener, useCapture);
  }

  this.addEventListener = addEventListener;
  this.dispatchEvent = dispatchEvent;
  this.removeEventListener = removeEventListener;
}

The usage stays the same