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events: _events should be an own property #2046

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events: _events should be an own property #2046

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21 changes: 14 additions & 7 deletions lib/events.js
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,17 @@ EventEmitter.prototype._maxListeners = undefined;
// added to it. This is a useful default which helps finding memory leaks.
EventEmitter.defaultMaxListeners = 10;

function ensureEvents(emitter) {
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@yosuke-furukawa yosuke-furukawa Jun 24, 2015

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ensureXxx function should not have a side-effect (no modified state).
I feel initializeXxx is better or remove side-effects like:

function ensureEvents(emitter) {
  if (!emitter._events ||
      emitter._events === Object.getPrototypeOf(emitter)._events) {
    return false;
  }
  return true;
}

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@vkurchatkin vkurchatkin Jun 24, 2015

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Disagree. ensure implies idempotency, nothing more

if (!emitter._events ||
emitter._events === Object.getPrototypeOf(emitter)._events) {
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@bnoordhuis bnoordhuis Jun 24, 2015

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This won't work for code like below, I think? Instances of B won't have an ._events property on their immediate prototype.

function A() { EventEmitter.call(this); }
A.prototype = Object.create(EventEmitter.prototype);
A.prototype._events = {};

function B() { A.call(this); }
B.prototype = Object.create(A.prototype);

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@vkurchatkin vkurchatkin Jun 24, 2015

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@bnoordhuis in this example instances of A and B would have own _events because of super constructor calls. If you omit those it should still be fine. Object.getPrototypeOf(emitter)._events returns inherited _events if immediate prototype has none.

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@bnoordhuis bnoordhuis Jun 24, 2015

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Good point. I wonder though, what is the reason for not just using Object.hasOwnProperty(emitter, '_events')?

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@vkurchatkin vkurchatkin Jun 24, 2015

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It was a bit faster at the time

emitter._events = {};
emitter._eventsCount = 0;
return false;
}

return true;
}

EventEmitter.init = function() {
this.domain = null;
if (EventEmitter.usingDomains) {
Expand All @@ -30,10 +41,7 @@ EventEmitter.init = function() {
}
}

if (!this._events || this._events === Object.getPrototypeOf(this)._events) {
this._events = {};
this._eventsCount = 0;
}
ensureEvents(this);

this._maxListeners = this._maxListeners || undefined;
};
Expand Down Expand Up @@ -197,9 +205,8 @@ EventEmitter.prototype.addListener = function addListener(type, listener) {
throw new TypeError('listener must be a function');

events = this._events;
if (!events) {
events = this._events = {};
this._eventsCount = 0;
if (!ensureEvents(this)) {
events = this._events;
} else {
// To avoid recursion in the case that type === "newListener"! Before
// adding it to the listeners, first emit "newListener".
Expand Down
13 changes: 13 additions & 0 deletions test/parallel/test-event-emitter-subclass.js
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Expand Up @@ -35,6 +35,8 @@ process.on('exit', function() {
});


// Incorrect subclassing should work

function MyEE2() {
EventEmitter.call(this);
}
Expand All @@ -47,3 +49,14 @@ var ee2 = new MyEE2();
ee1.on('x', function() {});

assert.equal(EventEmitter.listenerCount(ee2, 'x'), 0);

function MyEE3() {}

MyEE3.prototype = new EventEmitter();

var ee1 = new MyEE3();
var ee2 = new MyEE3();

ee1.on('x', function() {});

assert.equal(EventEmitter.listenerCount(ee2, 'x'), 0);