events: do not keep arrays with a single listener

Use the remaining listener directly if the array of listeners has only
one element after running `EventEmitter.prototype.removeListener()`.
Advantages:
- Better memory usage and better performance if no new listeners are
added for the same event.
Disadvantages:
- A new array must be created if new listeners are added for the same
event.
PR-URL: #12043
Backport-PR-URL: #13796
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>

Author: lpinca

lib/events.js

@@ -338,7 +338,7 @@ EventEmitter.prototype.removeListener =
 }elseif(typeoflist!=='function'){
 position=-1;
 
-for(i=list.length;i-->0;){
+for(i=list.length-1;i>=0;i--){
 if(list[i]===listener||list[i].listener===listener){
 originalListener=list[i].listener;
 position=i;
@@ -350,15 +350,20 @@ EventEmitter.prototype.removeListener =
 returnthis;
 
 if(list.length===1){
-list[0]=undefined;
 if(--this._eventsCount===0){
 this._events=newEventHandlers();
 returnthis;
 }else{
 deleteevents[type];
 }
+}elseif(position===0){
+list.shift();
+if(list.length===1)
+events[type]=list[0];
 }else{
 spliceOne(list,position);
+if(list.length===1)
+events[type]=list[0];
 }
 
 if(events.removeListener)
@@ -370,7 +375,7 @@ EventEmitter.prototype.removeListener =
 
 EventEmitter.prototype.removeAllListeners=
 functionremoveAllListeners(type){
-varlisteners,events;
+varlisteners,events,i;
 
 events=this._events;
 if(!events)
@@ -393,7 +398,8 @@ EventEmitter.prototype.removeAllListeners =
 // emit removeListener for all listeners on all events
 if(arguments.length===0){
 varkeys=Object.keys(events);
-for(vari=0,key;i<keys.length;++i){
+varkey;
+for(i=0;i<keys.length;++i){
 key=keys[i];
 if(key==='removeListener')continue;
 this.removeAllListeners(key);
@@ -410,9 +416,9 @@ EventEmitter.prototype.removeAllListeners =
 this.removeListener(type,listeners);
 }elseif(listeners){
 // LIFO order
-do{
-this.removeListener(type,listeners[listeners.length-1]);
-}while(listeners[0]);
+for(i=listeners.length-1;i>=0;i--){
+this.removeListener(type,listeners[i]);
+}
 }
 
 returnthis;

test/parallel/test-event-emitter-remove-listeners.js

@@ -136,3 +136,20 @@ assert.throws(() => {
 conste=ee.removeListener('foo',listener);
 assert.strictEqual(e,ee);
 }
+
+{
+constee=newEventEmitter();
+
+ee.on('foo',listener1);
+ee.on('foo',listener2);
+assert.deepStrictEqual(ee.listeners('foo'),[listener1,listener2]);
+
+ee.removeListener('foo',listener1);
+assert.strictEqual(ee._events.foo,listener2);
+
+ee.on('foo',listener1);
+assert.deepStrictEqual(ee.listeners('foo'),[listener2,listener1]);
+
+ee.removeListener('foo',listener1);
+assert.strictEqual(ee._events.foo,listener2);
+}