timers: improvements to TimersList management

No longer use delete to remove keys on unrefed & refed lists,
instead simply set to undefined.

Move all the TimersList instantiation code into the constructor.

Compare values to undefined & null as appropriate instead of
truthy or falsy.

lib/timers.js

@@ -55,7 +55,7 @@ const activateImmediateCheck = process._activateImmediateCheck;
 delete process._activateImmediateCheck;
 
 // Timeout values > TIMEOUT_MAX are set to 1.
-const TIMEOUT_MAX = 2147483647; // 2^31-1
+const TIMEOUT_MAX = 2 ** 31 - 1;
 
 
 // HOW and WHY the timers implementation works the way it does.
@@ -173,9 +173,9 @@ function insert(item, unrefed) {
 
   // Use an existing list if there is one, otherwise we need to make a new one.
   var list = lists[msecs];
-  if (!list) {
+  if (list === undefined) {
     debug('no %d list was found in insert, creating a new one', msecs);
-    lists[msecs] = list = createTimersList(msecs, unrefed);
+    lists[msecs] = list = new TimersList(msecs, unrefed);
   }
 
   if (!item[async_id_symbol] || item._destroyed) {
@@ -194,28 +194,21 @@ function insert(item, unrefed) {
   assert(!L.isEmpty(list)); // list is not empty
 }
 
-function createTimersList(msecs, unrefed) {
-  // Make a new linked list of timers, and create a TimerWrap to schedule
-  // processing for the list.
-  const list = new TimersList(msecs, unrefed);
-  L.init(list);
-  list._timer._list = list;
-
-  if (unrefed === true) list._timer.unref();
-  list._timer.start(msecs);
-
-  list._timer[kOnTimeout] = listOnTimeout;
-
-  return list;
-}
-
 function TimersList(msecs, unrefed) {
-  this._idleNext = null; // Create the list with the linkedlist properties to
-  this._idlePrev = null; // prevent any unnecessary hidden class changes.
-  this._timer = new TimerWrap();
+  this._idleNext = this; // Create the list with the linkedlist properties to
+  this._idlePrev = this; // prevent any unnecessary hidden class changes.
   this._unrefed = unrefed;
   this.msecs = msecs;
   this.nextTick = false;
+
+  const timer = this._timer = new TimerWrap();
+  timer._list = this;
+
+  if (unrefed === true)
+    timer.unref();
+  timer.start(msecs);
+
+  timer[kOnTimeout] = listOnTimeout;
 }
 
 function listOnTimeout() {
@@ -359,7 +352,7 @@ function reuse(item) {
 
   var list = refedLists[item._idleTimeout];
   // if empty - reuse the watcher
-  if (list && L.isEmpty(list)) {
+  if (list !== undefined && L.isEmpty(list)) {
     debug('reuse hit');
     list._timer.stop();
     delete refedLists[item._idleTimeout];
@@ -382,7 +375,7 @@ const unenroll = exports.unenroll = function(item) {
   }
 
   var handle = reuse(item);
-  if (handle) {
+  if (handle !== null) {
     debug('unenroll: list empty');
     handle.close();
   }
@@ -610,7 +603,7 @@ Timeout.prototype.unref = function() {
     }
 
     var handle = reuse(this);
-    if (handle) {
+    if (handle !== null) {
       handle._list = undefined;
     }