timers: use Maps for internal duration pooling

Refs: nodejs#10925

lib/timers.js

@@ -37,14 +37,14 @@ const TIMEOUT_MAX = 2147483647; // 2^31-1
 // operations as close to constant-time as possible.
 // (So that performance is not impacted by the number of scheduled timers.)
 //
-// Object maps are kept which contain linked lists keyed by their duration in
+// Maps are kept which contain linked lists keyed by their duration in
 // milliseconds.
 // The linked lists within also have some meta-properties, one of which is a
 // TimerWrap C++ handle, which makes the call after the duration to process the
 // list it is attached to.
 //
 //
-// ╔════ > Object Map
+// ╔════ > Map
 // ║
 // ╠══
 // ║ refedLists: { '40': { }, '320': { etc } } (keys of millisecond duration)
@@ -85,18 +85,17 @@ const TIMEOUT_MAX = 2147483647; // 2^31-1
 // other alternative timers architectures.
 
 
-// Object maps containing linked lists of timers, keyed and sorted by their
+// Maps containing linked lists of timers, keyed and sorted by their
 // duration in milliseconds.
 //
-// The difference between these two objects is that the former contains timers
+// The difference between these two Maps is that the former contains timers
 // that will keep the process open if they are the only thing left, while the
 // latter will not.
 //
 // - key = time in milliseconds
 // - value = linked list
-const refedLists = Object.create(null);
-const unrefedLists = Object.create(null);
-
+const refedLists = new Map();
+const unrefedLists = new Map();
 
 // Schedule or re-schedule a timer.
 // The item must have been enroll()'d first.
@@ -125,10 +124,11 @@ function insert(item, unrefed) {
   const lists = unrefed === true ? unrefedLists : refedLists;
 
   // Use an existing list if there is one, otherwise we need to make a new one.
-  var list = lists[msecs];
+  var list = lists.get(msecs);
   if (!list) {
     debug('no %d list was found in insert, creating a new one', msecs);
-    lists[msecs] = list = createTimersList(msecs, unrefed);
+    list = createTimersList(msecs, unrefed);
+    lists.set(msecs, list);
   }
 
   L.append(list, item);
@@ -220,10 +220,10 @@ function listOnTimeout() {
   // Either refedLists[msecs] or unrefedLists[msecs] may have been removed and
   // recreated since the reference to `list` was created. Make sure they're
   // the same instance of the list before destroying.
-  if (list._unrefed === true && list === unrefedLists[msecs]) {
-    delete unrefedLists[msecs];
-  } else if (list === refedLists[msecs]) {
-    delete refedLists[msecs];
+  if (list._unrefed === true && list === unrefedLists.get(msecs)) {
+    unrefedLists.delete(msecs);
+  } else if (list === refedLists.get(msecs)) {
+    refedLists.delete(msecs);
   }
 }
 
@@ -265,12 +265,12 @@ function listOnTimeoutNT(list) {
 function reuse(item) {
   L.remove(item);
 
-  var list = refedLists[item._idleTimeout];
+  var list = refedLists.get(item._idleTimeout);
   // if empty - reuse the watcher
   if (list && L.isEmpty(list)) {
     debug('reuse hit');
     list._timer.stop();
-    delete refedLists[item._idleTimeout];
+    refedLists.delete(item._idleTimeout);
     return list._timer;
   }