events: use bitset to save memory

PR-URL: #43700
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/event_target.js

@@ -394,6 +394,13 @@ function weakListeners() {
   return { registry: weakListenersState, map: objectToWeakListenerMap };
 }
 
+const kFlagOnce = 1 << 0;
+const kFlagCapture = 1 << 1;
+const kFlagPassive = 1 << 2;
+const kFlagNodeStyle = 1 << 3;
+const kFlagWeak = 1 << 4;
+const kFlagRemoved = 1 << 5;
+
 // The listeners for an EventTarget are maintained as a linked list.
 // Unfortunately, the way EventTarget is defined, listeners are accounted
 // using the tuple [handler,capture], and even if we don't actually make
@@ -409,13 +416,21 @@ class Listener {
       previous.next = this;
     this.previous = previous;
     this.listener = listener;
-    // TODO(benjamingr) these 4 can be 'flags' to save 3 slots
-    this.once = once;
-    this.capture = capture;
-    this.passive = passive;
-    this.isNodeStyleListener = isNodeStyleListener;
+
+    let flags = 0b0;
+    if (once)
+      flags |= kFlagOnce;
+    if (capture)
+      flags |= kFlagCapture;
+    if (passive)
+      flags |= kFlagPassive;
+    if (isNodeStyleListener)
+      flags |= kFlagNodeStyle;
+    if (weak)
+      flags |= kFlagWeak;
+    this.flags = flags;
+
     this.removed = false;
-    this.weak = Boolean(weak); // Don't retain the object
 
     if (this.weak) {
       this.callback = new SafeWeakRef(listener);
@@ -435,6 +450,31 @@ class Listener {
     }
   }
 
+  get once() {
+    return Boolean(this.flags & kFlagOnce);
+  }
+  get capture() {
+    return Boolean(this.flags & kFlagCapture);
+  }
+  get passive() {
+    return Boolean(this.flags & kFlagPassive);
+  }
+  get isNodeStyleListener() {
+    return Boolean(this.flags & kFlagNodeStyle);
+  }
+  get weak() {
+    return Boolean(this.flags & kFlagWeak);
+  }
+  get removed() {
+    return Boolean(this.flags & kFlagRemoved);
+  }
+  set removed(value) {
+    if (value)
+      this.flags |= kFlagRemoved;
+    else
+      this.flags &= ~kFlagRemoved;
+  }
+
   same(listener, capture) {
     const myListener = this.weak ? this.listener.deref() : this.listener;
     return myListener === listener && this.capture === capture;