From d12c1d47ddd1b694c8c22800f256c8906a95d76d Mon Sep 17 00:00:00 2001
From: "Nicholas C. Zakas" <nicholas@humanwhocodes.com>
Date: Thu, 7 Jan 2021 14:07:16 -0800
Subject: [PATCH] Chore: Refactor rejection tracking and add comments

---
 src/rejection-tracker.js | 65 ++++++++++++++++++++++++++++++----------
 1 file changed, 49 insertions(+), 16 deletions(-)

diff --git a/src/rejection-tracker.js b/src/rejection-tracker.js
index 8f5289b..3ece67a 100644
--- a/src/rejection-tracker.js
+++ b/src/rejection-tracker.js
@@ -9,17 +9,15 @@ import { PledgeSymbol } from "./pledge-symbol";
 // https://html.spec.whatwg.org/multipage/webappapis.html#promiserejectionevent
 //-----------------------------------------------------------------------------
 
-const canceled = Symbol("canceled");
-
 class PledgeRejectionEvent {
     constructor(pledge, reason) {
         this.pledge = pledge;
         this.reason = reason;
-        this[canceled] = false;
+        this.returnValue = true;
     }
 
     preventDefault() {
-        this[canceled] = true;
+        this.returnValue = false;
     }
 }
 
@@ -28,23 +26,37 @@ export class RejectionTracker {
     constructor(logger = console) {
         
         /**
-         * This set keeps track of promises that were rejected but were not
-         * handled. All promises in this set will trigger an unhandledrejection
-         * event if they are still there during the monitoring phase of
-         * the rejection tracking.
+         * This set keeps track of pledges that were rejected but were not
+         * handled. All pledges in this set will trigger onUnhandledRejection
+         * if they are still there during the monitoring phase of the
+         * rejection tracking.
          * @type Set
          * @property aboutToBeNotified
          */
         this.aboutToBeNotified = new Set();
 
         /**
-         * This set keeps track of rejected promises that have been handled
-         * and will trigger a rejectionhandled event.
-         * @type Set
+         * This set keeps track of pledges that were unhandled, triggered
+         * onUnhandledRejection, and then had a rejection handler added. These
+         * pledges will trigger onRejectionHandled.
+         * @type WeakSet
          * @property outstandingRejections
          */
         this.outstandingRejections = new WeakSet();
+
+        /**
+         * A logger used to output unhandled pledge rejection notices. This
+         * can be overwritten for easier testing but otherwise is console.
+         * @type console
+         * @property logger
+         */
         this.logger = logger;
+
+        /**
+         * Tracks the interval identifier for monitoring pledges.
+         * @type int
+         * @property timeoutId
+         */
         this.timeoutId = 0;
     }
 
@@ -74,16 +86,22 @@ export class RejectionTracker {
                 const event = new PledgeRejectionEvent(p, p[PledgeSymbol.result]);
                 p.constructor.onUnhandledRejection(event);
 
-                const notHandled = !event[canceled];
-
+                /*
+                 * In the browser, "not handled" is a term that means both that
+                 * the promise hasn't been handled and that the event handler
+                 * did not cancel the event.
+                 */
+                const notHandled = event.returnValue;
+
+                /*
+                 * The onUnhandledRejection handler might have added a rejection
+                 * handler, so we need to double-check here.
+                 */
                 if (p[PledgeSymbol.isHandled] === false) {
                     this.outstandingRejections.add(p);
                 }
                 
                 if (notHandled) {
-                    
-                    // what to do here?
-
                     this.logger.error(`Pledge rejection was not caught: ${ p[PledgeSymbol.result] }`);
                 }
             }
@@ -97,20 +115,35 @@ export class RejectionTracker {
 
     track(pledge, operation) {
 
+        // the pledge has no rejection handler so we might need to log it
         if (operation === "reject") {
             this.aboutToBeNotified.add(pledge);
         }
 
+        // the pledge was unhandled but now is handled
         if (operation === "handle") {
+
+            /*
+             * If the pledge was going to trigger onUnhandledRejection, remove
+             * it from the list so that doesn't happen.
+             */
             if (this.aboutToBeNotified.has(pledge)) {
                 this.aboutToBeNotified.delete(pledge);
                 return;
             }
 
+            /*
+             * If the pledge isn't already flagged to trigger
+             * onRejectionHandled, then there's nothing else we need to do.
+             */
             if (!this.outstandingRejections.has(pledge)) {
                 return;
             }
             
+            /*
+             * If we made it here, the pledge was already flagged to trigger
+             * onRejectionHandled, so remove it and trigger the event.
+             */
             this.outstandingRejections.delete(pledge);
 
             setTimeout(() => {