process: refactor unhandled rejection handling

- Use constants instead of a dictionary and add comments
  about the behavior of each mode.
- Use switch cases to handle the unhandled rejection modes.
- Rename the run time value of the CLI option from `state`
  to `unhandledRejectionsMode`.
- Return in the call site of `emitWarning` when
  `--unhandled-rejections=none` instead of inside
  the function.

PR-URL: #28228
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/internal/process/promises.js

@@ -25,14 +25,24 @@ const pendingUnhandledRejections = [];
 const asyncHandledRejections = [];
 let lastPromiseId = 0;
 
-const states = {
-  none: 0,
-  warn: 1,
-  strict: 2,
-  default: 3
-};
-
-let state;
+// --unhandled-rejection=none:
+// Emit 'unhandledRejection', but do not emit any warning.
+const kIgnoreUnhandledRejections = 0;
+// --unhandled-rejection=warn:
+// Emit 'unhandledRejection', then emit 'UnhandledPromiseRejectionWarning'.
+const kAlwaysWarnUnhandledRejections = 1;
+// --unhandled-rejection=strict:
+// Emit 'uncaughtException'. If it's not handled, print the error to stderr
+// and exit the process.
+// Otherwise, emit 'unhandledRejection'. If 'unhandledRejection' is not
+// handled, emit 'UnhandledPromiseRejectionWarning'.
+const kThrowUnhandledRejections = 2;
+// --unhandled-rejection is unset:
+// Emit 'unhandledRejection', if it's handled, emit
+// 'UnhandledPromiseRejectionWarning', then emit deprecation warning.
+const kDefaultUnhandledRejections = 3;
+
+let unhandledRejectionsMode;
 
 function setHasRejectionToWarn(value) {
   tickInfo[kHasRejectionToWarn] = value ? 1 : 0;
@@ -42,10 +52,23 @@ function hasRejectionToWarn() {
   return tickInfo[kHasRejectionToWarn] === 1;
 }
 
+function getUnhandledRejectionsMode() {
+  const { getOptionValue } = require('internal/options');
+  switch (getOptionValue('--unhandled-rejections')) {
+    case 'none':
+      return kIgnoreUnhandledRejections;
+    case 'warn':
+      return kAlwaysWarnUnhandledRejections;
+    case 'strict':
+      return kThrowUnhandledRejections;
+    default:
+      return kDefaultUnhandledRejections;
+  }
+}
+
 function promiseRejectHandler(type, promise, reason) {
-  if (state === undefined) {
-    const { getOptionValue } = require('internal/options');
-    state = states[getOptionValue('--unhandled-rejections') || 'default'];
+  if (unhandledRejectionsMode === undefined) {
+    unhandledRejectionsMode = getUnhandledRejectionsMode();
   }
   switch (type) {
     case kPromiseRejectWithNoHandler:
@@ -104,9 +127,6 @@ function handledRejection(promise) {
 
 const unhandledRejectionErrName = 'UnhandledPromiseRejectionWarning';
 function emitWarning(uid, reason) {
-  if (state === states.none) {
-    return;
-  }
   const warning = getError(
     unhandledRejectionErrName,
     'Unhandled promise rejection. This error originated either by ' +
@@ -129,7 +149,8 @@ function emitWarning(uid, reason) {
 
 let deprecationWarned = false;
 function emitDeprecationWarning() {
-  if (state === states.default && !deprecationWarned) {
+  if (unhandledRejectionsMode === kDefaultUnhandledRejections &&
+      !deprecationWarned) {
     deprecationWarned = true;
     process.emitWarning(
       'Unhandled promise rejections are deprecated. In the future, ' +
@@ -161,13 +182,27 @@ function processPromiseRejections() {
     }
     promiseInfo.warned = true;
     const { reason, uid } = promiseInfo;
-    if (state === states.strict) {
-      fatalException(reason);
-    }
-    if (!process.emit('unhandledRejection', reason, promise) ||
-        // Always warn in case the user requested it.
-        state === states.warn) {
-      emitWarning(uid, reason);
+    switch (unhandledRejectionsMode) {
+      case kThrowUnhandledRejections: {
+        fatalException(reason);
+        const handled = process.emit('unhandledRejection', reason, promise);
+        if (!handled) emitWarning(uid, reason);
+        break;
+      }
+      case kIgnoreUnhandledRejections: {
+        process.emit('unhandledRejection', reason, promise);
+        break;
+      }
+      case kAlwaysWarnUnhandledRejections: {
+        process.emit('unhandledRejection', reason, promise);
+        emitWarning(uid, reason);
+        break;
+      }
+      case kDefaultUnhandledRejections: {
+        const handled = process.emit('unhandledRejection', reason, promise);
+        if (!handled) emitWarning(uid, reason);
+        break;
+      }
     }
     maybeScheduledTicksOrMicrotasks = true;
   }