Add a flag NODEJS_CATCH_REJECTION to control handling of unhandled rejections in node

src/settings.js

@@ -519,10 +519,18 @@ var EXCEPTION_CATCHING_WHITELIST = [];
 // longer do that, and exceptions work normally, which can be useful for libraries
 // or programs that don't need exit() to work.
 
-// For more explanations of this option, please visit
-// https://github.com/emscripten-core/emscripten/wiki/Asyncify
+// Emscripten uses an ExitStatus exception to halt when exit() is called.
+// With this option, we prevent that from showing up as an unhandled
+// exception.
 var NODEJS_CATCH_EXIT = 1;
 
+// Catch unhandled rejections in node. Without this, node may print the error,
+// and that this behavior will change in future node, wait a few seconds, and
+// then exit with 0 (which hides the error if you don't read the log). With
+// this, we catch any unhandled rejection and throw an actual error, which will
+// make the process exit immediately with a non-0 return code.
+var NODEJS_CATCH_REJECTION = 1;
+
 // Whether to enable asyncify transformation
 // This allows to inject some async functions to the C code that appear to be sync
 // e.g. emscripten_sleep

src/shell.js

@@ -167,9 +167,10 @@ if (ENVIRONMENT_IS_NODE) {
     }
   });
 #endif
-  // Currently node will swallow unhandled rejections, but this behavior is
-  // deprecated, and in the future it will exit with error status.
+
+#if NODEJS_CATCH_REJECTION
   process['on']('unhandledRejection', abort);
+#endif
 
   Module['quit'] = function(status) {
     process['exit'](status);