stream: fix Writable instanceof for subclasses

The current custom instanceof for `Writable` subclasses previously
returned false positives for instances of *other* subclasses of
`Writable` because it was inherited by these subclasses.

Fixes: https://github.com/nodejs/node/issues/14943
PR-URL: https://github.com/nodejs/node/pull/14945
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/_stream_writable.js b/lib/_stream_writable.js
index 6e0eaf45b5..7de77958d5 100644
--- a/lib/_stream_writable.js
+++ b/lib/_stream_writable.js
@@ -179,6 +179,8 @@ if (typeof Symbol === 'function' && Symbol.hasInstance) {
     value: function(object) {
       if (realHasInstance.call(this, object))
         return true;
+      if (this !== Writable)
+        return false;
 
       return object && object._writableState instanceof WritableState;
     }