From 66187fa0445019cee8a4684870a7ea76e93de6df Mon Sep 17 00:00:00 2001
From: Anna Henningsen <anna@addaleax.net>
Date: Sat, 19 Aug 2017 18:29:25 +0200
Subject: [PATCH] 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>
---
 lib/_stream_writable.js                  | 2 ++
 test/parallel/test-stream-inheritance.js | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/lib/_stream_writable.js b/lib/_stream_writable.js
index 6cf7390b841a33..8251e8e219a519 100644
--- a/lib/_stream_writable.js
+++ b/lib/_stream_writable.js
@@ -144,6 +144,8 @@ 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;
     }
diff --git a/test/parallel/test-stream-inheritance.js b/test/parallel/test-stream-inheritance.js
index 2c6daed0385d4a..aefb39af512646 100644
--- a/test/parallel/test-stream-inheritance.js
+++ b/test/parallel/test-stream-inheritance.js
@@ -49,3 +49,8 @@ Object.setPrototypeOf(CustomWritable.prototype, Writable.prototype);
 new CustomWritable();
 
 assert.throws(CustomWritable, /AssertionError: undefined does not inherit from CustomWritable/);
+
+class OtherCustomWritable extends Writable {}
+
+assert(!(new OtherCustomWritable() instanceof CustomWritable));
+assert(!(new CustomWritable() instanceof OtherCustomWritable));