From cbe2fce21a4bddbb623d3307c8f22f66fd7de0e4 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
---
 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 6e0eaf45b5464d..7de77958d56b4c 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;
     }
diff --git a/test/parallel/test-stream-inheritance.js b/test/parallel/test-stream-inheritance.js
index 2d9a1e83ef7528..a687ea9da054c9 100644
--- a/test/parallel/test-stream-inheritance.js
+++ b/test/parallel/test-stream-inheritance.js
@@ -56,3 +56,8 @@ common.expectsError(
     message: 'undefined does not inherit from CustomWritable'
   }
 );
+
+class OtherCustomWritable extends Writable {}
+
+assert(!(new OtherCustomWritable() instanceof CustomWritable));
+assert(!(new CustomWritable() instanceof OtherCustomWritable));