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: nodejs#14943
addaleax · Aug 23, 2017 · cbe2fce · cbe2fce
1 parent 5e7c697
commit cbe2fce
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diff --git 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
@@ -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));