stream: proper `instanceof` for `Writable`s

Use `[Symbol.hasInstance]()` to return `true` when asking for
`new Duplex() instanceof Writable`.

PR-URL: https://github.com/nodejs/node/pull/8834
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Calvin Metcalf <calvin.metcalf@gmail.com>

1 files changed, 24 insertions, 3 deletions

diff --git a/lib/_stream_writable.js b/lib/_stream_writable.js
index 7a728c9de2..9e462a8f60 100644
--- a/lib/_stream_writable.js
+++ b/lib/_stream_writable.js
@@ -134,11 +134,32 @@ Object.defineProperty(WritableState.prototype, 'buffer', {
      'instead.')
 });
 
+// Test _writableState for inheritance to account for Duplex streams,
+// whose prototype chain only points to Readable.
+var realHasInstance;
+if (typeof Symbol === 'function' && Symbol.hasInstance) {
+  realHasInstance = Function.prototype[Symbol.hasInstance];
+  Object.defineProperty(Writable, Symbol.hasInstance, {
+    value: function(object) {
+      // Trying to move the `realHasInstance` from the Writable constructor
+      // to here will break the Node.js LazyTransform implementation.
+      return object._writableState instanceof WritableState;
+    }
+  });
+} else {
+  realHasInstance = function(object) {
+    return object instanceof this;
+  };
+}
+
 function Writable(options) {
-  // Writable ctor is applied to Duplexes, though they're not
-  // instanceof Writable, they're instanceof Readable.
-  if (!(this instanceof Writable) && !(this instanceof Stream.Duplex))
+  // Writable ctor is applied to Duplexes, too.
+  // `realHasInstance` is necessary because using plain `instanceof`
+  // would return false, as no `_writableState` property is attached.
+  if (!(realHasInstance.call(Writable, this)) &&
+      !(this instanceof Stream.Duplex)) {
     return new Writable(options);
+  }
 
   this._writableState = new WritableState(options, this);