napi: change napi_instanceof() to use Symbol.hasInstance

Re nodejs/node#11975 (comment)
Fixes nodejs#182
Closes nodejs#185

src/node_api.cc

@@ -2042,7 +2042,6 @@ napi_status napi_instanceof(napi_env e,
   *result = false;
 
   v8::Local<v8::Object> v8Cons;
-  v8::Local<v8::String> prototypeString;
   v8::Isolate* isolate = v8impl::V8IsolateFromJsEnv(e);
   v8::Local<v8::Context> context = isolate->GetCurrentContext();
 
@@ -2054,6 +2053,49 @@ napi_status napi_instanceof(napi_env e,
     return napi_set_last_error(napi_function_expected);
   }
 
+  napi_value value, key, jsRes;
+  napi_status status;
+  napi_valuetype valueType;
+
+  // Get "Symbol" from the global object
+  status = napi_get_global(e, &value);
+  if (status != napi_ok) return status;
+  status = napi_create_string_utf8(e, "Symbol", 6, &key);
+  if (status != napi_ok) return status;
+  status = napi_get_property(e, value, key, &value);
+  if (status != napi_ok) return status;
+  status = napi_get_type_of_value(e, value, &valueType);
+  if (status != napi_ok) return status;
+
+  // Get "hasInstance" from Symbol
+  if (valueType == napi_function) {
+    status = napi_create_string_utf8(e, "hasInstance", 11, &key);
+    if (status != napi_ok) return status;
+    status = napi_get_property(e, value, key, &value);
+    if (status != napi_ok) return status;
+    status = napi_get_type_of_value(e, value, &valueType);
+    if (status != napi_ok) return status;
+
+    // Retrieve the function at the Symbol(hasInstance) key of the constructor
+    if (valueType == napi_symbol) {
+      status = napi_get_property(e, constructor, value, &value);
+      if (status != napi_ok) return status;
+      status = napi_get_type_of_value(e, value, &valueType);
+
+      // Call the function to determine whether the object is an instance of the
+      // constructor
+      if (valueType == napi_function) {
+        status = napi_call_function(e, constructor, value, 1, &object, &jsRes);
+        if (status != napi_ok) return status;
+        return napi_get_value_bool(e, jsRes, result);
+      }
+    }
+  }
+
+  // If running constructor[Symbol.hasInstance](object) did not work, we perform
+  // a traditional instanceof (early Node.js 6.x).
+
+  v8::Local<v8::String> prototypeString;
   CHECK_NEW_FROM_UTF8(isolate, prototypeString, "prototype");
 
   auto maybe = v8Cons->Get(context, prototypeString);

test/addons-napi/test_instanceof/test.js

@@ -47,3 +47,41 @@ testFile(
     path.join(path.resolve(__dirname, '..', '..', '..',
                            'deps', 'v8', 'test', 'mjsunit'),
               'instanceof-2.js'));
+
+// We can only perform this test if we have a working Symbol.hasInstance
+if (typeof Symbol !== 'undefined' && 'hasInstance' in Symbol &&
+    typeof Symbol.hasInstance === 'symbol') {
+
+  function compareToNative( object, constructor ) {
+    return (addon.doInstanceOf( object, constructor) ===
+      (object instanceof constructor ));
+  }
+
+  var MyClass = function MyClass() {}
+  Object.defineProperty(MyClass, Symbol.hasInstance, {
+    value: function( candidate ) {
+      return 'mark' in candidate;
+    }
+  } );
+
+  var MySubClass = function MySubClass() {}
+  MySubClass.prototype = new MyClass();
+
+  var x = new MySubClass();
+  var y = new MySubClass();
+  x.mark = true;
+
+  compareToNative(x, MySubClass);
+  compareToNative(y, MySubClass);
+  compareToNative(x, MyClass);
+  compareToNative(y, MyClass);
+
+  x = new MyClass();
+  y = new MyClass();
+  x.mark = true;
+
+  compareToNative(x, MySubClass);
+  compareToNative(y, MySubClass);
+  compareToNative(x, MyClass);
+  compareToNative(y, MyClass);
+}