Use better isPlainObject

Author: gaearon

src/utils/isPlainObject.js

@@ -1,3 +1,25 @@
+const fnToString = (fn) => Function.prototype.toString.call(fn);
+
+/**
+ * @param {any} obj The object to inspect.
+ * @returns {boolean} True if the argument appears to be a plain object.
+ */
 export default function isPlainObject(obj) {
-  return obj ? typeof obj === 'object' && Object.getPrototypeOf(obj) === Object.prototype : false;
+  if (!obj || typeof obj !== 'object') {
+    return false;
+  }
+
+  const proto = typeof obj.constructor === 'function' ?
+    Object.getPrototypeOf(obj) :
+    Object.prototype;
+
+  if (proto === null) {
+    return true;
+  }
+
+  const constructor = proto.constructor;
+
+  return typeof constructor === 'function'
+    && constructor instanceof constructor
+    && fnToString(constructor) === fnToString(Object);
 }