[react-devtools-shared] Added string type check for object name prop in getDisplayName function (#16798)

* [react-devtools-shared] Added string type check for object name prop in getDisplayName function from utils.js file; tests included;

* Re-added empty string check to getDisplayName()

* Tweaked tests to use real functions

This more closely simulates how the utility is being used in production, and would catch cases like anonymous functions (with empty string names).

packages/react-devtools-shared/src/__tests__/utils-test.js

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import {getDisplayName} from 'react-devtools-shared/src/utils';
+
+describe('utils', () => {
+  describe('getDisplayName', () => {
+    it('should return a function name', () => {
+      function FauxComponent() {}
+      expect(getDisplayName(FauxComponent)).toEqual('FauxComponent');
+    });
+
+    it('should return a displayName name if specified', () => {
+      function FauxComponent() {}
+      FauxComponent.displayName = 'OverrideDisplayName';
+      expect(getDisplayName(FauxComponent)).toEqual('OverrideDisplayName');
+    });
+
+    it('should return the fallback for anonymous functions', () => {
+      expect(getDisplayName(() => {}, 'Fallback')).toEqual('Fallback');
+    });
+
+    it('should return Anonymous for anonymous functions without a fallback', () => {
+      expect(getDisplayName(() => {})).toEqual('Anonymous');
+    });
+
+    // Simulate a reported bug:
+    // https://github.com/facebook/react/issues/16685
+    it('should return a fallback when the name prop is not a string', () => {
+      const FauxComponent = {name: {}};
+      expect(getDisplayName(FauxComponent, 'Fallback')).toEqual('Fallback');
+    });
+  });
+});

packages/react-devtools-shared/src/utils.js

@@ -46,17 +46,15 @@ export function getDisplayName(
     return nameFromCache;
   }
 
-  let displayName;
+  let displayName = fallbackName;
 
   // The displayName property is not guaranteed to be a string.
   // It's only safe to use for our purposes if it's a string.
   // github.com/facebook/react-devtools/issues/803
   if (typeof type.displayName === 'string') {
     displayName = type.displayName;
-  }
-
-  if (!displayName) {
-    displayName = type.name || fallbackName;
+  } else if (typeof type.name === 'string' && type.name !== '') {
+    displayName = type.name;
   }
 
   cachedDisplayNames.set(type, displayName);