solution

src/restoreNames.test.js

@@ -1,11 +1,59 @@
 'use strict';
 
 describe('restoreNames', () => {
-  // const { restoreNames } = require('./restoreNames');
-
-  it('should ', () => {
+  const { restoreNames } = require('./restoreNames');
 
+  it('should exist', () => {
+    expect(restoreNames).toBeInstanceOf(Function);
   });
 
   // write tests here
+
+  it('should not change existing names', () => {
+    const users = [
+      {
+        firstName: 'Jack',
+        lastName: 'Holy',
+        fullName: 'Jack Holy',
+      },
+      {
+        firstName: 'Mike',
+        lastName: 'Adams',
+        fullName: 'Mike Adams',
+      },
+    ];
+
+    restoreNames(users);
+
+    expect(users).toEqual([
+      {
+        firstName: 'Jack',
+        lastName: 'Holy',
+        fullName: 'Jack Holy',
+      },
+      {
+        firstName: 'Mike',
+        lastName: 'Adams',
+        fullName: 'Mike Adams',
+      },
+    ]);
+  });
+
+  it('should restore names', () => {
+    const users = [
+      {
+        firstName: undefined,
+        lastName: 'Holy',
+        fullName: 'Jack Holy',
+      },
+      {
+        lastName: 'Adams',
+        fullName: 'Mike Adams',
+      },
+    ];
+
+    restoreNames(users);
+    expect(users[0].firstName).toBe('Jack');
+    expect(users[1].firstName).toBe('Mike');
+  });
 });