diff --git a/test/auth.test.ts b/test/auth.test.ts
index 951b443..ed28bc1 100644
--- a/test/auth.test.ts
+++ b/test/auth.test.ts
@@ -1,14 +1,16 @@
 // Jest tests for ./src/auth.ts
 
 import fs from 'fs';
-import { google } from 'googleapis';
-import { OAuth2Client } from 'google-auth-library';
+import { authenticate } from '@google-cloud/local-auth';
+import { google, oauth2_v2 } from 'googleapis';
+import { OAuth2Client, JWT as JSONClient } from 'google-auth-library';
 
 import * as auth from '../src/auth';
 import { C2gError } from '../src/c2g-error';
 import { MESSAGES } from '../src/messages';
 
 jest.mock('fs');
+jest.mock('@google-cloud/local-auth');
 jest.mock('googleapis');
 jest.mock('google-auth-library');
 
@@ -169,10 +171,141 @@ describe('authorize', () => {
     // Arrange
     jest.spyOn(fs, 'existsSync').mockReturnValue(true); // isAuthorized
     jest.spyOn(fs, 'readFileSync').mockReturnValue(JSON.stringify(mockToken)); // loadSavedToken
-    jest.spyOn(google.auth, 'fromJSON').mockImplementation(); // loadSavedToken
+    jest
+      .spyOn(google.auth, 'fromJSON')
+      .mockImplementation(() => mockClient as unknown as JSONClient); // loadSavedToken
+    // Act
+    const result = await auth.authorize();
+    // Assert
+    expect(result).toEqual(mockClient);
+  });
+
+  it('should create and return a new OAuth2Client after running `authenticate` and `saveToken`', async () => {
+    // Arrange
+    jest
+      .spyOn(fs, 'existsSync')
+      .mockReturnValueOnce(true) // isAuthorized > fs.existsSync(CREDENTIALS_PATH)
+      .mockReturnValueOnce(false); // isAuthorized > fs.existsSync(TOKEN_PATH)
+    const mockAuthenticate = authenticate as jest.MockedFunction<
+      typeof authenticate
+    >;
+    mockAuthenticate.mockResolvedValue(mockClient);
+    jest
+      .spyOn(fs, 'readFileSync')
+      .mockReturnValue(JSON.stringify(mockCredentials)); // saveToken
+    jest.spyOn(fs, 'writeFileSync').mockImplementation(); // saveToken
     // Act
     const result = await auth.authorize();
     // Assert
-    expect(result).toEqual(mockToken);
+    expect(result).toEqual(mockClient);
+    expect(mockAuthenticate).toHaveBeenCalledTimes(1);
+    expect(fs.writeFileSync).toHaveBeenCalledTimes(1);
+    expect(fs.writeFileSync).toHaveBeenCalledWith(
+      auth.TOKEN_PATH,
+      JSON.stringify(mockToken),
+    );
+  });
+});
+
+describe('getUserEmail', () => {
+  afterEach(() => {
+    jest.restoreAllMocks();
+  });
+
+  it('should return null if the user is not authorized', async () => {
+    // Arrange
+    jest
+      .spyOn(fs, 'existsSync')
+      .mockReturnValueOnce(true) // isAuthorized > fs.existsSync(CREDENTIALS_PATH)
+      .mockReturnValueOnce(false); // isAuthorized > fs.existsSync(TOKEN_PATH)
+    // Act
+    const result = await auth.getUserEmail();
+    // Assert
+    expect(result).toBeNull();
+  });
+
+  it('should return the user email if the user is authorized', async () => {
+    // Arrange
+    const mockClient = {
+      credentials: {
+        access_token: 'mock-access-token',
+        refresh_token: 'mock-refresh-token',
+      },
+    } as unknown as OAuth2Client;
+    const mockCredentials: auth.Credentials = {
+      installed: {
+        client_id: 'mock-client-id',
+        client_secret: 'mock-client-secret',
+      } as unknown as auth.CredentialsKey,
+    };
+    const mockToken = {
+      type: 'authorized_user',
+      client_id: mockCredentials.installed?.client_id,
+      client_secret: mockCredentials.installed?.client_secret,
+      access_token: mockClient.credentials.access_token,
+      refresh_token: mockClient.credentials.refresh_token,
+    };
+    const mockUserEmail = 'mock@user.email.com';
+    jest.spyOn(fs, 'existsSync').mockReturnValue(true); // isAuthorized
+    jest.spyOn(fs, 'readFileSync').mockReturnValue(JSON.stringify(mockToken)); // loadSavedToken
+    jest
+      .spyOn(google.auth, 'fromJSON')
+      .mockImplementation(() => mockClient as unknown as JSONClient); // loadSavedToken
+    jest.spyOn(google, 'oauth2').mockImplementation(() => {
+      return {
+        userinfo: {
+          get: jest.fn().mockResolvedValue({
+            data: {
+              email: mockUserEmail,
+            },
+          }),
+        },
+      } as unknown as oauth2_v2.Oauth2;
+    });
+    // Act
+    const result = await auth.getUserEmail();
+    // Assert
+    expect(result).toEqual(mockUserEmail);
+  });
+
+  it('should return null if there is a error in retrieving user email', async () => {
+    // Arrange
+    jest.spyOn(google, 'oauth2').mockImplementation(() => {
+      return {
+        userinfo: {
+          get: jest.fn().mockRejectedValue(new Error('mock-error')),
+        },
+      } as unknown as oauth2_v2.Oauth2;
+    });
+    // Other arrangements are the same as the previous test:
+    // 'should return the user email if the user is authorized'
+    const mockClient = {
+      credentials: {
+        access_token: 'mock-access-token',
+        refresh_token: 'mock-refresh-token',
+      },
+    } as unknown as OAuth2Client;
+    const mockCredentials: auth.Credentials = {
+      installed: {
+        client_id: 'mock-client-id',
+        client_secret: 'mock-client-secret',
+      } as unknown as auth.CredentialsKey,
+    };
+    const mockToken = {
+      type: 'authorized_user',
+      client_id: mockCredentials.installed?.client_id,
+      client_secret: mockCredentials.installed?.client_secret,
+      access_token: mockClient.credentials.access_token,
+      refresh_token: mockClient.credentials.refresh_token,
+    };
+    jest.spyOn(fs, 'existsSync').mockReturnValue(true); // isAuthorized
+    jest.spyOn(fs, 'readFileSync').mockReturnValue(JSON.stringify(mockToken)); // loadSavedToken
+    jest
+      .spyOn(google.auth, 'fromJSON')
+      .mockImplementation(() => mockClient as unknown as JSONClient); // loadSavedToken
+    // Act
+    const result = await auth.getUserEmail();
+    // Assert
+    expect(result).toBeNull();
   });
 });