diff --git a/src/auth/session.service.ts b/src/auth/session.service.ts
index d530135b..ab2e1cbf 100644
--- a/src/auth/session.service.ts
+++ b/src/auth/session.service.ts
@@ -74,6 +74,21 @@ export class SessionService {
     );
   }
 
+  private async getSessionIdByRefreshToken(
+    refreshToken: string,
+  ): Promise<number> {
+    const auth = this.authService.verify(refreshToken);
+    const permissions = auth.permissions;
+    if (permissions[0] == null || permissions.length > 1) {
+      throw new NotRefreshTokenError();
+    }
+    const resouceIds = permissions[0].authorizedResource.resourceIds;
+    if (resouceIds == null || resouceIds[0] == null || resouceIds.length > 1) {
+      throw new NotRefreshTokenError();
+    }
+    return resouceIds[0];
+  }
+
   // After refreshing the session, the old refresh token is revoked.
   // Returns:
   //     item1: A new refresh token.
@@ -83,15 +98,7 @@ export class SessionService {
     refreshTokenValidSeconds: number = this.defaultRefreshTokenValidSeconds,
     accessTokenValidSeconds: number = this.defaultAccessTokenValidSeconds,
   ): Promise<[string, string]> {
-    const auth = this.authService.verify(oldRefreshToken);
-    if (
-      auth.permissions.length !== 1 ||
-      auth.permissions[0].authorizedResource.resourceIds == undefined ||
-      auth.permissions[0].authorizedResource.resourceIds.length !== 1
-    ) {
-      throw new NotRefreshTokenError();
-    }
-    const sessionId = auth.permissions[0].authorizedResource.resourceIds[0];
+    const sessionId = await this.getSessionIdByRefreshToken(oldRefreshToken);
     this.authService.audit(
       oldRefreshToken,
       AuthorizedAction.other,
@@ -157,15 +164,7 @@ export class SessionService {
   }
 
   async revokeSession(refreshToken: string): Promise<void> {
-    const auth = this.authService.verify(refreshToken);
-    if (
-      auth.permissions.length !== 1 ||
-      auth.permissions[0].authorizedResource.resourceIds == undefined ||
-      auth.permissions[0].authorizedResource.resourceIds.length !== 1
-    ) {
-      throw new NotRefreshTokenError();
-    }
-    const sessionId = auth.permissions[0].authorizedResource.resourceIds[0];
+    const sessionId = await this.getSessionIdByRefreshToken(refreshToken);
     this.authService.audit(
       refreshToken,
       AuthorizedAction.other,
diff --git a/src/common/helper/page.helper.ts b/src/common/helper/page.helper.ts
index af2abfbc..27d8523e 100644
--- a/src/common/helper/page.helper.ts
+++ b/src/common/helper/page.helper.ts
@@ -110,7 +110,7 @@ export class PageHelper {
     prevStart: number,
     idGetter: (item: TData) => number,
   ): [TData[], PageRespondDto] {
-    if (data.length == 0 || pageSize < 0) {
+    if (data[0] == null || pageSize < 0) {
       return [
         [],
         {
diff --git a/src/users/users.controller.ts b/src/users/users.controller.ts
index 44027392..ea9f5ce3 100644
--- a/src/users/users.controller.ts
+++ b/src/users/users.controller.ts
@@ -159,13 +159,9 @@ export class UsersController {
       .json(data);
   }
 
-  @Post('/auth/refresh-token')
-  async refreshToken(
-    @Headers('cookie') cookieHeader: string,
-    @Res() res: Response,
-    @Ip() ip: string,
-    @Headers('User-Agent') userAgent: string,
-  ): Promise<Response> {
+  private async getRefreshTokenFromCookieHeader(
+    cookieHeader: string | undefined,
+  ): Promise<string> {
     if (cookieHeader == undefined) {
       throw new AuthenticationRequiredError();
     }
@@ -177,6 +173,21 @@ export class UsersController {
       throw new AuthenticationRequiredError();
     }
     const refreshToken = refreshTokenCookie.split('=')[1];
+    if (refreshToken == undefined) {
+      throw new AuthenticationRequiredError();
+    }
+    return refreshToken;
+  }
+
+  @Post('/auth/refresh-token')
+  async refreshToken(
+    @Headers('cookie') cookieHeader: string | undefined,
+    @Res() res: Response,
+    @Ip() ip: string,
+    @Headers('User-Agent') userAgent: string | undefined,
+  ): Promise<Response> {
+    const refreshToken =
+      await this.getRefreshTokenFromCookieHeader(cookieHeader);
     const [newRefreshToken, accessToken] =
       await this.sessionService.refreshSession(refreshToken);
     const newRefreshTokenExpire = new Date(
@@ -209,19 +220,10 @@ export class UsersController {
 
   @Post('/auth/logout')
   async logout(
-    @Headers('cookie') cookieHeader: string,
+    @Headers('cookie') cookieHeader: string | undefined,
   ): Promise<BaseRespondDto> {
-    if (cookieHeader == undefined) {
-      throw new AuthenticationRequiredError();
-    }
-    const cookies = cookieHeader.split(';').map((cookie) => cookie.trim());
-    const refreshTokenCookie = cookies.find((cookie) =>
-      cookie.startsWith('REFRESH_TOKEN='),
-    );
-    if (refreshTokenCookie == undefined) {
-      throw new AuthenticationRequiredError();
-    }
-    const refreshToken = refreshTokenCookie.split('=')[1];
+    const refreshToken =
+      await this.getRefreshTokenFromCookieHeader(cookieHeader);
     await this.sessionService.revokeSession(refreshToken);
     return {
       code: 201,
diff --git a/test/answer.e2e-spec.ts b/test/answer.e2e-spec.ts
index 6827b762..8dbdd087 100644
--- a/test/answer.e2e-spec.ts
+++ b/test/answer.e2e-spec.ts
@@ -34,7 +34,7 @@ describe('Answers Module', () => {
       .send({ email });
     expect(respond.status).toBe(201);
     const verificationCode = (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
     ).mock.calls.at(-1)[1];
     const respond2 = await request(app.getHttpServer())
       .post('/users')
@@ -60,17 +60,13 @@ describe('Answers Module', () => {
   }, 20000);
 
   beforeEach(() => {
+    const mockedEmailService = MockedEmailService.mock.instances[0]!;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.calls.length = 0;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.results.length = 0;
+    (mockedEmailService.sendPasswordResetEmail as jest.Mock).mock.calls.length =
+      0;
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.results.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      mockedEmailService.sendPasswordResetEmail as jest.Mock
     ).mock.results.length = 0;
   });
 
@@ -88,13 +84,13 @@ describe('Answers Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
diff --git a/test/comment.e2e-spec.ts b/test/comment.e2e-spec.ts
index 1f457fc1..afd232cb 100644
--- a/test/comment.e2e-spec.ts
+++ b/test/comment.e2e-spec.ts
@@ -27,13 +27,12 @@ describe('comments Module', () => {
   let auxAdminAccessToken: string;
   // eslint-disable-next-line @typescript-eslint/no-unused-vars
   let auxAdminUserDto: any;
-  const CommentIds: number[] = [];
-  const TopicIds: number[] = [];
+  const commentIds: number[] = [];
+  const topicIds: number[] = [];
   const questionIds: number[] = [];
-  const TestCommentPrefix = `G${Math.floor(Math.random() * 1000000)}`;
+  const testCommentPrefix = `G${Math.floor(Math.random() * 1000000)}`;
 
   async function createAuxiliaryUser(): Promise<[number, string]> {
-    // returns [userId, accessToken]
     const email = `test-${Math.floor(Math.random() * 10000000000)}@ruc.edu.cn`;
     const respond = await request(app.getHttpServer())
       .post('/users/verify/email')
@@ -41,7 +40,7 @@ describe('comments Module', () => {
       .send({ email });
     expect(respond.status).toBe(201);
     const verificationCode = (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
     ).mock.calls.at(-1)[1];
     const respond2 = await request(app.getHttpServer())
       .post('/users')
@@ -54,7 +53,7 @@ describe('comments Module', () => {
         emailCode: verificationCode,
       });
     expect(respond2.status).toBe(201);
-    return [respond2.body.data.user, respond2.body.data.accessToken];
+    return [respond2.body.data.user.id, respond2.body.data.accessToken];
   }
 
   beforeAll(async () => {
@@ -67,17 +66,13 @@ describe('comments Module', () => {
   }, 20000);
 
   beforeEach(() => {
+    const mockedEmailService = MockedEmailService.mock.instances[0]!;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.calls.length = 0;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.results.length = 0;
+    (mockedEmailService.sendPasswordResetEmail as jest.Mock).mock.calls.length =
+      0;
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.results.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      mockedEmailService.sendPasswordResetEmail as jest.Mock
     ).mock.results.length = 0;
   });
 
@@ -95,13 +90,13 @@ describe('comments Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
@@ -134,7 +129,7 @@ describe('comments Module', () => {
         expect(respond.body.message).toBe('OK');
         expect(respond.status).toBe(201);
         expect(respond.body.code).toBe(201);
-        TopicIds.push(respond.body.data.id);
+        topicIds.push(respond.body.data.id);
       }
       await createTopic('数学');
       await createTopic('哥德巴赫猜想');
@@ -152,7 +147,7 @@ describe('comments Module', () => {
             title: `${TestQuestionPrefix} ${title}`,
             content,
             type: 0,
-            topics: [TopicIds[0], TopicIds[1]],
+            topics: [topicIds[0], topicIds[1]],
           });
         expect(respond.body.message).toBe('Created');
         expect(respond.body.code).toBe(201);
@@ -183,7 +178,7 @@ describe('comments Module', () => {
           content:
             '哥德巴赫猜想又名1+1=2，而显然1+1=2是成立的，所以哥德巴赫猜想是成立的。',
           type: 0,
-          topics: [TopicIds[0], TopicIds[1]],
+          topics: [topicIds[0], topicIds[1]],
         });
       expect(respond.body.message).toMatch(/^AuthenticationRequiredError: /);
       expect(respond.body.code).toBe(401);
@@ -204,28 +199,31 @@ describe('comments Module', () => {
     });
     it('should create some comments', async () => {
       async function createComment(
-        commentableId: number,
+        commentableId: number | undefined,
         commentableType: 'comment' | 'question' | 'answer',
         content: string,
       ) {
+        if (commentableId == undefined) {
+          throw new Error('commentableId is undefined');
+        }
         const respond = await request(app.getHttpServer())
           .post(`/comments/${commentableType}/${commentableId}`)
           .set('Authorization', `Bearer ${TestToken}`)
           .send({
-            content: `${TestCommentPrefix} ${content}`,
+            content: `${testCommentPrefix} ${content}`,
           });
 
         expect(respond.body.message).toBe('Comment created successfully');
         expect(respond.body.code).toBe(201);
         expect(respond.status).toBe(201);
         expect(respond.body.data.id).toBeTruthy();
-        CommentIds.push(respond.body.data.id);
+        commentIds.push(respond.body.data.id);
       }
       await createComment(questionIds[0], 'question', 'zfgg好帅');
       await createComment(questionIds[1], 'question', 'zfggnb');
       await createComment(questionIds[2], 'question', 'zfgg???????');
       await createComment(questionIds[3], 'question', '宵宫!');
-      await createComment(CommentIds[0], 'comment', '啦啦啦德玛西亚');
+      await createComment(commentIds[0], 'comment', '啦啦啦德玛西亚');
     }, 80000);
     it('should not create comment due to invalid commentableId', async () => {
       const content = 'what you gonna to know?';
@@ -233,7 +231,7 @@ describe('comments Module', () => {
         .post(`/comments/question/114514`)
         .set('Authorization', `Bearer ${TestToken}`)
         .send({
-          content: `${TestCommentPrefix} ${content}`,
+          content: `${testCommentPrefix} ${content}`,
         });
 
       expect(respond.body.message).toMatch(/^CommentableIdNotFoundError: /);
@@ -245,7 +243,7 @@ describe('comments Module', () => {
       const respond = await request(app.getHttpServer())
         .post(`/comments/question/${questionIds[0]}/`)
         .send({
-          content: `${TestCommentPrefix} ${content}`,
+          content: `${testCommentPrefix} ${content}`,
         });
       expect(respond.body.message).toMatch(/^AuthenticationRequiredError: /);
       expect(respond.body.code).toBe(401);
@@ -296,7 +294,7 @@ describe('comments Module', () => {
   describe('get comment list by id', () => {
     it('should get comment by id', async () => {
       const respond = await request(app.getHttpServer())
-        .get(`/comments/${CommentIds[0]}`)
+        .get(`/comments/${commentIds[0]}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
         .send();
       expect(respond.body.message).toBe('Details are as follows');
@@ -314,14 +312,14 @@ describe('comments Module', () => {
     });
     it('should get comment by id', async () => {
       const respond = await request(app.getHttpServer())
-        .get(`/comments/${CommentIds[4]}`)
+        .get(`/comments/${commentIds[4]}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
         .send();
       expect(respond.body.message).toBe('Details are as follows');
       expect(respond.status).toBe(200);
       expect(respond.body.code).toBe(200);
       expect(respond.body.data.comment.id).toBeDefined();
-      expect(respond.body.data.comment.commentable_id).toBe(CommentIds[0]);
+      expect(respond.body.data.comment.commentable_id).toBe(commentIds[0]);
       expect(respond.body.data.comment.commentable_type).toBe('COMMENT');
       expect(respond.body.data.comment.content).toContain('啦啦啦德玛西亚');
       expect(respond.body.data.comment.user).toStrictEqual(TestUserDto);
@@ -343,7 +341,7 @@ describe('comments Module', () => {
 
   describe('AttitudeToComment', () => {
     it('should agree to a comment', async () => {
-      const commentId = CommentIds[0];
+      const commentId = commentIds[0];
       const respond = await request(app.getHttpServer())
         .put(`/comments/${commentId}/attitude`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -355,7 +353,7 @@ describe('comments Module', () => {
       expect(respond.body.code).toBe(200);
     });
     it('should agree to a comment', async () => {
-      const commentId = CommentIds[4];
+      const commentId = commentIds[4];
       const respond = await request(app.getHttpServer())
         .put(`/comments/${commentId}/attitude`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -368,7 +366,7 @@ describe('comments Module', () => {
     });
     it('should get some difference from others', async () => {
       const respond = await request(app.getHttpServer())
-        .get(`/comments/${CommentIds[0]}`)
+        .get(`/comments/${commentIds[0]}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
         .send();
       expect(respond.body.message).toBe('Details are as follows');
@@ -386,7 +384,7 @@ describe('comments Module', () => {
     });
     it('should get some difference from self', async () => {
       const respond = await request(app.getHttpServer())
-        .get(`/comments/${CommentIds[0]}`)
+        .get(`/comments/${commentIds[0]}`)
         .set('Authorization', `Bearer ${TestToken}`)
         .send();
       expect(respond.body.message).toBe('Details are as follows');
@@ -403,7 +401,7 @@ describe('comments Module', () => {
       expect(respond.body.data.comment.attitude_type).toBe('AGREE');
     });
     it('should disagree to a comment', async () => {
-      const commentId = CommentIds[0];
+      const commentId = commentIds[0];
       const respond = await request(app.getHttpServer())
         .put(`/comments/${commentId}/attitude`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -416,7 +414,7 @@ describe('comments Module', () => {
     });
     it('should get some difference from others', async () => {
       const respond = await request(app.getHttpServer())
-        .get(`/comments/${CommentIds[0]}`)
+        .get(`/comments/${commentIds[0]}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
         .send();
       expect(respond.body.message).toBe('Details are as follows');
@@ -434,7 +432,7 @@ describe('comments Module', () => {
     });
     it('should get some difference from self', async () => {
       const respond = await request(app.getHttpServer())
-        .get(`/comments/${CommentIds[0]}`)
+        .get(`/comments/${commentIds[0]}`)
         .set('Authorization', `Bearer ${TestToken}`)
         .send();
       expect(respond.body.message).toBe('Details are as follows');
@@ -451,7 +449,7 @@ describe('comments Module', () => {
       expect(respond.body.data.comment.attitude_type).toBe('DISAGREE');
     });
     it('should return to InvalidAttitudeTypeError', async () => {
-      const commentId = CommentIds[1];
+      const commentId = commentIds[1];
       const respond = await request(app.getHttpServer())
         .put(`/comments/${commentId}/attitude`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -471,7 +469,7 @@ describe('comments Module', () => {
       expect(respond.body.code).toBe(404);
     });
     it('should return AuthenticationRequiredError', async () => {
-      const commentId = CommentIds[1];
+      const commentId = commentIds[1];
       const respond = await request(app.getHttpServer())
         .put(`/comments/${commentId}/attitude`)
         .send({ attitude_type: 'disagree' });
@@ -482,7 +480,7 @@ describe('comments Module', () => {
 
   describe('deleteComment', () => {
     it('should delete a comment', async () => {
-      const commentId = CommentIds[1];
+      const commentId = commentIds[1];
       const respond = await request(app.getHttpServer())
         .delete(`/comments/${commentId}`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -492,7 +490,7 @@ describe('comments Module', () => {
       expect(respond.body.code).toBe(204);
     });
     it('should not delete a comment when the user does not match', async () => {
-      const commentId = CommentIds[0];
+      const commentId = commentIds[0];
       const respond = await request(app.getHttpServer())
         .delete(`/comments/${commentId}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -512,7 +510,7 @@ describe('comments Module', () => {
       expect(respond.body.code).toBe(404);
     });
     it('should return AuthenticationRequiredError', async () => {
-      const commentId = CommentIds[0];
+      const commentId = commentIds[0];
       const respond = await request(app.getHttpServer())
         .delete(`/comments/${commentId}`)
         .send();
@@ -520,7 +518,7 @@ describe('comments Module', () => {
       expect(respond.body.code).toBe(401);
     });
     it('should delete a comment', async () => {
-      const commentId = CommentIds[4];
+      const commentId = commentIds[4];
       const respond = await request(app.getHttpServer())
         .delete(`/comments/${commentId}`)
         .set('Authorization', `Bearer ${TestToken}`)
diff --git a/test/groups.e2e-spec.ts b/test/groups.e2e-spec.ts
index dbcedb07..e12ae110 100644
--- a/test/groups.e2e-spec.ts
+++ b/test/groups.e2e-spec.ts
@@ -31,11 +31,12 @@ describe('Groups Module', () => {
   // eslint-disable-next-line @typescript-eslint/no-unused-vars
   let auxAdminUserDto: any;
 
-  const GroupIds: number[] = [];
-  const TestGroupPrefix = `G${Math.floor(Math.random() * 1000000)}`;
+  const groupIds: number[] = [];
+  const testGroupPrefix = `G${Math.floor(Math.random() * 1000000)}`;
 
-  async function createAuxiliaryUser(): Promise<[number, string]> {
-    // returns [userId, accessToken]
+  // ! This function is not the same as the ones in other test files.
+  // ! Returns [userDto, accessToken]
+  async function createAuxiliaryUser(): Promise<[any, string]> {
     const email = `test-${Math.floor(Math.random() * 10000000000)}@ruc.edu.cn`;
     const respond = await request(app.getHttpServer())
       .post('/users/verify/email')
@@ -43,7 +44,7 @@ describe('Groups Module', () => {
       .send({ email });
     expect(respond.status).toBe(201);
     const verificationCode = (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
     ).mock.calls.at(-1)[1];
     const respond2 = await request(app.getHttpServer())
       .post('/users')
@@ -69,17 +70,13 @@ describe('Groups Module', () => {
   }, 20000);
 
   beforeEach(() => {
+    const mockedEmailService = MockedEmailService.mock.instances[0]!;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.calls.length = 0;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.results.length = 0;
+    (mockedEmailService.sendPasswordResetEmail as jest.Mock).mock.calls.length =
+      0;
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.results.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      mockedEmailService.sendPasswordResetEmail as jest.Mock
     ).mock.results.length = 0;
   });
 
@@ -97,13 +94,13 @@ describe('Groups Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
@@ -129,7 +126,7 @@ describe('Groups Module', () => {
         const respond = await request(app.getHttpServer())
           .post('/groups')
           .set('Authorization', `Bearer ${TestToken}`)
-          .send({ name: TestGroupPrefix + name, intro, avatar });
+          .send({ name: testGroupPrefix + name, intro, avatar });
         expect(respond.body.message).toBe('Group created successfully');
         expect(respond.body.code).toBe(201);
         expect(respond.status).toBe(201);
@@ -147,7 +144,7 @@ describe('Groups Module', () => {
         expect(groupDto.is_owner).toBe(true);
         expect(groupDto.is_public).toBe(true);
         expect(groupDto.intro).toBe(intro);
-        GroupIds.push(groupDto.id);
+        groupIds.push(groupDto.id);
       }
       await createGroup('数学之神膜膜喵', '不如原神', '🥸');
       await createGroup('ICS膜膜膜', 'pwb txdy!', '🐂');
@@ -358,7 +355,7 @@ describe('Groups Module', () => {
 
   describe('get group', () => {
     it('should get a group', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -382,7 +379,7 @@ describe('Groups Module', () => {
     });
 
     it('should get a group for another user', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -418,7 +415,10 @@ describe('Groups Module', () => {
 
   describe('join group', () => {
     it('should join a group', async () => {
-      async function joinGroup(groupId: number) {
+      async function joinGroup(groupId: number | undefined) {
+        if (groupId == null) {
+          throw new Error('groupId is undefined');
+        }
         const respond = await request(app.getHttpServer())
           .post(`/groups/${groupId}/members`)
           .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -427,11 +427,11 @@ describe('Groups Module', () => {
         expect(respond.status).toBe(201);
         expect(respond.body.code).toBe(201);
       }
-      await joinGroup(GroupIds[0]);
-      await joinGroup(GroupIds[1]);
+      await joinGroup(groupIds[0]);
+      await joinGroup(groupIds[1]);
     });
     it('should return a group with is_member true', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -463,7 +463,7 @@ describe('Groups Module', () => {
       expect(respond.body.code).toBe(404);
     });
     it('should return GroupAlreadyJoinedError when user is already in the group', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .post(`/groups/${TestGroupId}/members`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -476,12 +476,12 @@ describe('Groups Module', () => {
 
   describe('update group', () => {
     it('should update a group', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .put(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${TestToken}`)
         .send({
-          name: TestGroupPrefix + '关注幻城谢谢喵',
+          name: testGroupPrefix + '关注幻城谢谢喵',
           intro: '湾原审万德',
           avatar: '🤣',
         });
@@ -490,7 +490,7 @@ describe('Groups Module', () => {
       expect(respond.body.code).toBe(200);
     });
     it('should return a group with updated info from another user', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -517,7 +517,7 @@ describe('Groups Module', () => {
         .put('/groups/0')
         .set('Authorization', `Bearer ${TestToken}`)
         .send({
-          name: TestGroupPrefix + '关注幻城谢谢喵',
+          name: testGroupPrefix + '关注幻城谢谢喵',
           intro: '湾原审万德',
           avatar: '🤣',
         });
@@ -526,12 +526,12 @@ describe('Groups Module', () => {
       expect(respond.body.message).toMatch(/^GroupIdNotFoundError: /);
     });
     it('should return GroupNameAlreadyUsedError when group name is already used', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .put(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${TestToken}`)
         .send({
-          name: TestGroupPrefix + 'ICS膜膜膜',
+          name: testGroupPrefix + 'ICS膜膜膜',
           intro: '湾原审万德',
           avatar: '🤣',
         });
@@ -541,12 +541,12 @@ describe('Groups Module', () => {
     });
     // TODO: add permission control
     it('should return CannotDeleteGroupError when user is not the owner', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .put(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
         .send({
-          name: TestGroupPrefix + '关注幻城谢谢喵',
+          name: testGroupPrefix + '关注幻城谢谢喵',
           intro: '湾原审万德',
           avatar: '🤣',
         });
@@ -558,7 +558,7 @@ describe('Groups Module', () => {
 
   describe('leave group', () => {
     it('should leave a group', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .delete(`/groups/${TestGroupId}/members`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -568,7 +568,7 @@ describe('Groups Module', () => {
       expect(respond.body.code).toBe(200);
     });
     it('should return a group with is_member false', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -600,7 +600,7 @@ describe('Groups Module', () => {
       expect(respond.body.code).toBe(404);
     }, 10000);
     it('should return GroupNotJoinedError when user is not in the group', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .delete(`/groups/${TestGroupId}/members`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -614,7 +614,7 @@ describe('Groups Module', () => {
 
   describe('delete group', () => {
     it('should delete a group', async () => {
-      const TestGroupId = GroupIds[3];
+      const TestGroupId = groupIds[3];
       const respond = await request(app.getHttpServer())
         .delete(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -624,7 +624,7 @@ describe('Groups Module', () => {
       expect(respond.body.code).toBe(200);
     });
     it('should return GroupIdNotFoundError after deletion', async () => {
-      const TestGroupId = GroupIds[3];
+      const TestGroupId = groupIds[3];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -643,7 +643,7 @@ describe('Groups Module', () => {
       expect(respond.body.code).toBe(404);
     });
     it('should return CannotDeleteGroupError when user is not the owner', async () => {
-      const TestGroupId = GroupIds[1];
+      const TestGroupId = groupIds[1];
       const respond = await request(app.getHttpServer())
         .delete(`/groups/${TestGroupId}`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -656,7 +656,7 @@ describe('Groups Module', () => {
 
   describe('get group members', () => {
     it('should get group members', async () => {
-      const TestGroupId = GroupIds[1];
+      const TestGroupId = groupIds[1];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}/members`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -675,7 +675,7 @@ describe('Groups Module', () => {
       expect(respond.body.data.page.next_start).toBe(auxUserDto.id);
     });
     it('should get group members from a specific user', async () => {
-      const TestGroupId = GroupIds[1];
+      const TestGroupId = groupIds[1];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}/members`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -694,7 +694,7 @@ describe('Groups Module', () => {
       expect(respond.body.data.page.next_start).toBeFalsy();
     });
     it('should get group members from a specific user even quited', async () => {
-      const TestGroupId = GroupIds[0];
+      const TestGroupId = groupIds[0];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}/members`)
         .set('Authorization', `Bearer ${TestToken}`)
@@ -712,7 +712,7 @@ describe('Groups Module', () => {
       expect(respond.body.data.page.next_start).toBeFalsy();
     });
     it('should get group members for another user', async () => {
-      const TestGroupId = GroupIds[1];
+      const TestGroupId = groupIds[1];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}/members`)
         .set('Authorization', `Bearer ${auxAccessToken}`)
@@ -740,7 +740,7 @@ describe('Groups Module', () => {
       expect(respond.body.code).toBe(404);
     });
     it('should return empty list when page_size is not positive', async () => {
-      const TestGroupId = GroupIds[1];
+      const TestGroupId = groupIds[1];
       const respond = await request(app.getHttpServer())
         .get(`/groups/${TestGroupId}/members`)
         .set('Authorization', `Bearer ${TestToken}`)
diff --git a/test/question.e2e-spec.ts b/test/question.e2e-spec.ts
index 4215211e..a82bde51 100644
--- a/test/question.e2e-spec.ts
+++ b/test/question.e2e-spec.ts
@@ -32,7 +32,6 @@ describe('Questions Module', () => {
   let auxAccessToken: string;
 
   async function createAuxiliaryUser(): Promise<[number, string]> {
-    // returns [userId, accessToken]
     const email = `test-${Math.floor(Math.random() * 10000000000)}@ruc.edu.cn`;
     const respond = await request(app.getHttpServer())
       .post('/users/verify/email')
@@ -40,7 +39,7 @@ describe('Questions Module', () => {
       .send({ email });
     expect(respond.status).toBe(201);
     const verificationCode = (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
     ).mock.calls.at(-1)[1];
     const respond2 = await request(app.getHttpServer())
       .post('/users')
@@ -66,17 +65,13 @@ describe('Questions Module', () => {
   }, 20000);
 
   beforeEach(() => {
+    const mockedEmailService = MockedEmailService.mock.instances[0]!;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.calls.length = 0;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.results.length = 0;
+    (mockedEmailService.sendPasswordResetEmail as jest.Mock).mock.calls.length =
+      0;
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.results.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      mockedEmailService.sendPasswordResetEmail as jest.Mock
     ).mock.results.length = 0;
   });
 
@@ -94,13 +89,13 @@ describe('Questions Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
@@ -585,7 +580,10 @@ describe('Questions Module', () => {
       expect(respond4.body.data.page.next_start).toBe(0);
     });
     it('should unfollow questions', async () => {
-      async function unfollow(questionId: number) {
+      async function unfollow(questionId: number | undefined) {
+        if (questionId == undefined) {
+          throw new Error('questionId is undefined');
+        }
         const respond = await request(app.getHttpServer())
           .delete(`/questions/${questionId}/followers`)
           .set('Authorization', `Bearer ${TestToken}`)
diff --git a/test/topic.e2e-spec.ts b/test/topic.e2e-spec.ts
index 71fede9a..047831e8 100644
--- a/test/topic.e2e-spec.ts
+++ b/test/topic.e2e-spec.ts
@@ -35,17 +35,13 @@ describe('Topic Module', () => {
   }, 20000);
 
   beforeEach(() => {
+    const mockedEmailService = MockedEmailService.mock.instances[0]!;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.calls.length = 0;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.results.length = 0;
+    (mockedEmailService.sendPasswordResetEmail as jest.Mock).mock.calls.length =
+      0;
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.results.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      mockedEmailService.sendPasswordResetEmail as jest.Mock
     ).mock.results.length = 0;
   });
 
@@ -63,13 +59,13 @@ describe('Topic Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
diff --git a/test/user.e2e-spec.ts b/test/user.e2e-spec.ts
index d0373803..39944aea 100644
--- a/test/user.e2e-spec.ts
+++ b/test/user.e2e-spec.ts
@@ -34,17 +34,13 @@ describe('User Module', () => {
   }, 20000);
 
   beforeEach(() => {
+    const mockedEmailService = MockedEmailService.mock.instances[0]!;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.calls.length = 0;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.results.length = 0;
+    (mockedEmailService.sendPasswordResetEmail as jest.Mock).mock.calls.length =
+      0;
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.results.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      mockedEmailService.sendPasswordResetEmail as jest.Mock
     ).mock.results.length = 0;
   });
 
@@ -115,13 +111,13 @@ describe('User Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
 
       jest.advanceTimersByTime(11 * 60 * 1000);
@@ -157,13 +153,13 @@ describe('User Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
 
       jest.advanceTimersByTime(9 * 60 * 1000);
@@ -215,13 +211,13 @@ describe('User Module', () => {
       });
       expect(respond.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith('another-' + TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       return (
         request(app.getHttpServer())
@@ -284,13 +280,13 @@ describe('User Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith('another-' + TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
@@ -324,13 +320,13 @@ describe('User Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith('another-' + TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
@@ -362,13 +358,13 @@ describe('User Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith('another-' + TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
@@ -400,13 +396,13 @@ describe('User Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith('another-' + TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
@@ -443,12 +439,15 @@ describe('User Module', () => {
       expect(respond.body.code).toBe(201);
       expect(respond.body.data.user.username).toBe(TestUsername);
       expect(respond.body.data.user.nickname).toBe('test_user');
-      expect(respond.header['set-cookie'][0]).toMatch(
+      expect(respond.header['set-cookie']?.[0]).toMatch(
         /^REFRESH_TOKEN=.+; Path=\/users\/auth; Expires=.+; HttpOnly; SameSite=Strict$/,
       );
-      TestRefreshToken = respond.header['set-cookie'][0]
-        .split(';')[0]
-        .split('=')[1];
+      const TestRefreshToken2 = respond?.header?.['set-cookie']?.[0]
+        ?.split(';')?.[0]
+        ?.split('=')?.[1];
+      expect(TestRefreshToken2).toBeDefined();
+      TestRefreshToken = TestRefreshToken2!;
+
       expect(respond.body.data.accessToken).toBeDefined();
       TestToken = respond.body.data.accessToken;
     });
@@ -465,9 +464,12 @@ describe('User Module', () => {
       expect(respond2.body.code).toBe(201);
       expect(respond2.body.data.accessToken).toBeDefined();
       TestRefreshTokenOld = TestRefreshToken;
-      TestRefreshToken = respond2.header['set-cookie'][0]
-        .split(';')[0]
-        .split('=')[1];
+      const TestRefreshToken2 = respond2?.header?.['set-cookie']?.[0]
+        ?.split(';')?.[0]
+        ?.split('=')?.[1];
+      expect(TestRefreshToken2).toBeDefined();
+      TestRefreshToken = TestRefreshToken2!;
+
       TestToken = respond2.body.data.accessToken;
       expect(respond2.body.data.user.username).toBe(TestUsername);
       expect(respond2.body.data.user.nickname).toBe('test_user');
@@ -517,15 +519,17 @@ describe('User Module', () => {
       expect(respond2.body.data.accessToken).toBeDefined();
       expect(respond2.body.data.user.username).toBe(TestUsername);
       expect(respond2.body.data.user.nickname).toBe('test_user');
-      const refreshToken = respond2.header['set-cookie'][0]
-        .split(';')[0]
-        .split('=')[1];
+      const refreshToken = respond2?.header?.['set-cookie']?.[0]
+        ?.split(';')?.[0]
+        ?.split('=')?.[1];
+      expect(refreshToken).toBeDefined();
+
       jest.setSystemTime(Date.now() + 31 * 24 * 60 * 60 * 1000);
       const respond3 = await request(app.getHttpServer())
         .post('/users/auth/refresh-token')
         .set(
           'Cookie',
-          `some_cookie=12345;    REFRESH_TOKEN=${refreshToken};    other_cookie=value`,
+          `some_cookie=12345;    REFRESH_TOKEN=${refreshToken!};    other_cookie=value`,
         )
         .send();
       expect(respond3.body.message).toMatch(/^TokenExpiredError: /);
@@ -546,12 +550,14 @@ describe('User Module', () => {
       expect(respond.body.code).toBe(201);
       expect(respond.body.data.user.username).toBe(TestUsername);
       expect(respond.body.data.user.nickname).toBe('test_user');
-      expect(respond.header['set-cookie'][0]).toMatch(
+      expect(respond.header?.['set-cookie']?.[0]).toMatch(
         /^REFRESH_TOKEN=.+; Path=\/users\/auth; Expires=.+; HttpOnly; SameSite=Strict$/,
       );
-      TestRefreshToken = respond.header['set-cookie'][0]
-        .split(';')[0]
-        .split('=')[1];
+      const TestRefreshToken2 = respond?.header?.['set-cookie']?.[0]
+        ?.split(';')?.[0]
+        ?.split('=')?.[1];
+      expect(TestRefreshToken2).toBeDefined();
+      TestRefreshToken = TestRefreshToken2!;
       expect(respond.body.data.accessToken).toBeDefined();
       TestToken = respond.body.data.accessToken;
     });
@@ -571,9 +577,12 @@ describe('User Module', () => {
         expect(respond2.status).toBe(201);
         expect(respond2.body.code).toBe(201);
         expect(respond2.body.data.accessToken).toBeDefined();
-        refreshToken = respond2.header['set-cookie'][0]
-          .split(';')[0]
-          .split('=')[1];
+        const refreshToken2 = respond2?.header?.['set-cookie']?.[0]
+          ?.split(';')?.[0]
+          ?.split('=')?.[1];
+        expect(refreshToken2).toBeDefined();
+        refreshToken = refreshToken2!;
+
         expect(respond2.body.data.user.username).toBe(TestUsername);
         expect(respond2.body.data.user.nickname).toBe('test_user');
       }
@@ -603,12 +612,15 @@ describe('User Module', () => {
       expect(respond.body.code).toBe(201);
       expect(respond.body.data.user.username).toBe(TestUsername);
       expect(respond.body.data.user.nickname).toBe('test_user');
-      expect(respond.header['set-cookie'][0]).toMatch(
+      expect(respond?.header?.['set-cookie']?.[0]).toMatch(
         /^REFRESH_TOKEN=.+; Path=\/users\/auth; Expires=.+; HttpOnly; SameSite=Strict$/,
       );
-      TestRefreshToken = respond.header['set-cookie'][0]
-        .split(';')[0]
-        .split('=')[1];
+      const TestRefreshToken2 = respond?.header?.['set-cookie']?.[0]
+        ?.split(';')?.[0]
+        ?.split('=')?.[1];
+      expect(TestRefreshToken2).toBeDefined();
+      TestRefreshToken = TestRefreshToken2!;
+
       expect(respond.body.data.accessToken).toBeDefined();
       TestToken = respond.body.data.accessToken;
     });
@@ -625,9 +637,12 @@ describe('User Module', () => {
       expect(respond2.body.code).toBe(201);
       expect(respond2.body.data.accessToken).toBeDefined();
       TestRefreshTokenOld = TestRefreshToken;
-      TestRefreshToken = respond2.header['set-cookie'][0]
-        .split(';')[0]
-        .split('=')[1];
+      const TestRefreshToken2 = respond2?.header?.['set-cookie']?.[0]
+        ?.split(';')?.[0]
+        ?.split('=')?.[1];
+      expect(TestRefreshToken2).toBeDefined();
+      TestRefreshToken = TestRefreshToken2!;
+
       TestToken = respond2.body.data.accessToken;
       expect(respond2.body.data.user.username).toBe(TestUsername);
       expect(respond2.body.data.user.nickname).toBe('test_user');
@@ -771,13 +786,14 @@ describe('User Module', () => {
       expect(respond.body.code).toBe(201);
       expect(respond.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail,
+        MockedEmailService.mock.instances[0]?.sendPasswordResetEmail,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail,
+        MockedEmailService.mock.instances[0]?.sendPasswordResetEmail,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const token = (
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+        MockedEmailService.mock.instances[0]
+          ?.sendPasswordResetEmail as jest.Mock
       ).mock.calls[0][1];
 
       jest.advanceTimersByTime(9 * 60 * 1000);
@@ -834,13 +850,14 @@ describe('User Module', () => {
       expect(respond.body.code).toBe(201);
       expect(respond.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail,
+        MockedEmailService.mock.instances[0]?.sendPasswordResetEmail,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail,
+        MockedEmailService.mock.instances[0]?.sendPasswordResetEmail,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const token = (
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+        MockedEmailService.mock.instances[0]
+          ?.sendPasswordResetEmail as jest.Mock
       ).mock.calls[0][1];
 
       jest.advanceTimersByTime(11 * 60 * 1000);
@@ -872,13 +889,14 @@ describe('User Module', () => {
       expect(respond.body.code).toBe(201);
       expect(respond.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail,
+        MockedEmailService.mock.instances[0]?.sendPasswordResetEmail,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail,
+        MockedEmailService.mock.instances[0]?.sendPasswordResetEmail,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const token = (
-        MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+        MockedEmailService.mock.instances[0]
+          ?.sendPasswordResetEmail as jest.Mock
       ).mock.calls[0][1];
 
       jest.advanceTimersByTime(9 * 60 * 1000);
diff --git a/test/user.follow.e2e-spec.ts b/test/user.follow.e2e-spec.ts
index 6e90478c..52fdf9af 100644
--- a/test/user.follow.e2e-spec.ts
+++ b/test/user.follow.e2e-spec.ts
@@ -34,7 +34,7 @@ describe('Following Submodule of User Module', () => {
       .send({ email });
     expect(respond.status).toBe(201);
     const verificationCode = (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
     ).mock.calls.at(-1)[1];
     const respond2 = await request(app.getHttpServer())
       .post('/users')
@@ -61,16 +61,16 @@ describe('Following Submodule of User Module', () => {
 
   beforeEach(() => {
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
     ).mock.calls.length = 0;
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
     ).mock.results.length = 0;
     (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendPasswordResetEmail as jest.Mock
     ).mock.calls.length = 0;
     (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      MockedEmailService.mock.instances[0]?.sendPasswordResetEmail as jest.Mock
     ).mock.results.length = 0;
   });
 
@@ -88,13 +88,13 @@ describe('Following Submodule of User Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
@@ -136,6 +136,9 @@ describe('Following Submodule of User Module', () => {
     });
 
     it('should return UserIdNotFoundError', async () => {
+      if (tempUserIds[0] == null) {
+        throw new Error('tempUserIds[0] is undefined.');
+      }
       const respond = await request(app.getHttpServer())
         .post(`/users/${tempUserIds[0] + 1000000000}/followers`)
         //.set('User-Agent', 'PostmanRuntime/7.26.8')
diff --git a/test/user.profile.e2e-spec.ts b/test/user.profile.e2e-spec.ts
index 66a7c6bf..6e3d6709 100644
--- a/test/user.profile.e2e-spec.ts
+++ b/test/user.profile.e2e-spec.ts
@@ -33,17 +33,13 @@ describe('Profile Submodule of User Module', () => {
   }, 20000);
 
   beforeEach(() => {
+    const mockedEmailService = MockedEmailService.mock.instances[0]!;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.calls.length = 0;
+    (mockedEmailService.sendRegisterCode as jest.Mock).mock.results.length = 0;
+    (mockedEmailService.sendPasswordResetEmail as jest.Mock).mock.calls.length =
+      0;
     (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
-    ).mock.results.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
-    ).mock.calls.length = 0;
-    (
-      MockedEmailService.mock.instances[0].sendPasswordResetEmail as jest.Mock
+      mockedEmailService.sendPasswordResetEmail as jest.Mock
     ).mock.results.length = 0;
   });
 
@@ -61,13 +57,13 @@ describe('Profile Submodule of User Module', () => {
       });
       expect(respond1.status).toBe(201);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveReturnedTimes(1);
       expect(
-        MockedEmailService.mock.instances[0].sendRegisterCode,
+        MockedEmailService.mock.instances[0]?.sendRegisterCode,
       ).toHaveBeenCalledWith(TestEmail, expect.any(String));
       const verificationCode = (
-        MockedEmailService.mock.instances[0].sendRegisterCode as jest.Mock
+        MockedEmailService.mock.instances[0]?.sendRegisterCode as jest.Mock
       ).mock.calls[0][1];
       const req = request(app.getHttpServer())
         .post('/users')
diff --git a/tsconfig.json b/tsconfig.json
index 1110e975..47cdccdf 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -15,6 +15,7 @@
     "strictNullChecks": true,
     "strictPropertyInitialization": false,
     "noImplicitAny": true,
+    "noUncheckedIndexedAccess": true,
     "strictBindCallApply": false,
     "forceConsistentCasingInFileNames": true,
     "noFallthroughCasesInSwitch": false,