diff --git a/backend/src/app.ts b/backend/src/app.ts
index c8658cd5..f4c941e5 100644
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -26,7 +26,7 @@ app.use('/api', apiRouter);
 // Swagger
 app.use('/api-docs', swaggerUi.serve, swaggerMiddleware);
 
-async function startServer() {
+async function startServer(): Promise<void> {
     await initORM();
 
     app.listen(port, () => {
diff --git a/backend/src/controllers/students.ts b/backend/src/controllers/students.ts
index 96cdd4dd..d71571e5 100644
--- a/backend/src/controllers/students.ts
+++ b/backend/src/controllers/students.ts
@@ -47,7 +47,7 @@ export async function getStudentHandler(req: Request, res: Response): Promise<vo
     res.status(201).json(user);
 }
 
-export async function createStudentHandler(req: Request, res: Response) {
+export async function createStudentHandler(req: Request, res: Response): Promise<void> {
     const userData = req.body as StudentDTO;
 
     if (!userData.username || !userData.firstName || !userData.lastName) {
@@ -61,7 +61,7 @@ export async function createStudentHandler(req: Request, res: Response) {
     res.status(201).json(newUser);
 }
 
-export async function deleteStudentHandler(req: Request, res: Response) {
+export async function deleteStudentHandler(req: Request, res: Response): Promise<void> {
     const username = req.params.username;
 
     if (!username) {
diff --git a/backend/src/controllers/submissions.ts b/backend/src/controllers/submissions.ts
index 1e66dbe9..cb8f2de2 100644
--- a/backend/src/controllers/submissions.ts
+++ b/backend/src/controllers/submissions.ts
@@ -30,7 +30,7 @@ export async function getSubmissionHandler(req: Request<SubmissionParams>, res:
     res.json(submission);
 }
 
-export async function createSubmissionHandler(req: Request, res: Response) {
+export async function createSubmissionHandler(req: Request, res: Response): Promise<void> {
     const submissionDTO = req.body as SubmissionDTO;
 
     const submission = await createSubmission(submissionDTO);
@@ -42,7 +42,7 @@ export async function createSubmissionHandler(req: Request, res: Response) {
     }
 }
 
-export async function deleteSubmissionHandler(req: Request, res: Response) {
+export async function deleteSubmissionHandler(req: Request, res: Response): Promise<void> {
     const hruid = req.params.hruid;
     const submissionNumber = +req.params.id;
 
diff --git a/backend/src/controllers/teachers.ts b/backend/src/controllers/teachers.ts
index 7bcbb5f4..e5f1fc87 100644
--- a/backend/src/controllers/teachers.ts
+++ b/backend/src/controllers/teachers.ts
@@ -50,7 +50,7 @@ export async function getTeacherHandler(req: Request, res: Response): Promise<vo
     res.status(201).json(user);
 }
 
-export async function createTeacherHandler(req: Request, res: Response) {
+export async function createTeacherHandler(req: Request, res: Response): Promise<void> {
     const userData = req.body as TeacherDTO;
 
     if (!userData.username || !userData.firstName || !userData.lastName) {
@@ -64,7 +64,7 @@ export async function createTeacherHandler(req: Request, res: Response) {
     res.status(201).json(newUser);
 }
 
-export async function deleteTeacherHandler(req: Request, res: Response) {
+export async function deleteTeacherHandler(req: Request, res: Response): Promise<void> {
     const username = req.params.username;
 
     if (!username) {
diff --git a/backend/src/controllers/themes.ts b/backend/src/controllers/themes.ts
index 61a1a834..1ef113cd 100644
--- a/backend/src/controllers/themes.ts
+++ b/backend/src/controllers/themes.ts
@@ -8,7 +8,7 @@ interface Translations {
     };
 }
 
-export function getThemes(req: Request, res: Response) {
+export function getThemes(req: Request, res: Response): void {
     const language = (req.query.language as string)?.toLowerCase() || 'nl';
     const translations = loadTranslations<Translations>(language);
     const themeList = themes.map((theme) => ({
@@ -21,7 +21,7 @@ export function getThemes(req: Request, res: Response) {
     res.json(themeList);
 }
 
-export function getThemeByTitle(req: Request, res: Response) {
+export function getThemeByTitle(req: Request, res: Response): void {
     const themeKey = req.params.theme;
     const theme = themes.find((t) => t.title === themeKey);
 
diff --git a/backend/src/controllers/users.ts b/backend/src/controllers/users.ts
index ac3f824b..7706e2ee 100644
--- a/backend/src/controllers/users.ts
+++ b/backend/src/controllers/users.ts
@@ -47,7 +47,7 @@ export async function getUserHandler<T extends User>(req: Request, res: Response
     }
 }
 
-export async function createUserHandler<T extends User>(req: Request, res: Response, service: UserService<T>, userClass: new () => T) {
+export async function createUserHandler<T extends User>(req: Request, res: Response, service: UserService<T>, userClass: new () => T): Promise<void> {
     try {
         getLogger().debug({ req: req });
         const userData = req.body as UserDTO;
@@ -67,7 +67,7 @@ export async function createUserHandler<T extends User>(req: Request, res: Respo
     }
 }
 
-export async function deleteUserHandler<T extends User>(req: Request, res: Response, service: UserService<T>) {
+export async function deleteUserHandler<T extends User>(req: Request, res: Response, service: UserService<T>): Promise<void> {
     try {
         const username = req.params.username;
 
diff --git a/backend/src/data/assignments/assignment-repository.ts b/backend/src/data/assignments/assignment-repository.ts
index c3c457d3..3de5031d 100644
--- a/backend/src/data/assignments/assignment-repository.ts
+++ b/backend/src/data/assignments/assignment-repository.ts
@@ -3,13 +3,13 @@ import { Assignment } from '../../entities/assignments/assignment.entity.js';
 import { Class } from '../../entities/classes/class.entity.js';
 
 export class AssignmentRepository extends DwengoEntityRepository<Assignment> {
-    public findByClassAndId(within: Class, id: number): Promise<Assignment | null> {
+    public async findByClassAndId(within: Class, id: number): Promise<Assignment | null> {
         return this.findOne({ within: within, id: id });
     }
-    public findAllAssignmentsInClass(within: Class): Promise<Assignment[]> {
+    public async findAllAssignmentsInClass(within: Class): Promise<Assignment[]> {
         return this.findAll({ where: { within: within } });
     }
-    public deleteByClassAndId(within: Class, id: number): Promise<void> {
+    public async deleteByClassAndId(within: Class, id: number): Promise<void> {
         return this.deleteWhere({ within: within, id: id });
     }
 }
diff --git a/backend/src/data/assignments/group-repository.ts b/backend/src/data/assignments/group-repository.ts
index eb1b09e2..f06080f7 100644
--- a/backend/src/data/assignments/group-repository.ts
+++ b/backend/src/data/assignments/group-repository.ts
@@ -4,7 +4,7 @@ import { Assignment } from '../../entities/assignments/assignment.entity.js';
 import { Student } from '../../entities/users/student.entity.js';
 
 export class GroupRepository extends DwengoEntityRepository<Group> {
-    public findByAssignmentAndGroupNumber(assignment: Assignment, groupNumber: number): Promise<Group | null> {
+    public async findByAssignmentAndGroupNumber(assignment: Assignment, groupNumber: number): Promise<Group | null> {
         return this.findOne(
             {
                 assignment: assignment,
@@ -13,16 +13,16 @@ export class GroupRepository extends DwengoEntityRepository<Group> {
             { populate: ['members'] }
         );
     }
-    public findAllGroupsForAssignment(assignment: Assignment): Promise<Group[]> {
+    public async findAllGroupsForAssignment(assignment: Assignment): Promise<Group[]> {
         return this.findAll({
             where: { assignment: assignment },
             populate: ['members'],
         });
     }
-    public findAllGroupsWithStudent(student: Student): Promise<Group[]> {
+    public async findAllGroupsWithStudent(student: Student): Promise<Group[]> {
         return this.find({ members: student }, { populate: ['members'] });
     }
-    public deleteByAssignmentAndGroupNumber(assignment: Assignment, groupNumber: number) {
+    public async deleteByAssignmentAndGroupNumber(assignment: Assignment, groupNumber: number): Promise<void> {
         return this.deleteWhere({
             assignment: assignment,
             groupNumber: groupNumber,
diff --git a/backend/src/data/assignments/submission-repository.ts b/backend/src/data/assignments/submission-repository.ts
index 251823fa..f5090adc 100644
--- a/backend/src/data/assignments/submission-repository.ts
+++ b/backend/src/data/assignments/submission-repository.ts
@@ -5,7 +5,10 @@ import { LearningObjectIdentifier } from '../../entities/content/learning-object
 import { Student } from '../../entities/users/student.entity.js';
 
 export class SubmissionRepository extends DwengoEntityRepository<Submission> {
-    public findSubmissionByLearningObjectAndSubmissionNumber(loId: LearningObjectIdentifier, submissionNumber: number): Promise<Submission | null> {
+    public async findSubmissionByLearningObjectAndSubmissionNumber(
+        loId: LearningObjectIdentifier,
+        submissionNumber: number
+    ): Promise<Submission | null> {
         return this.findOne({
             learningObjectHruid: loId.hruid,
             learningObjectLanguage: loId.language,
@@ -14,7 +17,7 @@ export class SubmissionRepository extends DwengoEntityRepository<Submission> {
         });
     }
 
-    public findMostRecentSubmissionForStudent(loId: LearningObjectIdentifier, submitter: Student): Promise<Submission | null> {
+    public async findMostRecentSubmissionForStudent(loId: LearningObjectIdentifier, submitter: Student): Promise<Submission | null> {
         return this.findOne(
             {
                 learningObjectHruid: loId.hruid,
@@ -26,7 +29,7 @@ export class SubmissionRepository extends DwengoEntityRepository<Submission> {
         );
     }
 
-    public findMostRecentSubmissionForGroup(loId: LearningObjectIdentifier, group: Group): Promise<Submission | null> {
+    public async findMostRecentSubmissionForGroup(loId: LearningObjectIdentifier, group: Group): Promise<Submission | null> {
         return this.findOne(
             {
                 learningObjectHruid: loId.hruid,
@@ -38,15 +41,15 @@ export class SubmissionRepository extends DwengoEntityRepository<Submission> {
         );
     }
 
-    public findAllSubmissionsForGroup(group: Group): Promise<Submission[]> {
+    public async findAllSubmissionsForGroup(group: Group): Promise<Submission[]> {
         return this.find({ onBehalfOf: group });
     }
 
-    public findAllSubmissionsForStudent(student: Student): Promise<Submission[]> {
+    public async findAllSubmissionsForStudent(student: Student): Promise<Submission[]> {
         return this.find({ submitter: student });
     }
 
-    public deleteSubmissionByLearningObjectAndSubmissionNumber(loId: LearningObjectIdentifier, submissionNumber: number): Promise<void> {
+    public async deleteSubmissionByLearningObjectAndSubmissionNumber(loId: LearningObjectIdentifier, submissionNumber: number): Promise<void> {
         return this.deleteWhere({
             learningObjectHruid: loId.hruid,
             learningObjectLanguage: loId.language,
diff --git a/backend/src/data/classes/class-join-request-repository.ts b/backend/src/data/classes/class-join-request-repository.ts
index c1443c1c..1cd0288c 100644
--- a/backend/src/data/classes/class-join-request-repository.ts
+++ b/backend/src/data/classes/class-join-request-repository.ts
@@ -4,13 +4,13 @@ import { ClassJoinRequest } from '../../entities/classes/class-join-request.enti
 import { Student } from '../../entities/users/student.entity.js';
 
 export class ClassJoinRequestRepository extends DwengoEntityRepository<ClassJoinRequest> {
-    public findAllRequestsBy(requester: Student): Promise<ClassJoinRequest[]> {
+    public async findAllRequestsBy(requester: Student): Promise<ClassJoinRequest[]> {
         return this.findAll({ where: { requester: requester } });
     }
-    public findAllOpenRequestsTo(clazz: Class): Promise<ClassJoinRequest[]> {
+    public async findAllOpenRequestsTo(clazz: Class): Promise<ClassJoinRequest[]> {
         return this.findAll({ where: { class: clazz } });
     }
-    public deleteBy(requester: Student, clazz: Class): Promise<void> {
+    public async deleteBy(requester: Student, clazz: Class): Promise<void> {
         return this.deleteWhere({ requester: requester, class: clazz });
     }
 }
diff --git a/backend/src/data/classes/class-repository.ts b/backend/src/data/classes/class-repository.ts
index 0ceed98e..f4e0723f 100644
--- a/backend/src/data/classes/class-repository.ts
+++ b/backend/src/data/classes/class-repository.ts
@@ -4,20 +4,20 @@ import { Student } from '../../entities/users/student.entity.js';
 import { Teacher } from '../../entities/users/teacher.entity';
 
 export class ClassRepository extends DwengoEntityRepository<Class> {
-    public findById(id: string): Promise<Class | null> {
+    public async findById(id: string): Promise<Class | null> {
         return this.findOne({ classId: id }, { populate: ['students', 'teachers'] });
     }
-    public deleteById(id: string): Promise<void> {
+    public async deleteById(id: string): Promise<void> {
         return this.deleteWhere({ classId: id });
     }
-    public findByStudent(student: Student): Promise<Class[]> {
+    public async findByStudent(student: Student): Promise<Class[]> {
         return this.find(
             { students: student },
             { populate: ['students', 'teachers'] } // Voegt student en teacher objecten toe
         );
     }
 
-    public findByTeacher(teacher: Teacher): Promise<Class[]> {
+    public async findByTeacher(teacher: Teacher): Promise<Class[]> {
         return this.find({ teachers: teacher }, { populate: ['students', 'teachers'] });
     }
 }
diff --git a/backend/src/data/classes/teacher-invitation-repository.ts b/backend/src/data/classes/teacher-invitation-repository.ts
index 6b94deec..ce059ca8 100644
--- a/backend/src/data/classes/teacher-invitation-repository.ts
+++ b/backend/src/data/classes/teacher-invitation-repository.ts
@@ -4,16 +4,16 @@ import { TeacherInvitation } from '../../entities/classes/teacher-invitation.ent
 import { Teacher } from '../../entities/users/teacher.entity.js';
 
 export class TeacherInvitationRepository extends DwengoEntityRepository<TeacherInvitation> {
-    public findAllInvitationsForClass(clazz: Class): Promise<TeacherInvitation[]> {
+    public async findAllInvitationsForClass(clazz: Class): Promise<TeacherInvitation[]> {
         return this.findAll({ where: { class: clazz } });
     }
-    public findAllInvitationsBy(sender: Teacher): Promise<TeacherInvitation[]> {
+    public async findAllInvitationsBy(sender: Teacher): Promise<TeacherInvitation[]> {
         return this.findAll({ where: { sender: sender } });
     }
-    public findAllInvitationsFor(receiver: Teacher): Promise<TeacherInvitation[]> {
+    public async findAllInvitationsFor(receiver: Teacher): Promise<TeacherInvitation[]> {
         return this.findAll({ where: { receiver: receiver } });
     }
-    public deleteBy(clazz: Class, sender: Teacher, receiver: Teacher): Promise<void> {
+    public async deleteBy(clazz: Class, sender: Teacher, receiver: Teacher): Promise<void> {
         return this.deleteWhere({
             sender: sender,
             receiver: receiver,
diff --git a/backend/src/data/content/attachment-repository.ts b/backend/src/data/content/attachment-repository.ts
index 95c5ab1c..73baa943 100644
--- a/backend/src/data/content/attachment-repository.ts
+++ b/backend/src/data/content/attachment-repository.ts
@@ -4,7 +4,7 @@ import { Language } from '../../entities/content/language';
 import { LearningObjectIdentifier } from '../../entities/content/learning-object-identifier';
 
 export class AttachmentRepository extends DwengoEntityRepository<Attachment> {
-    public findByLearningObjectIdAndName(learningObjectId: LearningObjectIdentifier, name: string): Promise<Attachment | null> {
+    public async findByLearningObjectIdAndName(learningObjectId: LearningObjectIdentifier, name: string): Promise<Attachment | null> {
         return this.findOne({
             learningObject: {
                 hruid: learningObjectId.hruid,
@@ -15,7 +15,11 @@ export class AttachmentRepository extends DwengoEntityRepository<Attachment> {
         });
     }
 
-    public findByMostRecentVersionOfLearningObjectAndName(hruid: string, language: Language, attachmentName: string): Promise<Attachment | null> {
+    public async findByMostRecentVersionOfLearningObjectAndName(
+        hruid: string,
+        language: Language,
+        attachmentName: string
+    ): Promise<Attachment | null> {
         return this.findOne(
             {
                 learningObject: {
diff --git a/backend/src/data/content/learning-object-repository.ts b/backend/src/data/content/learning-object-repository.ts
index 49b4c536..4684c6cc 100644
--- a/backend/src/data/content/learning-object-repository.ts
+++ b/backend/src/data/content/learning-object-repository.ts
@@ -5,7 +5,7 @@ import { Language } from '../../entities/content/language.js';
 import { Teacher } from '../../entities/users/teacher.entity.js';
 
 export class LearningObjectRepository extends DwengoEntityRepository<LearningObject> {
-    public findByIdentifier(identifier: LearningObjectIdentifier): Promise<LearningObject | null> {
+    public async findByIdentifier(identifier: LearningObjectIdentifier): Promise<LearningObject | null> {
         return this.findOne(
             {
                 hruid: identifier.hruid,
@@ -18,7 +18,7 @@ export class LearningObjectRepository extends DwengoEntityRepository<LearningObj
         );
     }
 
-    public findLatestByHruidAndLanguage(hruid: string, language: Language) {
+    public async findLatestByHruidAndLanguage(hruid: string, language: Language): Promise<LearningObject | null> {
         return this.findOne(
             {
                 hruid: hruid,
@@ -33,7 +33,7 @@ export class LearningObjectRepository extends DwengoEntityRepository<LearningObj
         );
     }
 
-    public findAllByTeacher(teacher: Teacher): Promise<LearningObject[]> {
+    public async findAllByTeacher(teacher: Teacher): Promise<LearningObject[]> {
         return this.find(
             { admins: teacher },
             { populate: ['admins'] } // Make sure to load admin relations
diff --git a/backend/src/data/content/learning-path-repository.ts b/backend/src/data/content/learning-path-repository.ts
index a2f9b47e..e34508ec 100644
--- a/backend/src/data/content/learning-path-repository.ts
+++ b/backend/src/data/content/learning-path-repository.ts
@@ -3,7 +3,7 @@ import { LearningPath } from '../../entities/content/learning-path.entity.js';
 import { Language } from '../../entities/content/language.js';
 
 export class LearningPathRepository extends DwengoEntityRepository<LearningPath> {
-    public findByHruidAndLanguage(hruid: string, language: Language): Promise<LearningPath | null> {
+    public async findByHruidAndLanguage(hruid: string, language: Language): Promise<LearningPath | null> {
         return this.findOne({ hruid: hruid, language: language }, { populate: ['nodes', 'nodes.transitions'] });
     }
 
diff --git a/backend/src/data/dwengo-entity-repository.ts b/backend/src/data/dwengo-entity-repository.ts
index 6538d6f5..f64b8408 100644
--- a/backend/src/data/dwengo-entity-repository.ts
+++ b/backend/src/data/dwengo-entity-repository.ts
@@ -1,12 +1,12 @@
 import { EntityRepository, FilterQuery } from '@mikro-orm/core';
 
 export abstract class DwengoEntityRepository<T extends object> extends EntityRepository<T> {
-    public async save(entity: T) {
+    public async save(entity: T): Promise<void> {
         const em = this.getEntityManager();
         em.persist(entity);
         await em.flush();
     }
-    public async deleteWhere(query: FilterQuery<T>) {
+    public async deleteWhere(query: FilterQuery<T>): Promise<void> {
         const toDelete = await this.findOne(query);
         const em = this.getEntityManager();
         if (toDelete) {
diff --git a/backend/src/data/questions/answer-repository.ts b/backend/src/data/questions/answer-repository.ts
index a28342bd..a50bfd28 100644
--- a/backend/src/data/questions/answer-repository.ts
+++ b/backend/src/data/questions/answer-repository.ts
@@ -4,7 +4,7 @@ import { Question } from '../../entities/questions/question.entity.js';
 import { Teacher } from '../../entities/users/teacher.entity.js';
 
 export class AnswerRepository extends DwengoEntityRepository<Answer> {
-    public createAnswer(answer: { toQuestion: Question; author: Teacher; content: string }): Promise<Answer> {
+    public async createAnswer(answer: { toQuestion: Question; author: Teacher; content: string }): Promise<Answer> {
         const answerEntity = this.create({
             toQuestion: answer.toQuestion,
             author: answer.author,
@@ -13,13 +13,13 @@ export class AnswerRepository extends DwengoEntityRepository<Answer> {
         });
         return this.insert(answerEntity);
     }
-    public findAllAnswersToQuestion(question: Question): Promise<Answer[]> {
+    public async findAllAnswersToQuestion(question: Question): Promise<Answer[]> {
         return this.findAll({
             where: { toQuestion: question },
             orderBy: { sequenceNumber: 'ASC' },
         });
     }
-    public removeAnswerByQuestionAndSequenceNumber(question: Question, sequenceNumber: number): Promise<void> {
+    public async removeAnswerByQuestionAndSequenceNumber(question: Question, sequenceNumber: number): Promise<void> {
         return this.deleteWhere({
             toQuestion: question,
             sequenceNumber: sequenceNumber,
diff --git a/backend/src/data/questions/question-repository.ts b/backend/src/data/questions/question-repository.ts
index 9207e1dd..596b562c 100644
--- a/backend/src/data/questions/question-repository.ts
+++ b/backend/src/data/questions/question-repository.ts
@@ -5,7 +5,7 @@ import { Student } from '../../entities/users/student.entity.js';
 import { LearningObject } from '../../entities/content/learning-object.entity.js';
 
 export class QuestionRepository extends DwengoEntityRepository<Question> {
-    public createQuestion(question: { loId: LearningObjectIdentifier; author: Student; content: string }): Promise<Question> {
+    public async createQuestion(question: { loId: LearningObjectIdentifier; author: Student; content: string }): Promise<Question> {
         const questionEntity = this.create({
             learningObjectHruid: question.loId.hruid,
             learningObjectLanguage: question.loId.language,
@@ -21,7 +21,7 @@ export class QuestionRepository extends DwengoEntityRepository<Question> {
         questionEntity.content = question.content;
         return this.insert(questionEntity);
     }
-    public findAllQuestionsAboutLearningObject(loId: LearningObjectIdentifier): Promise<Question[]> {
+    public async findAllQuestionsAboutLearningObject(loId: LearningObjectIdentifier): Promise<Question[]> {
         return this.findAll({
             where: {
                 learningObjectHruid: loId.hruid,
@@ -33,7 +33,7 @@ export class QuestionRepository extends DwengoEntityRepository<Question> {
             },
         });
     }
-    public removeQuestionByLearningObjectAndSequenceNumber(loId: LearningObjectIdentifier, sequenceNumber: number): Promise<void> {
+    public async removeQuestionByLearningObjectAndSequenceNumber(loId: LearningObjectIdentifier, sequenceNumber: number): Promise<void> {
         return this.deleteWhere({
             learningObjectHruid: loId.hruid,
             learningObjectLanguage: loId.language,
diff --git a/backend/src/data/repositories.ts b/backend/src/data/repositories.ts
index cdeb50c1..852d9d6a 100644
--- a/backend/src/data/repositories.ts
+++ b/backend/src/data/repositories.ts
@@ -34,7 +34,7 @@ let entityManager: EntityManager | undefined;
 /**
  * Execute all the database operations within the function f in a single transaction.
  */
-export function transactional<T>(f: () => Promise<T>) {
+export function transactional<T>(f: () => Promise<T>): void {
     entityManager?.transactional(f);
 }
 
diff --git a/backend/src/data/users/student-repository.ts b/backend/src/data/users/student-repository.ts
index 34b24b8b..2efca048 100644
--- a/backend/src/data/users/student-repository.ts
+++ b/backend/src/data/users/student-repository.ts
@@ -2,10 +2,10 @@ import { Student } from '../../entities/users/student.entity.js';
 import { DwengoEntityRepository } from '../dwengo-entity-repository.js';
 
 export class StudentRepository extends DwengoEntityRepository<Student> {
-    public findByUsername(username: string): Promise<Student | null> {
+    public async findByUsername(username: string): Promise<Student | null> {
         return this.findOne({ username: username });
     }
-    public deleteByUsername(username: string): Promise<void> {
+    public async deleteByUsername(username: string): Promise<void> {
         return this.deleteWhere({ username: username });
     }
 }
diff --git a/backend/src/data/users/teacher-repository.ts b/backend/src/data/users/teacher-repository.ts
index 825b4d18..aa915627 100644
--- a/backend/src/data/users/teacher-repository.ts
+++ b/backend/src/data/users/teacher-repository.ts
@@ -2,10 +2,10 @@ import { Teacher } from '../../entities/users/teacher.entity.js';
 import { DwengoEntityRepository } from '../dwengo-entity-repository.js';
 
 export class TeacherRepository extends DwengoEntityRepository<Teacher> {
-    public findByUsername(username: string): Promise<Teacher | null> {
+    public async findByUsername(username: string): Promise<Teacher | null> {
         return this.findOne({ username: username });
     }
-    public deleteByUsername(username: string): Promise<void> {
+    public async deleteByUsername(username: string): Promise<void> {
         return this.deleteWhere({ username: username });
     }
 }
diff --git a/backend/src/data/users/user-repository.ts b/backend/src/data/users/user-repository.ts
index 21497b79..44eb0bc7 100644
--- a/backend/src/data/users/user-repository.ts
+++ b/backend/src/data/users/user-repository.ts
@@ -2,10 +2,10 @@ import { DwengoEntityRepository } from '../dwengo-entity-repository.js';
 import { User } from '../../entities/users/user.entity.js';
 
 export class UserRepository<T extends User> extends DwengoEntityRepository<T> {
-    public findByUsername(username: string): Promise<T | null> {
+    public async findByUsername(username: string): Promise<T | null> {
         return this.findOne({ username } as Partial<T>);
     }
-    public deleteByUsername(username: string): Promise<void> {
+    public async deleteByUsername(username: string): Promise<void> {
         return this.deleteWhere({ username } as Partial<T>);
     }
 }
diff --git a/backend/src/entities/content/learning-object-identifier.ts b/backend/src/entities/content/learning-object-identifier.ts
index 3c020bd7..9234afa7 100644
--- a/backend/src/entities/content/learning-object-identifier.ts
+++ b/backend/src/entities/content/learning-object-identifier.ts
@@ -5,5 +5,7 @@ export class LearningObjectIdentifier {
         public hruid: string,
         public language: Language,
         public version: number
-    ) {}
+    ) {
+        // Do nothing
+    }
 }
diff --git a/backend/src/logging/initalize.ts b/backend/src/logging/initalize.ts
index 7375aa37..2d3ec44d 100644
--- a/backend/src/logging/initalize.ts
+++ b/backend/src/logging/initalize.ts
@@ -28,7 +28,7 @@ function initializeLogger(): Logger {
         level: LOG_LEVEL,
         json: true,
         format: format.combine(format.timestamp(), format.json()),
-        onConnectionError: (err) => {
+        onConnectionError: (err): void => {
             // eslint-disable-next-line no-console
             console.error(`Connection error: ${err}`);
         },
diff --git a/backend/src/logging/mikroOrmLogger.ts b/backend/src/logging/mikroOrmLogger.ts
index 25bbac13..f6907691 100644
--- a/backend/src/logging/mikroOrmLogger.ts
+++ b/backend/src/logging/mikroOrmLogger.ts
@@ -5,7 +5,7 @@ import { LokiLabels } from 'loki-logger-ts';
 export class MikroOrmLogger extends DefaultLogger {
     private logger: Logger = getLogger();
 
-    log(namespace: LoggerNamespace, message: string, context?: LogContext) {
+    log(namespace: LoggerNamespace, message: string, context?: LogContext): void {
         if (!this.isEnabled(namespace, context)) {
             return;
         }
@@ -48,7 +48,7 @@ export class MikroOrmLogger extends DefaultLogger {
         }
     }
 
-    private createMessage(namespace: LoggerNamespace, messageArg: string, context?: LogContext) {
+    private createMessage(namespace: LoggerNamespace, messageArg: string, context?: LogContext): unknown {
         const labels: LokiLabels = {
             service: 'ORM',
         };
diff --git a/backend/src/logging/responseTimeLogger.ts b/backend/src/logging/responseTimeLogger.ts
index c1bb1e33..7fcc6c93 100644
--- a/backend/src/logging/responseTimeLogger.ts
+++ b/backend/src/logging/responseTimeLogger.ts
@@ -1,7 +1,7 @@
 import { getLogger, Logger } from './initalize.js';
 import { Request, Response } from 'express';
 
-export function responseTimeLogger(req: Request, res: Response, time: number) {
+export function responseTimeLogger(req: Request, res: Response, time: number): void {
     const logger: Logger = getLogger();
 
     const method = req.method;
diff --git a/backend/src/middleware/auth/auth.ts b/backend/src/middleware/auth/auth.ts
index 5389dc77..345bfa59 100644
--- a/backend/src/middleware/auth/auth.ts
+++ b/backend/src/middleware/auth/auth.ts
@@ -1,9 +1,9 @@
 import { envVars, getEnvVar } from '../../util/envVars.js';
 import { expressjwt } from 'express-jwt';
+import * as jwt from 'jsonwebtoken';
 import { JwtPayload } from 'jsonwebtoken';
 import jwksClient from 'jwks-rsa';
 import * as express from 'express';
-import * as jwt from 'jsonwebtoken';
 import { AuthenticatedRequest } from './authenticated-request.js';
 import { AuthenticationInfo } from './authentication-info.js';
 import { ForbiddenException, UnauthorizedException } from '../../exceptions.js';
@@ -74,7 +74,7 @@ const verifyJwtToken = expressjwt({
  */
 function getAuthenticationInfo(req: AuthenticatedRequest): AuthenticationInfo | undefined {
     if (!req.jwtPayload) {
-        return;
+        return undefined;
     }
     const issuer = req.jwtPayload.iss;
     let accountType: 'student' | 'teacher';
@@ -84,8 +84,9 @@ function getAuthenticationInfo(req: AuthenticatedRequest): AuthenticationInfo |
     } else if (issuer === idpConfigs.teacher.issuer) {
         accountType = 'teacher';
     } else {
-        return;
+        return undefined;
     }
+
     return {
         accountType: accountType,
         username: req.jwtPayload[JWT_PROPERTY_NAMES.username]!,
@@ -100,10 +101,10 @@ function getAuthenticationInfo(req: AuthenticatedRequest): AuthenticationInfo |
  * Add the AuthenticationInfo object with the information about the current authentication to the request in order
  * to avoid that the routers have to deal with the JWT token.
  */
-const addAuthenticationInfo = (req: AuthenticatedRequest, _res: express.Response, next: express.NextFunction) => {
+function addAuthenticationInfo(req: AuthenticatedRequest, _res: express.Response, next: express.NextFunction): void {
     req.auth = getAuthenticationInfo(req);
     next();
-};
+}
 
 export const authenticateUser = [verifyJwtToken, addAuthenticationInfo];
 
@@ -113,9 +114,8 @@ export const authenticateUser = [verifyJwtToken, addAuthenticationInfo];
  * @param accessCondition Predicate over the current AuthenticationInfo. Access is only granted when this evaluates
  *                        to true.
  */
-export const authorize =
-    (accessCondition: (auth: AuthenticationInfo) => boolean) =>
-    (req: AuthenticatedRequest, _res: express.Response, next: express.NextFunction): void => {
+export function authorize(accessCondition: (auth: AuthenticationInfo) => boolean) {
+    return (req: AuthenticatedRequest, _res: express.Response, next: express.NextFunction): void => {
         if (!req.auth) {
             throw new UnauthorizedException();
         } else if (!accessCondition(req.auth)) {
@@ -124,6 +124,7 @@ export const authorize =
             next();
         }
     };
+}
 
 /**
  * Middleware which rejects all unauthenticated users, but accepts all authenticated users.
diff --git a/backend/src/mikro-orm.config.ts b/backend/src/mikro-orm.config.ts
index d53e0398..ffa12c82 100644
--- a/backend/src/mikro-orm.config.ts
+++ b/backend/src/mikro-orm.config.ts
@@ -54,7 +54,7 @@ function config(testingMode: boolean = false): Options {
 
             // Workaround: vitest: `TypeError: Unknown file extension ".ts"` (ERR_UNKNOWN_FILE_EXTENSION)
             // (see https://mikro-orm.io/docs/guide/project-setup#testing-the-endpoint)
-            dynamicImportProvider: (id) => import(id),
+            dynamicImportProvider: async (id) => import(id),
         };
     }
 
diff --git a/backend/src/orm.ts b/backend/src/orm.ts
index 8ab5258a..419d5990 100644
--- a/backend/src/orm.ts
+++ b/backend/src/orm.ts
@@ -4,7 +4,7 @@ import { envVars, getEnvVar } from './util/envVars.js';
 import { getLogger, Logger } from './logging/initalize.js';
 
 let orm: MikroORM | undefined;
-export async function initORM(testingMode: boolean = false) {
+export async function initORM(testingMode: boolean = false): Promise<void> {
     const logger: Logger = getLogger();
 
     logger.info('Initializing ORM');
diff --git a/backend/src/services/assignments.ts b/backend/src/services/assignments.ts
index c637b822..7ac52844 100644
--- a/backend/src/services/assignments.ts
+++ b/backend/src/services/assignments.ts
@@ -79,7 +79,7 @@ export async function getAssignmentsSubmissions(classid: string, assignmentNumbe
     const groups = await groupRepository.findAllGroupsForAssignment(assignment);
 
     const submissionRepository = getSubmissionRepository();
-    const submissions = (await Promise.all(groups.map((group) => submissionRepository.findAllSubmissionsForGroup(group)))).flat();
+    const submissions = (await Promise.all(groups.map(async (group) => submissionRepository.findAllSubmissionsForGroup(group)))).flat();
 
     return submissions.map(mapToSubmissionDTO);
 }
diff --git a/backend/src/services/class.ts b/backend/src/services/class.ts
index 9f6e1efe..4f3cbc9d 100644
--- a/backend/src/services/class.ts
+++ b/backend/src/services/class.ts
@@ -24,11 +24,15 @@ export async function getAllClasses(full: boolean): Promise<ClassDTO[] | string[
 export async function createClass(classData: ClassDTO): Promise<Class | null> {
     const teacherRepository = getTeacherRepository();
     const teacherUsernames = classData.teachers || [];
-    const teachers = (await Promise.all(teacherUsernames.map((id) => teacherRepository.findByUsername(id)))).filter((teacher) => teacher != null);
+    const teachers = (await Promise.all(teacherUsernames.map(async (id) => teacherRepository.findByUsername(id)))).filter(
+        (teacher) => teacher != null
+    );
 
     const studentRepository = getStudentRepository();
     const studentUsernames = classData.students || [];
-    const students = (await Promise.all(studentUsernames.map((id) => studentRepository.findByUsername(id)))).filter((student) => student != null);
+    const students = (await Promise.all(studentUsernames.map(async (id) => studentRepository.findByUsername(id)))).filter(
+        (student) => student != null
+    );
 
     //Const cls = mapToClass(classData, teachers, students);
 
diff --git a/backend/src/services/groups.ts b/backend/src/services/groups.ts
index ecc92293..6abcd3fc 100644
--- a/backend/src/services/groups.ts
+++ b/backend/src/services/groups.ts
@@ -43,7 +43,9 @@ export async function createGroup(groupData: GroupDTO, classid: string, assignme
     const studentRepository = getStudentRepository();
 
     const memberUsernames = (groupData.members as string[]) || []; // TODO check if groupdata.members is a list
-    const members = (await Promise.all([...memberUsernames].map((id) => studentRepository.findByUsername(id)))).filter((student) => student != null);
+    const members = (await Promise.all([...memberUsernames].map(async (id) => studentRepository.findByUsername(id)))).filter(
+        (student) => student != null
+    );
 
     getLogger().debug(members);
 
diff --git a/backend/src/services/learning-objects/attachment-service.ts b/backend/src/services/learning-objects/attachment-service.ts
index aacc7187..4ff4ec47 100644
--- a/backend/src/services/learning-objects/attachment-service.ts
+++ b/backend/src/services/learning-objects/attachment-service.ts
@@ -3,7 +3,7 @@ import { Attachment } from '../../entities/content/attachment.entity.js';
 import { LearningObjectIdentifier } from '../../interfaces/learning-content.js';
 
 const attachmentService = {
-    getAttachment(learningObjectId: LearningObjectIdentifier, attachmentName: string): Promise<Attachment | null> {
+    async getAttachment(learningObjectId: LearningObjectIdentifier, attachmentName: string): Promise<Attachment | null> {
         const attachmentRepo = getAttachmentRepository();
 
         if (learningObjectId.version) {
diff --git a/backend/src/services/learning-objects/database-learning-object-provider.ts b/backend/src/services/learning-objects/database-learning-object-provider.ts
index bab0b9b1..03dbfdff 100644
--- a/backend/src/services/learning-objects/database-learning-object-provider.ts
+++ b/backend/src/services/learning-objects/database-learning-object-provider.ts
@@ -41,7 +41,7 @@ function convertLearningObject(learningObject: LearningObject | null): FilteredL
     };
 }
 
-function findLearningObjectEntityById(id: LearningObjectIdentifier): Promise<LearningObject | null> {
+async function findLearningObjectEntityById(id: LearningObjectIdentifier): Promise<LearningObject | null> {
     const learningObjectRepo = getLearningObjectRepository();
 
     return learningObjectRepo.findLatestByHruidAndLanguage(id.hruid, id.language as Language);
@@ -69,7 +69,7 @@ const databaseLearningObjectProvider: LearningObjectProvider = {
         if (!learningObject) {
             return null;
         }
-        return await processingService.render(learningObject, (id) => findLearningObjectEntityById(id));
+        return await processingService.render(learningObject, async (id) => findLearningObjectEntityById(id));
     },
 
     /**
@@ -96,7 +96,7 @@ const databaseLearningObjectProvider: LearningObjectProvider = {
             throw new NotFoundError('The learning path with the given ID could not be found.');
         }
         const learningObjects = await Promise.all(
-            learningPath.nodes.map((it) => {
+            learningPath.nodes.map(async (it) => {
                 const learningObject = learningObjectService.getLearningObjectById({
                     hruid: it.learningObjectHruid,
                     language: it.language,
diff --git a/backend/src/services/learning-objects/learning-object-service.ts b/backend/src/services/learning-objects/learning-object-service.ts
index 95e5daff..59ffb643 100644
--- a/backend/src/services/learning-objects/learning-object-service.ts
+++ b/backend/src/services/learning-objects/learning-object-service.ts
@@ -18,28 +18,28 @@ const learningObjectService = {
     /**
      * Fetches a single learning object by its HRUID
      */
-    getLearningObjectById(id: LearningObjectIdentifier): Promise<FilteredLearningObject | null> {
+    async getLearningObjectById(id: LearningObjectIdentifier): Promise<FilteredLearningObject | null> {
         return getProvider(id).getLearningObjectById(id);
     },
 
     /**
      * Fetch full learning object data (metadata)
      */
-    getLearningObjectsFromPath(id: LearningPathIdentifier): Promise<FilteredLearningObject[]> {
+    async getLearningObjectsFromPath(id: LearningPathIdentifier): Promise<FilteredLearningObject[]> {
         return getProvider(id).getLearningObjectsFromPath(id);
     },
 
     /**
      * Fetch only learning object HRUIDs
      */
-    getLearningObjectIdsFromPath(id: LearningPathIdentifier): Promise<string[]> {
+    async getLearningObjectIdsFromPath(id: LearningPathIdentifier): Promise<string[]> {
         return getProvider(id).getLearningObjectIdsFromPath(id);
     },
 
     /**
      * Obtain a HTML-rendering of the learning object with the given identifier (as a string).
      */
-    getLearningObjectHTML(id: LearningObjectIdentifier): Promise<string | null> {
+    async getLearningObjectHTML(id: LearningObjectIdentifier): Promise<string | null> {
         return getProvider(id).getLearningObjectHTML(id);
     },
 };
diff --git a/backend/src/services/learning-objects/processing/extern/extern-processor.ts b/backend/src/services/learning-objects/processing/extern/extern-processor.ts
index 453e998b..162f80d2 100644
--- a/backend/src/services/learning-objects/processing/extern/extern-processor.ts
+++ b/backend/src/services/learning-objects/processing/extern/extern-processor.ts
@@ -15,7 +15,7 @@ class ExternProcessor extends StringProcessor {
         super(DwengoContentType.EXTERN);
     }
 
-    override renderFn(externURL: string) {
+    override renderFn(externURL: string): string {
         if (!isValidHttpUrl(externURL)) {
             throw new ProcessingError('The url is not valid: ' + externURL);
         }
diff --git a/backend/src/services/learning-objects/processing/gift/gift-processor.ts b/backend/src/services/learning-objects/processing/gift/gift-processor.ts
index 5396236a..8d548f56 100644
--- a/backend/src/services/learning-objects/processing/gift/gift-processor.ts
+++ b/backend/src/services/learning-objects/processing/gift/gift-processor.ts
@@ -32,7 +32,7 @@ class GiftProcessor extends StringProcessor {
         super(DwengoContentType.GIFT);
     }
 
-    override renderFn(giftString: string) {
+    override renderFn(giftString: string): string {
         const quizQuestions: GIFTQuestion[] = parse(giftString);
 
         let html = "<div class='learning-object-gift'>\n";
diff --git a/backend/src/services/learning-objects/processing/image/block-image-processor.ts b/backend/src/services/learning-objects/processing/image/block-image-processor.ts
index f4f8a773..6ad427df 100644
--- a/backend/src/services/learning-objects/processing/image/block-image-processor.ts
+++ b/backend/src/services/learning-objects/processing/image/block-image-processor.ts
@@ -10,7 +10,7 @@ class BlockImageProcessor extends InlineImageProcessor {
         super();
     }
 
-    override renderFn(imageUrl: string) {
+    override renderFn(imageUrl: string): string {
         const inlineHtml = super.render(imageUrl);
         return DOMPurify.sanitize(`<div>${inlineHtml}</div>`);
     }
diff --git a/backend/src/services/learning-objects/processing/image/inline-image-processor.ts b/backend/src/services/learning-objects/processing/image/inline-image-processor.ts
index 478ce326..22e6313e 100644
--- a/backend/src/services/learning-objects/processing/image/inline-image-processor.ts
+++ b/backend/src/services/learning-objects/processing/image/inline-image-processor.ts
@@ -13,7 +13,7 @@ class InlineImageProcessor extends StringProcessor {
         super(contentType);
     }
 
-    override renderFn(imageUrl: string) {
+    override renderFn(imageUrl: string): string {
         if (!isValidHttpUrl(imageUrl)) {
             throw new ProcessingError(`Image URL is invalid: ${imageUrl}`);
         }
diff --git a/backend/src/services/learning-objects/processing/markdown/markdown-processor.ts b/backend/src/services/learning-objects/processing/markdown/markdown-processor.ts
index 39f2adc8..77673cbf 100644
--- a/backend/src/services/learning-objects/processing/markdown/markdown-processor.ts
+++ b/backend/src/services/learning-objects/processing/markdown/markdown-processor.ts
@@ -14,7 +14,7 @@ class MarkdownProcessor extends StringProcessor {
         super(DwengoContentType.TEXT_MARKDOWN);
     }
 
-    override renderFn(mdText: string) {
+    override renderFn(mdText: string): string {
         try {
             marked.use({ renderer: dwengoMarkedRenderer });
             const html = marked(mdText, { async: false });
@@ -24,7 +24,7 @@ class MarkdownProcessor extends StringProcessor {
         }
     }
 
-    replaceLinks(html: string) {
+    replaceLinks(html: string): string {
         const proc = new InlineImageProcessor();
         html = html.replace(
             /<img.*?src="(.*?)".*?(alt="(.*?)")?.*?(title="(.*?)")?.*?>/g,
diff --git a/backend/src/services/learning-objects/processing/pdf/pdf-processor.ts b/backend/src/services/learning-objects/processing/pdf/pdf-processor.ts
index 26cb4d94..c41de802 100644
--- a/backend/src/services/learning-objects/processing/pdf/pdf-processor.ts
+++ b/backend/src/services/learning-objects/processing/pdf/pdf-processor.ts
@@ -15,7 +15,7 @@ class PdfProcessor extends StringProcessor {
         super(DwengoContentType.APPLICATION_PDF);
     }
 
-    override renderFn(pdfUrl: string) {
+    override renderFn(pdfUrl: string): string {
         if (!isValidHttpUrl(pdfUrl)) {
             throw new ProcessingError(`PDF URL is invalid: ${pdfUrl}`);
         }
diff --git a/backend/src/services/learning-objects/processing/processor.ts b/backend/src/services/learning-objects/processing/processor.ts
index a11c4416..1dcca255 100644
--- a/backend/src/services/learning-objects/processing/processor.ts
+++ b/backend/src/services/learning-objects/processing/processor.ts
@@ -9,7 +9,9 @@ import { DwengoContentType } from './content-type.js';
  * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/processor.js
  */
 abstract class Processor<T> {
-    protected constructor(public contentType: DwengoContentType) {}
+    protected constructor(public contentType: DwengoContentType) {
+        // Do nothing
+    }
 
     /**
      * Render the given object.
diff --git a/backend/src/services/learning-objects/processing/text/text-processor.ts b/backend/src/services/learning-objects/processing/text/text-processor.ts
index db6d37df..a2c84150 100644
--- a/backend/src/services/learning-objects/processing/text/text-processor.ts
+++ b/backend/src/services/learning-objects/processing/text/text-processor.ts
@@ -11,7 +11,7 @@ class TextProcessor extends StringProcessor {
         super(DwengoContentType.TEXT_PLAIN);
     }
 
-    override renderFn(text: string) {
+    override renderFn(text: string): string {
         // Sanitize plain text to prevent xss.
         return DOMPurify.sanitize(text);
     }
diff --git a/backend/src/services/learning-paths/database-learning-path-provider.ts b/backend/src/services/learning-paths/database-learning-path-provider.ts
index bbcb6485..cc12c44c 100644
--- a/backend/src/services/learning-paths/database-learning-path-provider.ts
+++ b/backend/src/services/learning-paths/database-learning-path-provider.ts
@@ -18,7 +18,7 @@ async function getLearningObjectsForNodes(nodes: LearningPathNode[]): Promise<Ma
     // Its corresponding learning object.
     const nullableNodesToLearningObjects = new Map<LearningPathNode, FilteredLearningObject | null>(
         await Promise.all(
-            nodes.map((node) =>
+            nodes.map(async (node) =>
                 learningObjectService
                     .getLearningObjectById({
                         hruid: node.learningObjectHruid,
@@ -117,7 +117,7 @@ async function convertNodes(
     nodesToLearningObjects: Map<LearningPathNode, FilteredLearningObject>,
     personalizedFor?: PersonalizationTarget
 ): Promise<LearningObjectNode[]> {
-    const nodesPromise = Array.from(nodesToLearningObjects.entries()).map((entry) =>
+    const nodesPromise = Array.from(nodesToLearningObjects.entries()).map(async (entry) =>
         convertNode(entry[0], entry[1], personalizedFor, nodesToLearningObjects)
     );
     return await Promise.all(nodesPromise);
@@ -179,11 +179,11 @@ const databaseLearningPathProvider: LearningPathProvider = {
     ): Promise<LearningPathResponse> {
         const learningPathRepo = getLearningPathRepository();
 
-        const learningPaths = (await Promise.all(hruids.map((hruid) => learningPathRepo.findByHruidAndLanguage(hruid, language)))).filter(
+        const learningPaths = (await Promise.all(hruids.map(async (hruid) => learningPathRepo.findByHruidAndLanguage(hruid, language)))).filter(
             (learningPath) => learningPath !== null
         );
         const filteredLearningPaths = await Promise.all(
-            learningPaths.map((learningPath, index) => convertLearningPath(learningPath, index, personalizedFor))
+            learningPaths.map(async (learningPath, index) => convertLearningPath(learningPath, index, personalizedFor))
         );
 
         return {
@@ -200,7 +200,7 @@ const databaseLearningPathProvider: LearningPathProvider = {
         const learningPathRepo = getLearningPathRepository();
 
         const searchResults = await learningPathRepo.findByQueryStringAndLanguage(query, language);
-        return await Promise.all(searchResults.map((result, index) => convertLearningPath(result, index, personalizedFor)));
+        return await Promise.all(searchResults.map(async (result, index) => convertLearningPath(result, index, personalizedFor)));
     },
 };
 
diff --git a/backend/src/services/learning-paths/learning-path-service.ts b/backend/src/services/learning-paths/learning-path-service.ts
index 4dd21407..f8e328b2 100644
--- a/backend/src/services/learning-paths/learning-path-service.ts
+++ b/backend/src/services/learning-paths/learning-path-service.ts
@@ -49,7 +49,9 @@ const learningPathService = {
      * Search learning paths in the data source using the given search string.
      */
     async searchLearningPaths(query: string, language: Language, personalizedFor?: PersonalizationTarget): Promise<LearningPath[]> {
-        const providerResponses = await Promise.all(allProviders.map((provider) => provider.searchLearningPaths(query, language, personalizedFor)));
+        const providerResponses = await Promise.all(
+            allProviders.map(async (provider) => provider.searchLearningPaths(query, language, personalizedFor))
+        );
         return providerResponses.flat();
     },
 };
diff --git a/backend/src/services/questions.ts b/backend/src/services/questions.ts
index 0433b69f..bb42aabd 100644
--- a/backend/src/services/questions.ts
+++ b/backend/src/services/questions.ts
@@ -2,7 +2,7 @@ import { getAnswerRepository, getQuestionRepository } from '../data/repositories
 import { mapToQuestionDTO, mapToQuestionId, QuestionDTO, QuestionId } from '../interfaces/question.js';
 import { Question } from '../entities/questions/question.entity.js';
 import { Answer } from '../entities/questions/answer.entity.js';
-import { mapToAnswerDTO, mapToAnswerId } from '../interfaces/answer.js';
+import { AnswerDTO, AnswerId, mapToAnswerDTO, mapToAnswerId } from '../interfaces/answer.js';
 import { QuestionRepository } from '../data/questions/question-repository.js';
 import { LearningObjectIdentifier } from '../entities/content/learning-object-identifier.js';
 import { mapToStudent } from '../interfaces/student.js';
@@ -45,7 +45,7 @@ export async function getQuestion(questionId: QuestionId): Promise<QuestionDTO |
     return mapToQuestionDTO(question);
 }
 
-export async function getAnswersByQuestion(questionId: QuestionId, full: boolean) {
+export async function getAnswersByQuestion(questionId: QuestionId, full: boolean): Promise<AnswerDTO[] | AnswerId[]> {
     const answerRepository = getAnswerRepository();
     const question = await fetchQuestion(questionId);
 
@@ -68,7 +68,7 @@ export async function getAnswersByQuestion(questionId: QuestionId, full: boolean
     return answersDTO.map(mapToAnswerId);
 }
 
-export async function createQuestion(questionDTO: QuestionDTO) {
+export async function createQuestion(questionDTO: QuestionDTO): Promise<QuestionDTO | null> {
     const questionRepository = getQuestionRepository();
 
     const author = mapToStudent(questionDTO.author);
@@ -86,7 +86,7 @@ export async function createQuestion(questionDTO: QuestionDTO) {
     return questionDTO;
 }
 
-export async function deleteQuestion(questionId: QuestionId) {
+export async function deleteQuestion(questionId: QuestionId): Promise<Question | null> {
     const questionRepository = getQuestionRepository();
 
     const question = await fetchQuestion(questionId);
diff --git a/backend/src/services/submissions.ts b/backend/src/services/submissions.ts
index 153ce169..b47e04bf 100644
--- a/backend/src/services/submissions.ts
+++ b/backend/src/services/submissions.ts
@@ -2,6 +2,7 @@ import { getSubmissionRepository } from '../data/repositories.js';
 import { Language } from '../entities/content/language.js';
 import { LearningObjectIdentifier } from '../entities/content/learning-object-identifier.js';
 import { mapToSubmission, mapToSubmissionDTO, SubmissionDTO } from '../interfaces/submission.js';
+import { Submission } from '../entities/assignments/submission.entity.js';
 
 export async function getSubmission(
     learningObjectHruid: string,
@@ -21,7 +22,7 @@ export async function getSubmission(
     return mapToSubmissionDTO(submission);
 }
 
-export async function createSubmission(submissionDTO: SubmissionDTO) {
+export async function createSubmission(submissionDTO: SubmissionDTO): Promise<Submission | null> {
     const submissionRepository = getSubmissionRepository();
     const submission = mapToSubmission(submissionDTO);
 
@@ -35,7 +36,12 @@ export async function createSubmission(submissionDTO: SubmissionDTO) {
     return submission;
 }
 
-export async function deleteSubmission(learningObjectHruid: string, language: Language, version: number, submissionNumber: number) {
+export async function deleteSubmission(
+    learningObjectHruid: string,
+    language: Language,
+    version: number,
+    submissionNumber: number
+): Promise<SubmissionDTO | null> {
     const submissionRepository = getSubmissionRepository();
 
     const submission = getSubmission(learningObjectHruid, language, version, submissionNumber);
diff --git a/backend/src/services/teachers.ts b/backend/src/services/teachers.ts
index 1949152f..1b098778 100644
--- a/backend/src/services/teachers.ts
+++ b/backend/src/services/teachers.ts
@@ -77,7 +77,7 @@ export async function getClassIdsByTeacher(username: string): Promise<string[]>
     return classes.map((cls) => cls.id);
 }
 
-export async function fetchStudentsByTeacher(username: string) {
+export async function fetchStudentsByTeacher(username: string): Promise<StudentDTO[]> {
     const classes = await getClassIdsByTeacher(username);
 
     return (await Promise.all(classes.map(async (id) => getClassStudents(id)))).flat();
diff --git a/backend/src/util/async.ts b/backend/src/util/async.ts
index a5fc9b82..7b7b8818 100644
--- a/backend/src/util/async.ts
+++ b/backend/src/util/async.ts
@@ -6,7 +6,11 @@
  * @param regex
  * @param replacementFn
  */
-export async function replaceAsync(str: string, regex: RegExp, replacementFn: (match: string, ...args: string[]) => Promise<string>) {
+export async function replaceAsync(
+    str: string,
+    regex: RegExp,
+    replacementFn: (match: string, ...args: string[]) => Promise<string>
+): Promise<string> {
     const promises: Promise<string>[] = [];
 
     // First run through matches: add all Promises resulting from the replacement function
diff --git a/backend/src/util/links.ts b/backend/src/util/links.ts
index c4b54c72..ff334eb5 100644
--- a/backend/src/util/links.ts
+++ b/backend/src/util/links.ts
@@ -9,7 +9,7 @@ export function isValidHttpUrl(url: string): boolean {
     }
 }
 
-export function getUrlStringForLearningObject(learningObjectId: LearningObjectIdentifier) {
+export function getUrlStringForLearningObject(learningObjectId: LearningObjectIdentifier): string {
     let url = `/learningObject/${learningObjectId.hruid}/html?language=${learningObjectId.language}`;
     if (learningObjectId.version) {
         url += `&version=${learningObjectId.version}`;
diff --git a/backend/tests/services/learning-objects/database-learning-object-provider.test.ts b/backend/tests/services/learning-objects/database-learning-object-provider.test.ts
index 4750e4bd..2c3b7dfc 100644
--- a/backend/tests/services/learning-objects/database-learning-object-provider.test.ts
+++ b/backend/tests/services/learning-objects/database-learning-object-provider.test.ts
@@ -78,7 +78,7 @@ describe('DatabaseLearningObjectProvider', () => {
         });
         it('should throw an error if queried with a path identifier for which there is no learning path', async () => {
             await expect(
-                (async () => {
+                (async (): Promise<void> => {
                     await databaseLearningObjectProvider.getLearningObjectIdsFromPath({
                         hruid: 'non_existing_hruid',
                         language: Language.Dutch,
@@ -97,7 +97,7 @@ describe('DatabaseLearningObjectProvider', () => {
         });
         it('should throw an error if queried with a path identifier for which there is no learning path', async () => {
             await expect(
-                (async () => {
+                (async (): Promise<void> => {
                     await databaseLearningObjectProvider.getLearningObjectsFromPath({
                         hruid: 'non_existing_hruid',
                         language: Language.Dutch,
diff --git a/backend/tests/services/learning-path/database-learning-path-provider.test.ts b/backend/tests/services/learning-path/database-learning-path-provider.test.ts
index 04782df3..8d9dd530 100644
--- a/backend/tests/services/learning-path/database-learning-path-provider.test.ts
+++ b/backend/tests/services/learning-path/database-learning-path-provider.test.ts
@@ -124,7 +124,7 @@ describe('DatabaseLearningPathProvider', () => {
 
             const learningObjectsOnPath = (
                 await Promise.all(
-                    example.learningPath.nodes.map((node) =>
+                    example.learningPath.nodes.map(async (node) =>
                         learningObjectService.getLearningObjectById({
                             hruid: node.learningObjectHruid,
                             version: node.version,
diff --git a/backend/tests/setup-tests.ts b/backend/tests/setup-tests.ts
index 9502bcb8..016099f3 100644
--- a/backend/tests/setup-tests.ts
+++ b/backend/tests/setup-tests.ts
@@ -14,7 +14,7 @@ import { makeTestQuestions } from './test_assets/questions/questions.testdata.js
 import { makeTestAnswers } from './test_assets/questions/answers.testdata.js';
 import { makeTestSubmissions } from './test_assets/assignments/submission.testdata.js';
 
-export async function setupTestApp() {
+export async function setupTestApp(): Promise<void> {
     dotenv.config({ path: '.env.test' });
     await initORM(true);
 
diff --git a/backend/tests/test-assets/learning-objects/dummy/dummy-learning-object-example.ts b/backend/tests/test-assets/learning-objects/dummy/dummy-learning-object-example.ts
index b1bd3089..f810d57a 100644
--- a/backend/tests/test-assets/learning-objects/dummy/dummy-learning-object-example.ts
+++ b/backend/tests/test-assets/learning-objects/dummy/dummy-learning-object-example.ts
@@ -11,7 +11,7 @@ import { envVars, getEnvVar } from '../../../../src/util/envVars';
  */
 export function dummyLearningObject(hruid: string, language: Language, title: string): LearningObjectExample {
     return {
-        createLearningObject: () => {
+        createLearningObject: (): LearningObject => {
             const learningObject = new LearningObject();
             learningObject.hruid = getEnvVar(envVars.UserContentPrefix) + hruid;
             learningObject.language = language;
diff --git a/backend/tests/test-assets/learning-paths/learning-path-utils.ts b/backend/tests/test-assets/learning-paths/learning-path-utils.ts
index c567de66..eb786fe1 100644
--- a/backend/tests/test-assets/learning-paths/learning-path-utils.ts
+++ b/backend/tests/test-assets/learning-paths/learning-path-utils.ts
@@ -3,7 +3,12 @@ import { LearningPathTransition } from '../../../src/entities/content/learning-p
 import { LearningPathNode } from '../../../src/entities/content/learning-path-node.entity';
 import { LearningPath } from '../../../src/entities/content/learning-path.entity';
 
-export function createLearningPathTransition(node: LearningPathNode, transitionNumber: number, condition: string | null, to: LearningPathNode) {
+export function createLearningPathTransition(
+    node: LearningPathNode,
+    transitionNumber: number,
+    condition: string | null,
+    to: LearningPathNode
+): LearningPathTransition {
     const trans = new LearningPathTransition();
     trans.node = node;
     trans.transitionNumber = transitionNumber;
@@ -19,7 +24,7 @@ export function createLearningPathNode(
     version: number,
     language: Language,
     startNode: boolean
-) {
+): LearningPathNode {
     const node = new LearningPathNode();
     node.learningPath = learningPath;
     node.nodeNumber = nodeNumber;
diff --git a/backend/tests/test-utils/expectations.ts b/backend/tests/test-utils/expectations.ts
index 0fe63811..347f4b40 100644
--- a/backend/tests/test-utils/expectations.ts
+++ b/backend/tests/test-utils/expectations.ts
@@ -69,7 +69,7 @@ export function expectToBeCorrectEntity<T extends object>(actual: { entity: T; n
  * @param filtered the representation as FilteredLearningObject
  * @param original the original entity added to the database
  */
-export function expectToBeCorrectFilteredLearningObject(filtered: FilteredLearningObject, original: LearningObject) {
+export function expectToBeCorrectFilteredLearningObject(filtered: FilteredLearningObject, original: LearningObject): void {
     expect(filtered.uuid).toEqual(original.uuid);
     expect(filtered.version).toEqual(original.version);
     expect(filtered.language).toEqual(original.language);
@@ -105,7 +105,7 @@ export function expectToBeCorrectLearningPath(
     learningPath: LearningPath,
     expectedEntity: LearningPathEntity,
     learningObjectsOnPath: FilteredLearningObject[]
-) {
+): void {
     expect(learningPath.hruid).toEqual(expectedEntity.hruid);
     expect(learningPath.language).toEqual(expectedEntity.language);
     expect(learningPath.description).toEqual(expectedEntity.description);
diff --git a/eslint.config.ts b/eslint.config.ts
index e4534398..3596633a 100644
--- a/eslint.config.ts
+++ b/eslint.config.ts
@@ -23,11 +23,20 @@ export default [
         languageOptions: {
             ecmaVersion: 'latest',
             sourceType: 'module',
+            parserOptions: {
+                projectService: true,
+                tsconfigRootDir: import.meta.dirname,
+            },
         },
         linterOptions: {
             reportUnusedInlineConfigs: 'error',
         },
         rules: {
+            'consistent-return': 'off',
+            '@typescript-eslint/consistent-return': 'off',
+
+            '@typescript-eslint/explicit-function-return-type': 'warn',
+
             '@typescript-eslint/naming-convention': [
                 'warn',
                 { // Enforce that all variables, functions and properties are camelCase
@@ -49,6 +58,14 @@ export default [
                 }
             ],
 
+            // 'no-empty-function': 'off',
+            '@typescript-eslint/no-empty-function': 'error',
+
+            'no-loop-func': 'off',
+            '@typescript-eslint/no-loop-func': 'error',
+
+            '@typescript-eslint/no-unsafe-function-type': 'error',
+
             'no-unused-expressions': 'off',
             '@typescript-eslint/no-unused-expressions': 'warn',
             'no-unused-vars': 'off',
@@ -66,6 +83,10 @@ export default [
             'no-use-before-define': 'off',
             '@typescript-eslint/no-use-before-define': 'off',
 
+            '@typescript-eslint/prefer-function-type': 'error',
+
+            '@typescript-eslint/promise-function-async': 'warn',
+
             'no-await-in-loop': 'warn',
             'no-constructor-return': 'error',
             'no-duplicate-imports': 'error',
@@ -110,7 +131,6 @@ export default [
             'no-iterator': 'error',
             'no-label-var': 'warn',
             'no-labels': 'warn',
-            'no-loop-func': 'error',
             'no-multi-assign': 'error',
             'no-nested-ternary': 'error',
             'no-object-constructor': 'error',