diff --git a/src/app/data-structure/has-id-and-title.ts b/src/app/data-structure/has-id-and-title.ts
index 5d8d44c..531bf38 100644
--- a/src/app/data-structure/has-id-and-title.ts
+++ b/src/app/data-structure/has-id-and-title.ts
@@ -1,5 +1,5 @@
 export class HasIdAndTitle {
-  public id: string;
-  public title: string;
-  public link: string;
+  public id: string = "";
+  public title: string = "";
+  public link: string = "";
 }
diff --git a/src/app/data-structure/login-info.ts b/src/app/data-structure/login-info.ts
index ec2a64b..cfc3ff7 100644
--- a/src/app/data-structure/login-info.ts
+++ b/src/app/data-structure/login-info.ts
@@ -5,15 +5,15 @@ export class LoginInfo {
     return plainToClass(LoginInfo, obj as object);
   }
 
-  public loggedIn: boolean;
-  public nickname: string;
+  public loggedIn: boolean = false;
+  public nickname: string = "";
 
-  public googleLinked: boolean;
-  public googleProfileUrl: string;
-  public gitHubLinked: boolean;
-  public gitHubProfileUrl: string;
-  public facebookLinked: boolean;
-  public facebookProfileUrl: string;
+  public googleLinked: boolean = false;
+  public googleProfileUrl: string = "";
+  public gitHubLinked: boolean = false;
+  public gitHubProfileUrl: string = "";
+  public facebookLinked: boolean = false;
+  public facebookProfileUrl: string = "";
 
-  public errorMessage: string;
+  public errorMessage: string = "";
 }
diff --git a/src/app/data-structure/quiz-question-and-answer.ts b/src/app/data-structure/quiz-question-and-answer.ts
index ab35468..3066c3c 100644
--- a/src/app/data-structure/quiz-question-and-answer.ts
+++ b/src/app/data-structure/quiz-question-and-answer.ts
@@ -4,8 +4,8 @@ import { QuizText } from "./quiz-text";
 
 export class QuizQuestionAndAnswer {
   @Type(() => QuizQuestion)
-  public question: QuizQuestion;
+  public question?: QuizQuestion;
 
   @Type(() => QuizText)
-  public answer: QuizText;
+  public answer?: QuizText;
 }
diff --git a/src/app/data-structure/quiz-question.ts b/src/app/data-structure/quiz-question.ts
index 2082040..457c87d 100644
--- a/src/app/data-structure/quiz-question.ts
+++ b/src/app/data-structure/quiz-question.ts
@@ -8,25 +8,25 @@ export class QuizQuestion extends HasIdAndTitle {
   }
 
   @Type(() => QuizText)
-  public text: QuizText;
+  public text?: QuizText;
 
-  public note: string;
-  public videoUrl: string;
-  public codeUrl: string;
+  public note: string = "";
+  public videoUrl: string = "";
+  public codeUrl: string = "";
 
   @Type(() => QuizText)
-  public choices: QuizText[];
+  public choices?: QuizText[];
 
-  public sectionId: string;
-  public subSectionId: string;
+  public sectionId: string = "";
+  public subSectionId: string = "";
 
-  public quizTitle: string;
+  public quizTitle: string = "";
 
   @Type(() => HasIdAndTitle)
-  public section: HasIdAndTitle;
+  public section?: HasIdAndTitle;
 
   @Type(() => HasIdAndTitle)
-  public subSection: HasIdAndTitle;
+  public subSection?: HasIdAndTitle;
 
-  public quizUsesMathML: boolean;
+  public quizUsesMathML: boolean = false;
 }
diff --git a/src/app/data-structure/quiz-section.ts b/src/app/data-structure/quiz-section.ts
index 53851b2..0636e6f 100644
--- a/src/app/data-structure/quiz-section.ts
+++ b/src/app/data-structure/quiz-section.ts
@@ -1,12 +1,11 @@
 import { plainToClass, Type } from "class-transformer";
-import {isUndefined} from "util";
 import { HasIdAndTitle } from "./has-id-and-title";
 import { QuizQuestionAndAnswer } from "./quiz-question-and-answer";
 import { QuizSubSection } from "./quiz-sub-section";
 
 export class QuizSection extends HasIdAndTitle {
-  public static fromJson(obj: any): QuizSection {
-    if (isUndefined(obj)) {
+  public static fromJson(obj: any): QuizSection | undefined {
+    if (!obj!) {
       return undefined;
     }
 
@@ -14,9 +13,9 @@ export class QuizSection extends HasIdAndTitle {
   }
 
   @Type(() => QuizSubSection)
-  public subSections: QuizSubSection[];
+  public subSections: QuizSubSection[] = [];
 
   // defaultChoices
   @Type(() => QuizQuestionAndAnswer)
-  public questions: QuizQuestionAndAnswer[];
+  public questions: QuizQuestionAndAnswer[] = [];
 }
diff --git a/src/app/data-structure/quiz-sub-section.ts b/src/app/data-structure/quiz-sub-section.ts
index 28f50f3..0120213 100644
--- a/src/app/data-structure/quiz-sub-section.ts
+++ b/src/app/data-structure/quiz-sub-section.ts
@@ -5,7 +5,7 @@ import { QuizQuestionAndAnswer } from "./quiz-question-and-answer";
 export class QuizSubSection extends HasIdAndTitle {
 
   @Type(() => QuizQuestionAndAnswer)
-  public questions: QuizQuestionAndAnswer[];
+  public questions: QuizQuestionAndAnswer[] = [];
 
   public addQuestion(qa: QuizQuestionAndAnswer): void {
     if (!this.questions) {
diff --git a/src/app/data-structure/quiz-text.ts b/src/app/data-structure/quiz-text.ts
index 96942f7..009ba2d 100644
--- a/src/app/data-structure/quiz-text.ts
+++ b/src/app/data-structure/quiz-text.ts
@@ -1,4 +1,4 @@
 export class QuizText {
-  public text: string;
-  public isHtml: boolean;
+  public text: string = "";
+  public isHtml: boolean = false;
 }
diff --git a/src/app/data-structure/quiz.ts b/src/app/data-structure/quiz.ts
index 4b73e45..b51a343 100644
--- a/src/app/data-structure/quiz.ts
+++ b/src/app/data-structure/quiz.ts
@@ -8,9 +8,9 @@ export class Quiz extends HasIdAndTitle {
     return plainToClass(Quiz, obj as object);
   }
 
-  public isPrivate: boolean;
-  public usesMathML: boolean;
+  public isPrivate: boolean = false;
+  public usesMathML: boolean = false;
 
   @Type(() => QuizSection)
-  public sections: QuizSection[];
+  public sections: QuizSection[] = [];
 }
diff --git a/src/app/data-structure/submission-result.ts b/src/app/data-structure/submission-result.ts
index 88d7557..8d7fa5d 100644
--- a/src/app/data-structure/submission-result.ts
+++ b/src/app/data-structure/submission-result.ts
@@ -7,11 +7,11 @@ export class SubmissionResult {
     return plainToClass(SubmissionResult, obj as object);
   }
 
-  public result: boolean;
+  public result: boolean = false;
 
   @Type(() => QuizText)
-  public correctAnswer: QuizText;
+  public correctAnswer?: QuizText = undefined;
 
   @Type(() => QuizQuestion)
-  public nextQuestion: QuizQuestion;
+  public nextQuestion?: QuizQuestion = undefined;
 }
diff --git a/src/app/data-structure/submission.ts b/src/app/data-structure/submission.ts
index 4d3ee87..596586c 100644
--- a/src/app/data-structure/submission.ts
+++ b/src/app/data-structure/submission.ts
@@ -1,3 +1,3 @@
 export class Submission {
-  public answer: string;
+  public answer: string = "";
 }
diff --git a/src/app/data-structure/user-history-quizzes.ts b/src/app/data-structure/user-history-quizzes.ts
index c2cf948..ed3bd04 100644
--- a/src/app/data-structure/user-history-quizzes.ts
+++ b/src/app/data-structure/user-history-quizzes.ts
@@ -7,5 +7,5 @@ export class UserHistoryQuizzes {
   }
 
   @Type(() => UserStats)
-  public stats: UserStats[];
+  public stats: UserStats[] = [];
 }
diff --git a/src/app/data-structure/user-history-sections.ts b/src/app/data-structure/user-history-sections.ts
index 8b66474..fceaa9d 100644
--- a/src/app/data-structure/user-history-sections.ts
+++ b/src/app/data-structure/user-history-sections.ts
@@ -17,27 +17,27 @@ export class UserHistorySections {
       }
 
       // After parsing, we will use only statsMap.
-      result.stats = undefined;
+      result.stats = [];
     }
 
     return result;
   }
 
   @Type(() => LoginInfo)
-  public loginInfo: LoginInfo;
+  public loginInfo?: LoginInfo = undefined;
 
-  public quizId: string;
-  public quizTitle: string;
+  public quizId: string = "";
+  public quizTitle: string = "";
 
   // Only used when parsing from JSON.
   @Type(() => UserStats)
-  public stats: UserStats[];
+  public stats: UserStats[] = [];
 
   // Map of section IDs to stats.
   // This is built from stats
-  public statsMap: Map<string, UserStats>;
+  public statsMap: Map<string, UserStats> = new Map<string, UserStats>();
 
-  public getUserStatsForSection(sectionId: string): UserStats {
+  public getUserStatsForSection(sectionId: string): UserStats | undefined {
     if (!this.statsMap) {
       return undefined;
     }
@@ -46,10 +46,6 @@ export class UserHistorySections {
   }
 
   public setUserStatsForSection(sectionId: string, userStats: UserStats) {
-    if (!this.statsMap) {
-      this.statsMap = new Map<string, UserStats>();
-    }
-
     // Update in the map.
     this.statsMap.set(sectionId, userStats);
   }
diff --git a/src/app/data-structure/user-question-history.ts b/src/app/data-structure/user-question-history.ts
index f0c01fa..525c403 100644
--- a/src/app/data-structure/user-question-history.ts
+++ b/src/app/data-structure/user-question-history.ts
@@ -15,13 +15,13 @@ export class UserQuestionHistory {
     return result;
   }
 
-  public questionId: string;
+  public questionId: string = "";
 
   @Type(() => QuizText)
-  public questionTitle: QuizText;
+  public questionTitle?: QuizText = undefined;
 
-  public sectionId: string;
-  public subSectionTitle: string;
+  public sectionId: string = "";
+  public subSectionTitle: string = "";
 
   public answeredCorrectlyOnce = false;
   public countAnsweredWrong = 0;
diff --git a/src/app/data-structure/user-stats.ts b/src/app/data-structure/user-stats.ts
index a7c4fb1..002f2a4 100644
--- a/src/app/data-structure/user-stats.ts
+++ b/src/app/data-structure/user-stats.ts
@@ -17,7 +17,7 @@ export class UserStats {
         result.questionHistoriesMap.set(questionId, questionHistory);
 
         // After this point, we will only use questionHistoriesMap.
-        result.questionHistories = undefined;
+        result.questionHistories = [];
       }
     }
 
@@ -41,10 +41,10 @@ export class UserStats {
     return Number((part / total) * 100).toFixed(2) + "%";
   }
 
-  public quizId: string;
-  public quizTitle: string;
-  public sectionId: string;
-  public sectionTitle: string;
+  public quizId: string = "";
+  public quizTitle: string = "";
+  public sectionId: string = "";
+  public sectionTitle: string = "";
 
   public answered = 0;
   public correct = 0;
@@ -52,18 +52,18 @@ export class UserStats {
   public countQuestionsAnsweredOnce = 0;
   public countQuestionsCorrectOnce = 0;
 
-  public problemQuestionHistoriesCount: number;
+  public problemQuestionHistoriesCount: number = 0;
 
   @Type(() => UserQuestionHistory)
-  public topProblemQuestionHistories: UserQuestionHistory[];
+  public topProblemQuestionHistories: UserQuestionHistory[] = [];
 
   // Only used for parsing from JSON.
   @Type(() => UserQuestionHistory)
-  public questionHistories: UserQuestionHistory[];
+  public questionHistories: UserQuestionHistory[] = [];
 
   // Built from questionHistories.
   @Exclude()
-  public questionHistoriesMap: Map<string, UserQuestionHistory>;
+  public questionHistoriesMap: Map<string, UserQuestionHistory> = new Map<string, UserQuestionHistory>();
 
   public percentAnsweredOnce(total: number): string {
     return UserStats.percentageString(total, this.countQuestionsAnsweredOnce);
@@ -83,7 +83,7 @@ export class UserStats {
       this.questionHistoriesMap = new Map<string, UserQuestionHistory>();
     }
 
-    let history: UserQuestionHistory = this.questionHistoriesMap.get(questionId);
+    let history: UserQuestionHistory | undefined = this.questionHistoriesMap.get(questionId);
 
     // Add a new one, if necessary:
     if (!history) {
@@ -114,7 +114,7 @@ export class UserStats {
   private updateTopProblemQuestions(): void {
     this.problemQuestionHistoriesCount = 0;
 
-    this.topProblemQuestionHistories = undefined;
+    this.topProblemQuestionHistories = [];
 
     if (!this.questionHistoriesMap) {
       return;
@@ -123,18 +123,10 @@ export class UserStats {
     // Copy the values of the array.
     this.questionHistoriesMap.forEach((value: UserQuestionHistory, key: string) => {
       if (value.countAnsweredWrong > 0) {
-        if (!this.topProblemQuestionHistories) {
-          this.topProblemQuestionHistories = [];
-        }
-
         this.topProblemQuestionHistories.push(value);
       }
     });
 
-    if (!this.topProblemQuestionHistories) {
-      return;
-    }
-
     // Sort the array.
     this.topProblemQuestionHistories.sort((a, b) => {
       if (!a) {
@@ -169,25 +161,17 @@ export class UserStats {
 
   private clearNonProblemQuestions() {
     // TODO: Use a Set when/if TypeScript has one.
-    let idsToRemove: Map<string, boolean>;
+    const idsToRemove: Map<string, boolean> = new Map<string, boolean>();
     for (const history of this.topProblemQuestionHistories) {
       if (!history) {
         continue;
       }
 
       if (history.countAnsweredWrong <= 0) {
-        if (!idsToRemove) {
-          idsToRemove = new Map<string, boolean>();
-        }
-
         idsToRemove.set(history.questionId, true);
       }
     }
 
-    if (!idsToRemove) {
-      return;
-    }
-
     const list: UserQuestionHistory[] = [];
     for (const history of this.topProblemQuestionHistories) {
       if (!history) {
diff --git a/src/app/history.component.ts b/src/app/history.component.ts
index dd1c317..9931358 100644
--- a/src/app/history.component.ts
+++ b/src/app/history.component.ts
@@ -13,10 +13,10 @@ import { UserHistorySectionsComponent } from "./user-history-sections.component"
 })
 export class HistoryComponent extends BaseComponent {
 
-  public quizId: string;
-  public quizTitle: string;
+  public quizId: string = "";
+  public quizTitle: string = "";
   @ViewChild(UserHistorySectionsComponent, { static: true })
-  private sectionsComponent: UserHistorySectionsComponent;
+  private sectionsComponent?: UserHistorySectionsComponent;
 
   constructor(private userHistoryService: UserHistoryService,
               titleService: Title) {
@@ -24,9 +24,13 @@ export class HistoryComponent extends BaseComponent {
   }
 
   public onViewChildJsonParsed() {
-    const sections: UserHistorySections = this.sectionsComponent.userHistorySections;
-    this.quizId = sections.quizId;
-    this.quizTitle = sections.quizTitle;
+    if (this.sectionsComponent) {
+      const sections: UserHistorySections | undefined = this.sectionsComponent.userHistorySections;
+      if (sections) {
+        this.quizId = sections.quizId;
+        this.quizTitle = sections.quizTitle;
+      }
+    }
 
     this.setTitle("History: " + this.quizTitle);
   }
diff --git a/src/app/login.component.ts b/src/app/login.component.ts
index f1b533e..669b222 100644
--- a/src/app/login.component.ts
+++ b/src/app/login.component.ts
@@ -1,6 +1,6 @@
 import { Component, Input, NgZone, OnInit } from "@angular/core";
 import { Title } from "@angular/platform-browser";
-import { ActivatedRoute, ParamMap } from "@angular/router";
+import {ActivatedRoute, ParamMap, Params} from "@angular/router";
 
 import { BaseComponent } from "./base.component";
 import { Config } from "./config";
@@ -20,10 +20,10 @@ export class LoginComponent extends BaseComponent implements OnInit {
   private static redirectSuffix = `?redirect=${Config.baseUrl}/user`;
   // We don't want to show the logout button in the header,
   // but we do want to show it on the user/profile page.
-  @Input() public showLogOutWhenAppropriate: boolean;
+  @Input() public showLogOutWhenAppropriate: boolean = false;
 
-  public loginInfo: LoginInfo;
-  public loginFailed: boolean;
+  public loginInfo?: LoginInfo;
+  public loginFailed: boolean = false;
   public googleLoginUrl = `${Config.googleLoginPrefix}${LoginComponent.redirectSuffix}`;
   public gitHubLoginUrl = `${Config.gitHubLoginPrefix}${LoginComponent.redirectSuffix}`;
   public facebookLoginUrl = `${Config.facebookLoginPrefix}${LoginComponent.redirectSuffix}`;
diff --git a/src/app/question-result-event.ts b/src/app/question-result-event.ts
index a99079c..ee930ba 100644
--- a/src/app/question-result-event.ts
+++ b/src/app/question-result-event.ts
@@ -1,6 +1,6 @@
 import { QuizQuestion } from "./data-structure/quiz-question";
 
 export class QuestionResultEvent {
-  public question: QuizQuestion;
-  public result: boolean;
+  public question?: QuizQuestion;
+  public result: boolean = false;
 }
diff --git a/src/app/question.component.ts b/src/app/question.component.ts
index ef34944..d10575e 100644
--- a/src/app/question.component.ts
+++ b/src/app/question.component.ts
@@ -2,6 +2,7 @@ import { Component } from "@angular/core";
 import { OnInit } from "@angular/core";
 import { Title } from "@angular/platform-browser";
 import { ActivatedRoute, ParamMap, Router } from "@angular/router";
+import { of } from "rxjs";
 import { switchMap } from "rxjs/operators";
 
 import { BaseComponent } from "./base.component";
@@ -24,7 +25,7 @@ import { QuestionResultsService } from "./question-results.service";
 })
 export class QuestionComponent extends BaseComponent implements OnInit {
 
-  private static titleForHasIdAndTitle(obj: HasIdAndTitle): string {
+  private static titleForHasIdAndTitle(obj?: HasIdAndTitle): string {
     if (!obj) {
       return "";
     }
@@ -35,7 +36,7 @@ export class QuestionComponent extends BaseComponent implements OnInit {
   /** Gets the title, if it is not HTML.
    * Otherwise, this returns an empty string.
    */
-  private static titleWithoutHtmlForText(text: QuizText): string {
+  private static titleWithoutHtmlForText(text?: QuizText): string {
     if (!text) {
       return "";
     }
@@ -51,7 +52,7 @@ export class QuestionComponent extends BaseComponent implements OnInit {
    * avoiding use of unparsed HTML * and avoiding duplication where the quiz,
    * section, and subsection titles are duplicates of each other.
    */
-  private static questionTitle(question: QuizQuestion): string {
+  private static questionTitle(question?: QuizQuestion): string {
     if (!question) {
       return "";
     }
@@ -76,20 +77,20 @@ export class QuestionComponent extends BaseComponent implements OnInit {
     return text;
   }
 
-  public quizId: string;
+  public quizId: string = "";
 
-  public question: QuizQuestion;
-  public submissionResult: SubmissionResult;
-  public showAnswer: boolean;
-  public chosenAnswer: string;
+  public question?: QuizQuestion = undefined;
+  public submissionResult?: SubmissionResult = undefined;
+  public showAnswer: boolean = false;
+  public chosenAnswer: string = "";
   public enableChoices = true;
 
-  public sections: QuizSection[];
+  public sections: QuizSection[] = [];
 
   // The section to show questions from.
   // Not just the section ID of the question.
-  public sectionId: string;
-  private questionId: string;
+  public sectionId: string = "";
+  private questionId: string = "";
 
   constructor(private quizService: QuizService,
               private questionService: QuestionService,
@@ -103,10 +104,10 @@ export class QuestionComponent extends BaseComponent implements OnInit {
 
   public ngOnInit(): void {
     this.route.queryParamMap.pipe(
-      switchMap((params: ParamMap) => {
-        this.quizId = params.get("quiz-id");
-        this.questionId = params.get("question-id");
-        this.sectionId = params.get("section-id");
+      switchMap((params: ParamMap, index: number) => {
+        this.quizId = params.get("quiz-id") || "";
+        this.questionId = params.get("question-id") || "";
+        this.sectionId = params.get("section-id") || "";
 
         if (this.submissionResult && this.submissionResult.nextQuestion &&
           this.submissionResult.nextQuestion.id === this.questionId) {
@@ -132,17 +133,17 @@ export class QuestionComponent extends BaseComponent implements OnInit {
             return this.questionService.getNextQuestion(this.quizId, this.sectionId);
             // TODO: Update the URL.
           } else {
-            // TODO: Show an error message?
-            return undefined;
+            return of({
+              error: "No question ID or quiz ID"});
           }
         }
       }))
       .subscribe(
-        (question) => {
+        (question: any) => {
           this.setServerSuccess();
 
           if (this.questionId) {
-            this.question = question;
+            this.question = question as QuizQuestion;
 
             this.setTitle("Question: " + QuestionComponent.questionTitle(this.question));
           } else {
@@ -156,8 +157,9 @@ export class QuestionComponent extends BaseComponent implements OnInit {
               },
             });
           }
+
         },
-        (err) => {
+        (err: any) => {
           this.setServerFailed();
           this.question = undefined;
         });
@@ -292,7 +294,7 @@ export class QuestionComponent extends BaseComponent implements OnInit {
     }
 
     // Wipe these:
-    this.chosenAnswer = undefined;
+    this.chosenAnswer = "";
     this.showAnswer = false;
     this.enableChoices = true;
 
diff --git a/src/app/quiz.component.ts b/src/app/quiz.component.ts
index a963b78..31a1574 100644
--- a/src/app/quiz.component.ts
+++ b/src/app/quiz.component.ts
@@ -15,7 +15,7 @@ import { QuizService } from "./rest-api-clients/quiz.service";
   templateUrl: "./quiz.component.html",
 })
 export class QuizComponent extends BaseComponent implements OnInit {
-  public quiz: Quiz;
+  public quiz?: Quiz = undefined;
 
   constructor(private quizService: QuizService,
               private route: ActivatedRoute,
@@ -26,7 +26,7 @@ export class QuizComponent extends BaseComponent implements OnInit {
   public ngOnInit(): void {
     this.setServerLoading();
     this.route.queryParamMap.pipe(
-      switchMap((params: ParamMap) => this.quizService.getQuiz(params.get("quiz-id"))))
+      switchMap((params: ParamMap) => this.quizService.getQuiz(params.get("quiz-id") || "")))
       .subscribe(
         (quiz) => {
           this.setServerSuccess();
diff --git a/src/app/quizzes.component.ts b/src/app/quizzes.component.ts
index 1a6328c..62c24ad 100644
--- a/src/app/quizzes.component.ts
+++ b/src/app/quizzes.component.ts
@@ -12,7 +12,7 @@ import { Quiz } from "./data-structure/quiz";
   templateUrl: "./quizzes.component.html",
 })
 export class QuizzesComponent extends BaseComponent implements OnInit {
-  public quizzes: Quiz[];
+  public quizzes: Quiz[] = [];
 
   constructor(private quizService: QuizService, titleService: Title) {
     super(titleService);
diff --git a/src/app/server-status.component.ts b/src/app/server-status.component.ts
index efef21b..f92f314 100644
--- a/src/app/server-status.component.ts
+++ b/src/app/server-status.component.ts
@@ -16,5 +16,5 @@ export class ServerStatusComponent {
   public ServerStateEnum = ServerState;
 
   // Note: For some reason we need to define an explicit set,
-  @Input() public state: ServerState;
+  @Input() public state?: ServerState;
 }
diff --git a/src/app/user-history-sections.component.ts b/src/app/user-history-sections.component.ts
index e975ed4..0e6651f 100644
--- a/src/app/user-history-sections.component.ts
+++ b/src/app/user-history-sections.component.ts
@@ -21,13 +21,13 @@ import { QuestionResultsService } from "./question-results.service";
 })
 export class UserHistorySectionsComponent extends BaseComponent implements OnInit, OnDestroy {
   @Output() public onJsonParsed = new EventEmitter<void>();
-  public userHistorySections: UserHistorySections;
+  public userHistorySections?: UserHistorySections;
 
   public readonly MAX_PROBLEM_QUESTIONS: number = 5;
 
-  public quizId: string;
-  private sectionId: string;
-  private subscriptionQuestionResultsService: Subscription;
+  public quizId: string = "";
+  private sectionId: string = "";
+  private subscriptionQuestionResultsService?: Subscription = undefined;
 
   constructor(private userHistoryService: UserHistoryService,
               private questionResultsService: QuestionResultsService,
@@ -39,7 +39,7 @@ export class UserHistorySectionsComponent extends BaseComponent implements OnIni
   public ngOnInit(): void {
     this.route.queryParamMap.pipe(
     switchMap((params: ParamMap) => {
-      const paramQuizId = params.get("quiz-id");
+      const paramQuizId = params.get("quiz-id") || "";
 
       if (this.quizId === paramQuizId && this.userHistorySections) {
         // Use the existing data:
@@ -48,7 +48,7 @@ export class UserHistorySectionsComponent extends BaseComponent implements OnIni
       }
 
       this.quizId = paramQuizId;
-      this.sectionId = params.get("section-id");
+      this.sectionId = params.get("section-id") || "";
 
       this.setServerLoading();
       return this.userHistoryService.getUserHistorySectionsForQuiz(this.quizId);
@@ -71,7 +71,9 @@ export class UserHistorySectionsComponent extends BaseComponent implements OnIni
   }
 
   public ngOnDestroy(): void {
-    this.subscriptionQuestionResultsService.unsubscribe();
+    if (this.subscriptionQuestionResultsService) {
+      this.subscriptionQuestionResultsService.unsubscribe();
+    }
   }
 
   // This is not static because it is difficult to call static method from the .html template.
@@ -118,7 +120,7 @@ export class UserHistorySectionsComponent extends BaseComponent implements OnIni
   }
 
   public generateQuestionLinkQueryParams(qh: UserQuestionHistory): object {
-    const result = {"quiz-id": this.quizId, "question-id": qh.questionId};
+    const result = {"quiz-id": this.quizId, "question-id": qh.questionId, "section-id": ""};
     if (this.sectionId && this.sectionId === qh.sectionId) {
       result["section-id"] = this.sectionId;
     }
@@ -131,11 +133,15 @@ export class UserHistorySectionsComponent extends BaseComponent implements OnIni
       return;
     }
 
+    if (!data.question) {
+      return;
+    }
+
     if (!data.question.sectionId) {
       return;
     }
 
-    const stats: UserStats = this.userHistorySections.getUserStatsForSection(data.question.sectionId);
+    const stats: UserStats | undefined = this.userHistorySections.getUserStatsForSection(data.question.sectionId);
     if (!stats) {
       return;
     }
diff --git a/src/app/user-status.component.ts b/src/app/user-status.component.ts
index b3497e6..2c63a67 100644
--- a/src/app/user-status.component.ts
+++ b/src/app/user-status.component.ts
@@ -14,9 +14,9 @@ export class UserStatusComponent implements OnInit {
   private static redirectSuffix = `?redirect=${Config.baseUrl}/user`;
   // We don't want to show the logout button in the header,
   // but we do want to show it on the user/profile page.
-  @Input() public showExtras: boolean;
+  @Input() public showExtras: boolean = false;
 
-  public loginInfo: LoginInfo;
+  public loginInfo?: LoginInfo = undefined;
   public logoutUrl = `${Config.baseApiUrl}/login/logout?redirect=${Config.baseUrl}/user`;
   public googleLoginUrl = `${Config.googleLoginPrefix}${UserStatusComponent.redirectSuffix}`;
   public gitHubLoginUrl = `${Config.gitHubLoginPrefix}${UserStatusComponent.redirectSuffix}`;
diff --git a/src/app/user.component.ts b/src/app/user.component.ts
index 3c942fb..d2c685e 100644
--- a/src/app/user.component.ts
+++ b/src/app/user.component.ts
@@ -11,7 +11,7 @@ import { UserHistoryService } from "./rest-api-clients/user-history.service";
   templateUrl: "./user.component.html",
 })
 export class UserComponent extends BaseComponent implements OnInit {
-  public userHistoryQuizzes: UserHistoryQuizzes;
+  public userHistoryQuizzes?: UserHistoryQuizzes;
 
   constructor(private userHistoryService: UserHistoryService,
               titleService: Title) {
diff --git a/tsconfig.json b/tsconfig.json
index b3ce562..fb9dc81 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -18,6 +18,7 @@
       "dom"
     ],
     "module": "esnext",
-    "baseUrl": "./"
+    "baseUrl": "./",
+    "strict": true
   }
-}
\ No newline at end of file
+}