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-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..7458613 100644
--- a/src/app/data-structure/quiz.ts
+++ b/src/app/data-structure/quiz.ts
@@ -8,8 +8,8 @@ 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[];
diff --git a/src/app/data-structure/submission-result.ts b/src/app/data-structure/submission-result.ts
index 88d7557..6262c12 100644
--- a/src/app/data-structure/submission-result.ts
+++ b/src/app/data-structure/submission-result.ts
@@ -7,7 +7,7 @@ export class SubmissionResult {
     return plainToClass(SubmissionResult, obj as object);
   }
 
-  public result: boolean;
+  public result: boolean = false;
 
   @Type(() => QuizText)
   public correctAnswer: QuizText;
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-sections.ts b/src/app/data-structure/user-history-sections.ts
index 8b66474..3617bcd 100644
--- a/src/app/data-structure/user-history-sections.ts
+++ b/src/app/data-structure/user-history-sections.ts
@@ -26,8 +26,8 @@ export class UserHistorySections {
   @Type(() => LoginInfo)
   public loginInfo: LoginInfo;
 
-  public quizId: string;
-  public quizTitle: string;
+  public quizId: string = "";
+  public quizTitle: string = "";
 
   // Only used when parsing from JSON.
   @Type(() => UserStats)
diff --git a/src/app/data-structure/user-question-history.ts b/src/app/data-structure/user-question-history.ts
index f0c01fa..b35066a 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 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..90f9bfa 100644
--- a/src/app/data-structure/user-stats.ts
+++ b/src/app/data-structure/user-stats.ts
@@ -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;
diff --git a/src/app/history.component.ts b/src/app/history.component.ts
index dd1c317..49ba892 100644
--- a/src/app/history.component.ts
+++ b/src/app/history.component.ts
@@ -13,8 +13,8 @@ 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;
 
diff --git a/src/app/question.component.ts b/src/app/question.component.ts
index ef34944..d2b63c2 100644
--- a/src/app/question.component.ts
+++ b/src/app/question.component.ts
@@ -76,7 +76,7 @@ export class QuestionComponent extends BaseComponent implements OnInit {
     return text;
   }
 
-  public quizId: string;
+  public quizId: string = "";
 
   public question: QuizQuestion;
   public submissionResult: SubmissionResult;
@@ -88,8 +88,8 @@ export class QuestionComponent extends BaseComponent implements OnInit {
 
   // 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,