MOBILE-3320 core: Always pass initial value to reduce function

src/addons/mod/assign/pages/submission-list/submission-list.page.ts

@@ -29,6 +29,7 @@ import {
     AddonModAssignSubmission,
     AddonModAssignProvider,
     AddonModAssign,
+    AddonModAssignGrade,
 } from '../../services/assign';
 import { AddonModAssignHelper, AddonModAssignSubmissionFormatted } from '../../services/assign-helper';
 import { AddonModAssignOffline } from '../../services/assign-offline';
@@ -252,7 +253,10 @@ export class AddonModAssignSubmissionListPage implements AfterViewInit, OnDestro
                             // Get the last grade of the submission.
                             const grade = grades
                                 .filter((grade) => grade.userid == submission.userid)
-                                .reduce((a, b) => (a.timemodified > b.timemodified ? a : b));
+                                .reduce(
+                                    (a, b) => (a && a.timemodified > b.timemodified ? a : b),
+                                    <AddonModAssignGrade | undefined> undefined,
+                                );
 
                             if (grade && grade.timemodified < submission.timemodified) {
                                 submission.gradingstatus = AddonModAssignProvider.GRADED_FOLLOWUP_SUBMIT;

src/addons/mod/lesson/services/lesson-offline.ts

@@ -190,7 +190,10 @@ export class AddonModLessonOfflineProvider {
             const attempts = await this.getRetakeAttemptsForPage(lessonId, retake, retakeData.lastquestionpage, siteId);
 
             // Return the attempt with highest timemodified.
-            return attempts.reduce((a, b) => a.timemodified > b.timemodified ? a : b);
+            return attempts.reduce(
+                (a, b) => a && a.timemodified > b.timemodified ? a : b,
+                <AddonModLessonPageAttemptRecord | undefined> undefined,
+            );
         } catch {
             // Error, return undefined.
         }

src/core/features/course/components/format/format.ts

@@ -632,7 +632,7 @@ export class CoreCourseFormatComponent implements OnInit, OnChanges, OnDestroy {
         const completionModules = (<CoreCourseModule[]> [])
             .concat(...this.sections!.map((section) => section.modules))
             .map((module) => module.completion && module.completion > 0 ? 1 : module.completion)
-            .reduce((accumulator, currentValue) => (accumulator || 0) + (currentValue || 0));
+            .reduce((accumulator, currentValue) => (accumulator || 0) + (currentValue || 0), 0);
 
         const moduleProgressPercent = 100 / (completionModules || 1);
         // Use min/max here to avoid floating point rounding errors over/under-flowing the progress bar.