diff --git a/packages/backend/src/core/chart/core.ts b/packages/backend/src/core/chart/core.ts
index 571702435..7a89233ed 100644
--- a/packages/backend/src/core/chart/core.ts
+++ b/packages/backend/src/core/chart/core.ts
@@ -627,7 +627,7 @@ export default abstract class Chart<T extends Schema> {
 			}
 
 		// 要求された範囲の最も古い箇所に位置するログが存在しなかったら
-		} else if (!isTimeSame(new Date(logs[logs.length - 1].date * 1000), gt)) {
+		} else if (!isTimeSame(new Date(logs.at(-1)!.date * 1000), gt)) {
 			// 要求された範囲の最も古い箇所時点での最も新しいログを持ってきて末尾に追加する
 			// (隙間埋めできないため)
 			const outdatedLog = await repository.findOne({
diff --git a/packages/backend/src/misc/prelude/array.ts b/packages/backend/src/misc/prelude/array.ts
index 0b2830cb7..2524eacfb 100644
--- a/packages/backend/src/misc/prelude/array.ts
+++ b/packages/backend/src/misc/prelude/array.ts
@@ -67,8 +67,9 @@ export function maximum(xs: number[]): number {
 export function groupBy<T>(f: EndoRelation<T>, xs: T[]): T[][] {
 	const groups = [] as T[][];
 	for (const x of xs) {
-		if (groups.length !== 0 && f(groups[groups.length - 1][0], x)) {
-			groups[groups.length - 1].push(x);
+		const lastGroup = groups.at(-1);
+		if (lastGroup !== undefined && f(lastGroup[0], x)) {
+			lastGroup.push(x);
 		} else {
 			groups.push([x]);
 		}
diff --git a/packages/backend/src/queue/processors/CleanRemoteFilesProcessorService.ts b/packages/backend/src/queue/processors/CleanRemoteFilesProcessorService.ts
index c54bf59ae..6f887089e 100644
--- a/packages/backend/src/queue/processors/CleanRemoteFilesProcessorService.ts
+++ b/packages/backend/src/queue/processors/CleanRemoteFilesProcessorService.ts
@@ -1,7 +1,7 @@
 import { Inject, Injectable } from '@nestjs/common';
 import { IsNull, MoreThan, Not } from 'typeorm';
 import { DI } from '@/di-symbols.js';
-import type { DriveFilesRepository } from '@/models/index.js';
+import type { DriveFile, DriveFilesRepository } from '@/models/index.js';
 import type { Config } from '@/config.js';
 import type Logger from '@/logger.js';
 import { DriveService } from '@/core/DriveService.js';
@@ -31,7 +31,7 @@ export class CleanRemoteFilesProcessorService {
 		this.logger.info('Deleting cached remote files...');
 
 		let deletedCount = 0;
-		let cursor: any = null;
+		let cursor: DriveFile['id'] | null = null;
 
 		while (true) {
 			const files = await this.driveFilesRepository.find({
@@ -51,7 +51,7 @@ export class CleanRemoteFilesProcessorService {
 				break;
 			}
 
-			cursor = files[files.length - 1].id;
+			cursor = files.at(-1)?.id ?? null;
 
 			await Promise.all(files.map(file => this.driveService.deleteFileSync(file, true)));
 
diff --git a/packages/backend/src/queue/processors/DeleteAccountProcessorService.ts b/packages/backend/src/queue/processors/DeleteAccountProcessorService.ts
index 65ded170b..b2886563f 100644
--- a/packages/backend/src/queue/processors/DeleteAccountProcessorService.ts
+++ b/packages/backend/src/queue/processors/DeleteAccountProcessorService.ts
@@ -70,7 +70,7 @@ export class DeleteAccountProcessorService {
 					break;
 				}
 
-				cursor = notes[notes.length - 1].id;
+				cursor = notes.at(-1)?.id ?? null;
 
 				await this.notesRepository.delete(notes.map(note => note.id));
 
@@ -101,7 +101,7 @@ export class DeleteAccountProcessorService {
 					break;
 				}
 
-				cursor = files[files.length - 1].id;
+				cursor = files.at(-1)?.id ?? null;
 
 				for (const file of files) {
 					await this.driveService.deleteFileSync(file);
diff --git a/packages/backend/src/queue/processors/DeleteDriveFilesProcessorService.ts b/packages/backend/src/queue/processors/DeleteDriveFilesProcessorService.ts
index 6772c5dc7..07e376233 100644
--- a/packages/backend/src/queue/processors/DeleteDriveFilesProcessorService.ts
+++ b/packages/backend/src/queue/processors/DeleteDriveFilesProcessorService.ts
@@ -1,7 +1,7 @@
 import { Inject, Injectable } from '@nestjs/common';
 import { MoreThan } from 'typeorm';
 import { DI } from '@/di-symbols.js';
-import type { UsersRepository, DriveFilesRepository } from '@/models/index.js';
+import type { UsersRepository, DriveFilesRepository, DriveFile } from '@/models/index.js';
 import type { Config } from '@/config.js';
 import type Logger from '@/logger.js';
 import { DriveService } from '@/core/DriveService.js';
@@ -40,7 +40,7 @@ export class DeleteDriveFilesProcessorService {
 		}
 
 		let deletedCount = 0;
-		let cursor: any = null;
+		let cursor: DriveFile['id'] | null = null;
 
 		while (true) {
 			const files = await this.driveFilesRepository.find({
@@ -59,7 +59,7 @@ export class DeleteDriveFilesProcessorService {
 				break;
 			}
 
-			cursor = files[files.length - 1].id;
+			cursor = files.at(-1)?.id ?? null;
 
 			for (const file of files) {
 				await this.driveService.deleteFileSync(file);
diff --git a/packages/backend/src/queue/processors/ExportBlockingProcessorService.ts b/packages/backend/src/queue/processors/ExportBlockingProcessorService.ts
index eb758e162..d100c6d09 100644
--- a/packages/backend/src/queue/processors/ExportBlockingProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportBlockingProcessorService.ts
@@ -3,7 +3,7 @@ import { Inject, Injectable } from '@nestjs/common';
 import { MoreThan } from 'typeorm';
 import { format as dateFormat } from 'date-fns';
 import { DI } from '@/di-symbols.js';
-import type { UsersRepository, BlockingsRepository } from '@/models/index.js';
+import type { UsersRepository, BlockingsRepository, Blocking } from '@/models/index.js';
 import type { Config } from '@/config.js';
 import type Logger from '@/logger.js';
 import { DriveService } from '@/core/DriveService.js';
@@ -53,7 +53,7 @@ export class ExportBlockingProcessorService {
 			const stream = fs.createWriteStream(path, { flags: 'a' });
 
 			let exportedCount = 0;
-			let cursor: any = null;
+			let cursor: Blocking['id'] | null = null;
 
 			while (true) {
 				const blockings = await this.blockingsRepository.find({
@@ -72,7 +72,7 @@ export class ExportBlockingProcessorService {
 					break;
 				}
 
-				cursor = blockings[blockings.length - 1].id;
+				cursor = blockings.at(-1)?.id ?? null;
 
 				for (const block of blockings) {
 					const u = await this.usersRepository.findOneBy({ id: block.blockeeId });
diff --git a/packages/backend/src/queue/processors/ExportFavoritesProcessorService.ts b/packages/backend/src/queue/processors/ExportFavoritesProcessorService.ts
index 76c38a6b8..2be42b1a7 100644
--- a/packages/backend/src/queue/processors/ExportFavoritesProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportFavoritesProcessorService.ts
@@ -94,7 +94,7 @@ export class ExportFavoritesProcessorService {
 					break;
 				}
 
-				cursor = favorites[favorites.length - 1].id;
+				cursor = favorites.at(-1)?.id ?? null;
 
 				for (const favorite of favorites) {
 					let poll: Poll | undefined;
diff --git a/packages/backend/src/queue/processors/ExportFollowingProcessorService.ts b/packages/backend/src/queue/processors/ExportFollowingProcessorService.ts
index 8726cb140..d54e5e0b3 100644
--- a/packages/backend/src/queue/processors/ExportFollowingProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportFollowingProcessorService.ts
@@ -79,7 +79,7 @@ export class ExportFollowingProcessorService {
 					break;
 				}
 
-				cursor = followings[followings.length - 1].id;
+				cursor = followings.at(-1)?.id ?? null;
 
 				for (const following of followings) {
 					const u = await this.usersRepository.findOneBy({ id: following.followeeId });
diff --git a/packages/backend/src/queue/processors/ExportMutingProcessorService.ts b/packages/backend/src/queue/processors/ExportMutingProcessorService.ts
index 0f11a9e84..030e38931 100644
--- a/packages/backend/src/queue/processors/ExportMutingProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportMutingProcessorService.ts
@@ -3,7 +3,7 @@ import { Inject, Injectable } from '@nestjs/common';
 import { IsNull, MoreThan } from 'typeorm';
 import { format as dateFormat } from 'date-fns';
 import { DI } from '@/di-symbols.js';
-import type { MutingsRepository, UsersRepository, BlockingsRepository } from '@/models/index.js';
+import type { MutingsRepository, UsersRepository, BlockingsRepository, Muting } from '@/models/index.js';
 import type { Config } from '@/config.js';
 import type Logger from '@/logger.js';
 import { DriveService } from '@/core/DriveService.js';
@@ -56,7 +56,7 @@ export class ExportMutingProcessorService {
 			const stream = fs.createWriteStream(path, { flags: 'a' });
 
 			let exportedCount = 0;
-			let cursor: any = null;
+			let cursor: Muting['id'] | null = null;
 
 			while (true) {
 				const mutes = await this.mutingsRepository.find({
@@ -76,7 +76,7 @@ export class ExportMutingProcessorService {
 					break;
 				}
 
-				cursor = mutes[mutes.length - 1].id;
+				cursor = mutes.at(-1)?.id ?? null;
 
 				for (const mute of mutes) {
 					const u = await this.usersRepository.findOneBy({ id: mute.muteeId });
diff --git a/packages/backend/src/queue/processors/ExportNotesProcessorService.ts b/packages/backend/src/queue/processors/ExportNotesProcessorService.ts
index 24fb33188..75f32ffee 100644
--- a/packages/backend/src/queue/processors/ExportNotesProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportNotesProcessorService.ts
@@ -90,7 +90,7 @@ export class ExportNotesProcessorService {
 					break;
 				}
 
-				cursor = notes[notes.length - 1].id;
+				cursor = notes.at(-1)?.id ?? null;
 
 				for (const note of notes) {
 					let poll: Poll | undefined;
diff --git a/packages/backend/src/server/ActivityPubServerService.ts b/packages/backend/src/server/ActivityPubServerService.ts
index f75170934..634f5f0a4 100644
--- a/packages/backend/src/server/ActivityPubServerService.ts
+++ b/packages/backend/src/server/ActivityPubServerService.ts
@@ -181,7 +181,7 @@ export class ActivityPubServerService {
 				undefined,
 				inStock ? `${partOf}?${url.query({
 					page: 'true',
-					cursor: followings[followings.length - 1].id,
+					cursor: followings.at(-1)!.id,
 				})}` : undefined,
 			);
 
@@ -273,7 +273,7 @@ export class ActivityPubServerService {
 				undefined,
 				inStock ? `${partOf}?${url.query({
 					page: 'true',
-					cursor: followings[followings.length - 1].id,
+					cursor: followings.at(-1)!.id,
 				})}` : undefined,
 			);
 
@@ -398,7 +398,7 @@ export class ActivityPubServerService {
 				})}` : undefined,
 				notes.length ? `${partOf}?${url.query({
 					page: 'true',
-					until_id: notes[notes.length - 1].id,
+					until_id: notes.at(-1)!.id,
 				})}` : undefined,
 			);
 
diff --git a/packages/backend/test/utils.ts b/packages/backend/test/utils.ts
index 48947072e..31ea3e5ab 100644
--- a/packages/backend/test/utils.ts
+++ b/packages/backend/test/utils.ts
@@ -447,12 +447,12 @@ export async function testPaginationConsistency<Entity extends { id: string, cre
 	for (const limit of [1, 5, 10, 100, undefined]) {
 		// 1. sinceId/DateとuntilId/Dateで両端を指定して取得した結果が期待通りになっていること
 		if (ordering === 'desc') {
-			const end = expected[expected.length - 1];
+			const end = expected.at(-1)!;
 			let last = await fetchEntities(rangeToParam({ limit, since: end }));
 			const actual: Entity[] = [];
 			while (last.length !== 0) {
 				actual.push(...last);
-				last = await fetchEntities(rangeToParam({ limit, until: last[last.length - 1], since: end }));
+				last = await fetchEntities(rangeToParam({ limit, until: last.at(-1), since: end }));
 			}
 			actual.push(end);
 			assert.deepStrictEqual(
@@ -467,7 +467,7 @@ export async function testPaginationConsistency<Entity extends { id: string, cre
 			const actual: Entity[] = [];
 			while (last.length !== 0) {
 				actual.push(...last);
-				last = await fetchEntities(rangeToParam({ limit, since: last[last.length - 1] }));
+				last = await fetchEntities(rangeToParam({ limit, since: last.at(-1) }));
 			}
 			assert.deepStrictEqual(
 				actual.map(({ id, createdAt }) => id + ':' + createdAt),
@@ -480,7 +480,7 @@ export async function testPaginationConsistency<Entity extends { id: string, cre
 			const actual: Entity[] = [];
 			while (last.length !== 0) {
 				actual.push(...last);
-				last = await fetchEntities(rangeToParam({ limit, until: last[last.length - 1] }));
+				last = await fetchEntities(rangeToParam({ limit, until: last.at(-1) }));
 			}
 			assert.deepStrictEqual(
 				actual.map(({ id, createdAt }) => id + ':' + createdAt),
diff --git a/packages/frontend/src/components/MkDrive.vue b/packages/frontend/src/components/MkDrive.vue
index 201a6ccdc..aff227da4 100644
--- a/packages/frontend/src/components/MkDrive.vue
+++ b/packages/frontend/src/components/MkDrive.vue
@@ -568,7 +568,7 @@ function fetchMoreFolders() {
 	os.api('drive/folders', {
 		folderId: folder.value ? folder.value.id : null,
 		type: props.type,
-		untilId: folders.value[folders.value.length - 1].id,
+		untilId: folders.value.at(-1)?.id,
 		limit: max + 1,
 	}).then(folders => {
 		if (folders.length === max + 1) {
@@ -591,7 +591,7 @@ function fetchMoreFiles() {
 	os.api('drive/files', {
 		folderId: folder.value ? folder.value.id : null,
 		type: props.type,
-		untilId: files.value[files.value.length - 1].id,
+		untilId: files.value.at(-1)?.id,
 		limit: max + 1,
 	}).then(files => {
 		if (files.length === max + 1) {
diff --git a/packages/frontend/src/components/MkMiniChart.vue b/packages/frontend/src/components/MkMiniChart.vue
index 89050e10f..e88445570 100644
--- a/packages/frontend/src/components/MkMiniChart.vue
+++ b/packages/frontend/src/components/MkMiniChart.vue
@@ -59,8 +59,8 @@ function draw(): void {
 
 	polygonPoints = `0,${ viewBoxY } ${ polylinePoints } ${ viewBoxX },${ viewBoxY }`;
 
-	headX = _polylinePoints[_polylinePoints.length - 1][0];
-	headY = _polylinePoints[_polylinePoints.length - 1][1];
+	headX = _polylinePoints.at(-1)![0];
+	headY = _polylinePoints.at(-1)![1];
 }
 
 watch(() => props.src, draw, { immediate: true });
diff --git a/packages/frontend/src/components/MkPageWindow.vue b/packages/frontend/src/components/MkPageWindow.vue
index 6318a9fd7..6e35ad424 100644
--- a/packages/frontend/src/components/MkPageWindow.vue
+++ b/packages/frontend/src/components/MkPageWindow.vue
@@ -120,7 +120,7 @@ const contextmenu = $computed(() => ([{
 
 function back() {
 	history.pop();
-	router.replace(history[history.length - 1].path, history[history.length - 1].key);
+	router.replace(history.at(-1)!.path, history.at(-1)!.key);
 }
 
 function reload() {
diff --git a/packages/frontend/src/components/MkPagination.vue b/packages/frontend/src/components/MkPagination.vue
index 661b04c36..b9a75f600 100644
--- a/packages/frontend/src/components/MkPagination.vue
+++ b/packages/frontend/src/components/MkPagination.vue
@@ -233,7 +233,7 @@ const fetchMore = async (): Promise<void> => {
 		...(props.pagination.offsetMode ? {
 			offset: offset.value,
 		} : {
-			untilId: Array.from(items.value.keys())[items.value.size - 1],
+			untilId: Array.from(items.value.keys()).at(-1),
 		}),
 	}).then(res => {
 		for (let i = 0; i < res.length; i++) {
@@ -297,7 +297,7 @@ const fetchMoreAhead = async (): Promise<void> => {
 		...(props.pagination.offsetMode ? {
 			offset: offset.value,
 		} : {
-			sinceId: Array.from(items.value.keys())[items.value.size - 1],
+			sinceId: Array.from(items.value.keys()).at(-1),
 		}),
 	}).then(res => {
 		if (res.length === 0) {
diff --git a/packages/frontend/src/scripts/array.ts b/packages/frontend/src/scripts/array.ts
index 4620c8b73..c9a146e70 100644
--- a/packages/frontend/src/scripts/array.ts
+++ b/packages/frontend/src/scripts/array.ts
@@ -78,8 +78,9 @@ export function maximum(xs: number[]): number {
 export function groupBy<T>(f: EndoRelation<T>, xs: T[]): T[][] {
 	const groups = [] as T[][];
 	for (const x of xs) {
-		if (groups.length !== 0 && f(groups[groups.length - 1][0], x)) {
-			groups[groups.length - 1].push(x);
+		const lastGroup = groups.at(-1);
+		if (lastGroup !== undefined && f(lastGroup[0], x)) {
+			lastGroup.push(x);
 		} else {
 			groups.push([x]);
 		}
diff --git a/packages/frontend/src/widgets/server-metric/cpu-mem.vue b/packages/frontend/src/widgets/server-metric/cpu-mem.vue
index c178ba517..b9ba400b4 100644
--- a/packages/frontend/src/widgets/server-metric/cpu-mem.vue
+++ b/packages/frontend/src/widgets/server-metric/cpu-mem.vue
@@ -121,10 +121,10 @@ function onStats(connStats) {
 	cpuPolygonPoints = `${viewBoxX - (stats.length - 1)},${viewBoxY} ${cpuPolylinePoints} ${viewBoxX},${viewBoxY}`;
 	memPolygonPoints = `${viewBoxX - (stats.length - 1)},${viewBoxY} ${memPolylinePoints} ${viewBoxX},${viewBoxY}`;
 
-	cpuHeadX = cpuPolylinePointsStats[cpuPolylinePointsStats.length - 1][0];
-	cpuHeadY = cpuPolylinePointsStats[cpuPolylinePointsStats.length - 1][1];
-	memHeadX = memPolylinePointsStats[memPolylinePointsStats.length - 1][0];
-	memHeadY = memPolylinePointsStats[memPolylinePointsStats.length - 1][1];
+	cpuHeadX = cpuPolylinePointsStats.at(-1)![0];
+	cpuHeadY = cpuPolylinePointsStats.at(-1)![1];
+	memHeadX = memPolylinePointsStats.at(-1)![0];
+	memHeadY = memPolylinePointsStats.at(-1)![1];
 
 	cpuP = (connStats.cpu * 100).toFixed(0);
 	memP = (connStats.mem.active / props.meta.mem.total * 100).toFixed(0);
diff --git a/packages/frontend/src/widgets/server-metric/net.vue b/packages/frontend/src/widgets/server-metric/net.vue
index 5a9134078..817a422e6 100644
--- a/packages/frontend/src/widgets/server-metric/net.vue
+++ b/packages/frontend/src/widgets/server-metric/net.vue
@@ -94,10 +94,10 @@ function onStats(connStats) {
 	inPolygonPoints = `${viewBoxX - (stats.length - 1)},${viewBoxY} ${inPolylinePoints} ${viewBoxX},${viewBoxY}`;
 	outPolygonPoints = `${viewBoxX - (stats.length - 1)},${viewBoxY} ${outPolylinePoints} ${viewBoxX},${viewBoxY}`;
 
-	inHeadX = inPolylinePointsStats[inPolylinePointsStats.length - 1][0];
-	inHeadY = inPolylinePointsStats[inPolylinePointsStats.length - 1][1];
-	outHeadX = outPolylinePointsStats[outPolylinePointsStats.length - 1][0];
-	outHeadY = outPolylinePointsStats[outPolylinePointsStats.length - 1][1];
+	inHeadX = inPolylinePointsStats.at(-1)![0];
+	inHeadY = inPolylinePointsStats.at(-1)![1];
+	outHeadX = outPolylinePointsStats.at(-1)![0];
+	outHeadY = outPolylinePointsStats.at(-1)![1];
 
 	inRecent = connStats.net.rx;
 	outRecent = connStats.net.tx;