Revert "refactor: 可読性のため一部でArray.prototype.atを使うように" (#115)

* Revert "refactor: 可読性のため一部で`Array.prototype.at`を使うように (#11274)"

This reverts commit 2b6dbd4fcb.

* Apply suggestions from code review

---------

Co-authored-by: okayurisotto <okayurisotto@proton.me>

packages/backend/src/misc/prelude/array.ts

@@ -67,9 +67,8 @@ 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) {
-		const lastGroup = groups.at(-1);
-		if (lastGroup !== undefined && f(lastGroup[0], x)) {
-			lastGroup.push(x);
+		if (groups.length !== 0 && f(groups[groups.length - 1][0], x)) {
+			groups[groups.length - 1].push(x);
 		} else {
 			groups.push([x]);
 		}