Bug of the week · 2026-10-26
Pagination that loses the last item
A list endpoint in a REST API · Logic · difficulty 1/3
A colleague asked the AI assistant: "Write a function paginate(items, page, pageSize) for our list endpoint. Pages start at 1. Return the items of the requested page plus pagination info."
The code came back in thirty seconds, with passing tests. The contract promised by the docstring:
pagestarts at 1;pageSizeis the maximum number of items per page.itemscontains the items of the requested page, in their original order.totalPagesis the number of pages needed for all items.hasNextistruewhen there is a page after the requested one.
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1export type Page<T> = {
2 items: T[];
3 page: number;
4 pageSize: number;
5 totalItems: number;
6 totalPages: number;
7 hasNext: boolean;
8};
9
10/**
11 * Returns page `page` (starting at 1) of `items`, with `pageSize` items per page.
12 */
13export function paginate<T>(items: T[], page: number, pageSize: number): Page<T> {
14 const totalItems = items.length;
15 const totalPages = Math.floor(totalItems / pageSize);
16 const start = (page - 1) * pageSize;
17 const end = start + pageSize;
18 return {
19 items: items.slice(start, end),
20 page,
21 pageSize,
22 totalItems,
23 totalPages,
24 hasNext: page < totalPages,
25 };
26}
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