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:
- `page` starts at 1; `pageSize` is the maximum number of items per page.
- `items` contains the items of the requested page, in their original order.
- `totalPages` is the number of pages needed for all items.
- `hasNext` is `true` when 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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