Pagination

Level: Intermediate 30–60 min

Concepts: Business LogicBoundariesValidationEdge Cases

---

Build a pagination calculator that takes a total item count, page size, and current page — then returns everything a UI needs to render pagination controls.

Requirements

Given:

• totalItems — total number of items in the dataset
• pageSize — number of items per page
• currentPage — the page being viewed (1-based)

Return a pagination result containing:

• totalPages — total number of pages
• currentPage — the current page (validated)
• pageSize — items per page
• startItem — the 1-based index of the first item on this page
• endItem — the 1-based index of the last item on this page
• hasPrevious — whether a previous page exists
• hasNext — whether a next page exists

Validation Rules

• totalItems must be ≥ 0
• pageSize must be ≥ 1
• If currentPage < 1, clamp to 1
• If currentPage > totalPages, clamp to totalPages
• If totalItems is 0, return a result with totalPages = 0 and no items

Test Cases

Total Items	Page Size	Current Page	Total Pages	Start	End	Prev	Next
100	10	1	10	1	10	false	true
100	10	5	10	41	50	true	true
100	10	10	10	91	100	true	false
95	10	10	10	91	95	true	false
1	10	1	1	1	1	false	false
0	10	1	0	0	0	false	false
100	10	0	10	1	10	false	true
100	10	99	10	91	100	true	false

Edge cases:

• Exactly divisible (100 items / 10 per page = 10 pages)
• Not divisible (95 items / 10 per page = 10 pages, last page has 5)
• Single item
• Zero items
• Page number too low (clamp to 1)
• Page number too high (clamp to last page)
• Page size of 1 (every item is its own page)

Step 2: Page Window

Add a pageWindow(windowSize) method that returns the page numbers to display in the UI:

Current Page	Total Pages	Window Size	Page Numbers
1	10	5	[1, 2, 3, 4, 5]
5	10	5	[3, 4, 5, 6, 7]
10	10	5	[6, 7, 8, 9, 10]
3	4	5	[1, 2, 3, 4]
1	2	5	[1, 2]

The window should be centered on the current page when possible, and shift when near the edges.

Bonus

• Add offset-based calculation: return offset and limit for SQL queries
• Add cursor-based pagination: given a sorted list and a cursor value, return the next/previous page
• Add firstPage and lastPage helpers for quick navigation

Hint

Start with totalPages — it’s just ceiling division. Then startItem and endItem — simple arithmetic from page number and page size. Add clamping next. The page window is a separate concern — build it after the basic calculator is solid.