easyLogic ErrorsJavaScript

The boundary miss

A developer asked an AI assistant to slice a results array into pages of 10 items for a paginated table. The suggested code compiles and looks reasonable, but every page is missing its last row, and users keep reporting gaps between pages.

paginate.js

function getPage(items, pageNumber, pageSize = 10) {
  const start = (pageNumber - 1) * pageSize;
  const end = start + pageSize - 1;
  return items.slice(start, end);
}

Why does every page returned by getPage have only 9 items instead of 10?

Array.prototype.slice's end argument is already exclusive, so subtracting 1 excludes the page's true last item.

pageNumber should start counting from 0, not 1.

pageSize only defaults to 10 when the argument is undefined, not when it's 0.

start is computed before pageNumber is validated as a positive integer.