Truncated Pagination with Variable Fixed Length

truncated-pagination-variable-fixed-length.ts

const MINIMUM_PAGE_SIZE = 5;

const getRange = (start: number, end: number) => {
  const length = end - start + 1;
  return Array.from({length}, (_, i) => start + i);
};

const clamp = (number: number, lower: number, upper: number) => {
  return Math.min(Math.max(number, lower), upper);
}

const pagination = (currentPage: number, pageCount: number, pageSize: number) => {
  let delta: number;
  currentPage = clamp(currentPage, 1, pageCount);
  pageSize = clamp(pageSize, MINIMUM_PAGE_SIZE, pageCount);
  const centerPageSize = pageSize - 5;
  const boundaryPageSize = pageSize - 3;
  if (pageCount <= pageSize) {
      delta = pageSize;
  } else {
      delta =
          currentPage < boundaryPageSize || currentPage > pageCount - boundaryPageSize
              ? boundaryPageSize
              : centerPageSize;
  }

  const range = {
      start: Math.round(currentPage - delta / 2),
      end: Math.round(currentPage + delta / 2),
  };

  if (range.start - 1 === 1 || range.end + 1 === pageCount) {
      range.start += 1;
      range.end += 1;
  }

  let pages: (string | number)[] =
      currentPage > delta
          ? getRange(
                Math.min(range.start, pageCount - delta),
                Math.min(range.end, pageCount),
            )
          : getRange(1, Math.min(pageCount, delta + 1));

  if (currentPage > pageCount - boundaryPageSize) {
      pages = getRange(pageCount - delta, pageCount);
  }

  const withDots = (value: number, pair: (string | number)[]) =>
      pages.length + 1 !== pageCount ? pair : [value];
  if (pages[0] !== 1) {
      pages = withDots(1, [
          1,
          '...',
      ]).concat(pages);
  }
  const lastPage = pages[pages.length - 1];
  if (lastPage && lastPage < pageCount) {
      pages = pages.concat(
          withDots(pageCount, [
              '...',
              pageCount,
          ]),
      );
  }

  return pages;
};