react-bootstrap-table2/packages/react-bootstrap-table2-paginator/src/page.js

const getNormalizedPage = (
  page,
  pageStartIndex
) => {
  const offset = Math.abs(1 - pageStartIndex);
  return page + offset;
};

const endIndex = (
  page,
  sizePerPage,
  pageStartIndex
) => (getNormalizedPage(page, pageStartIndex) * sizePerPage) - 1;

const startIndex = (
  end,
  sizePerPage,
) => end - (sizePerPage - 1);

export const alignPage = (
  data,
  page,
  sizePerPage,
  pageStartIndex
) => {
  const end = endIndex(page, sizePerPage, pageStartIndex);
  const dataSize = data.length;

  if (end - 1 > dataSize) {
    return pageStartIndex;
  }
  return page;
};

export const getByCurrPage = (
  data,
  page,
  sizePerPage,
  pageStartIndex
) => {
  const dataSize = data.length;
  if (!dataSize) return [];

  const end = endIndex(page, sizePerPage, pageStartIndex);
  const start = startIndex(end, sizePerPage);

  const result = [];
  for (let i = start; i <= end; i += 1) {
    result.push(data[i]);
    if (i + 1 === dataSize) break;
  }
  return result;
};