Generate bracketed page list for pagination; no loops or mutation

pagination.js

/**
 * Calculate start and end page numbers for page list in pagination
 *
 * Page list is expanded to the left and right of the current page by the
 * amount in [bracketing] argument but without exceeding outer bounds.
 *
 * For example (7 pages total, bracketing by 2):
 * [1] 2  3  4  5
 *  1 [2] 3  4  5
 *  1  2 [3] 4  5
 *  2  3 [4] 5  6
 *  3  4 [5] 6  7
 *  3  4  5 [6] 7
 *  3  4  5  6 [7]
 *
 * @param {number} page - current page
 * @param {number} pages - total pages
 * @param {number} bracketing - number of pages shown before/after current
 * @returns {number[]}
 */
function buildPageList(page, pages, bracketing = 2) {
  const expansion = bracketing * 2; // Number of extra pages in list
  const length = expansion + 1; // Total pages in list, incl. current

  if (pages <= length) {
    // Not enough pages for the sliding list, simply list them
    return fillPageList(1, pages);
  }

  // Pages must be within low and high boundary (can never be under 1 or over total)
  const maxBoundary = Math.min(page + bracketing, pages);
  const minBoundary = Math.max(maxBoundary - expansion, 1);

  const last = minBoundary + expansion;
  return fillPageList(minBoundary, last);
}

/**
 * Creates an array of dates for display based on a first and last value.
 * Basically just filling an array with values offset from index.
 *
 * @param {number} first
 * @param {number} last
 * @returns {number[]}
 */
function fillPageList(first, last) {
  return _.range(first, last + 1);
}