Simple pagination algorithm

simple-pagination.js

// Implementation in ES6
function pagination(c, m) {
    var current = c,
        last = m,
        delta = 2,
        left = current - delta,
        right = current + delta + 1,
        range = [],
        rangeWithDots = [],
        l;

    for (let i = 1; i <= last; i++) {
        if (i == 1 || i == last || i >= left && i < right) {
            range.push(i);
        }
    }

    for (let i of range) {
        if (l) {
            if (i - l === 2) {
                rangeWithDots.push(l + 1);
            } else if (i - l !== 1) {
                rangeWithDots.push('...');
            }
        }
        rangeWithDots.push(i);
        l = i;
    }

    return rangeWithDots;
}

/* 
Test it:

for (let i = 1, l = 20; i <= l; i++)
    console.log(`Selected page ${i}:`, pagination(i, l));

Expected output:

Selected page 1: [1, 2, 3, "...", 20]
Selected page 2: [1, 2, 3, 4, "...", 20]
Selected page 3: [1, 2, 3, 4, 5, "...", 20]
Selected page 4: [1, 2, 3, 4, 5, 6, "...", 20]
Selected page 5: [1, 2, 3, 4, 5, 6, 7, "...", 20]
Selected page 6: [1, "...", 4, 5, 6, 7, 8, "...", 20]
Selected page 7: [1, "...", 5, 6, 7, 8, 9, "...", 20]
Selected page 8: [1, "...", 6, 7, 8, 9, 10, "...", 20]
Selected page 9: [1, "...", 7, 8, 9, 10, 11, "...", 20]
Selected page 10: [1, "...", 8, 9, 10, 11, 12, "...", 20]
Selected page 11: [1, "...", 9, 10, 11, 12, 13, "...", 20]
Selected page 12: [1, "...", 10, 11, 12, 13, 14, "...", 20]
Selected page 13: [1, "...", 11, 12, 13, 14, 15, "...", 20]
Selected page 14: [1, "...", 12, 13, 14, 15, 16, "...", 20]
Selected page 15: [1, "...", 13, 14, 15, 16, 17, "...", 20]
Selected page 16: [1, "...", 14, 15, 16, 17, 18, 19, 20]
Selected page 17: [1, "...", 15, 16, 17, 18, 19, 20]
Selected page 18: [1, "...", 16, 17, 18, 19, 20]
Selected page 19: [1, "...", 17, 18, 19, 20]
Selected page 20: [1, "...", 18, 19, 20]
*/

Another Pagination which ensures a fixed number of items in the pagination array, including ellipses as necessary same as material UI pagination.

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

function pagination(currentPage, totalPages) {
  const leftSiblingIndex = Math.max(currentPage - 1, 1);
  const rightSiblingIndex = Math.min(currentPage + 1, totalPages);

  const shouldShowLeftEllipsis = leftSiblingIndex > 2;
  const shouldShowRightEllipsis = rightSiblingIndex < totalPages - 2;

  const firstPageIndex = 1;
  const lastPageIndex = totalPages;

  if (!shouldShowLeftEllipsis && shouldShowRightEllipsis) {
    const leftItemCount = 3;
    const leftRange = range(1, leftItemCount + 2);

    return [...leftRange, '...', totalPages];
  }

  if (shouldShowLeftEllipsis && !shouldShowRightEllipsis) {
    const rightItemCount = 3;
    const rightRange = range(totalPages - rightItemCount - 1, totalPages);

    return [firstPageIndex, '...', ...rightRange];
  }

  if (shouldShowLeftEllipsis && shouldShowRightEllipsis) {
    const middleRange = range(leftSiblingIndex, rightSiblingIndex);

    return [firstPageIndex, '...', ...middleRange, '...', lastPageIndex];
  }

  return range(1, totalPages);
}


// pagination(1,20) : [1, 2, 3, 4, 5, '...', 20]
// pagination(5,20) : [1, '...', 4, 5, 6, '...', 20]
// pagination(10,20) : [1, '...', 9, 10, 11, '...', 20]
// pagination(18,20) : [1, '...', 16, 17, 18, 19, 20]

After testing one of these great codes, I think that having the same number of elements is indeed very important, in terms of user experience