A simple an effective pagination algorithm with an O(n) time complexity where n is the number of pages in the range `(current - delta) ... (current + delta) + 2`

pagination.js

// Pagination algorithm 
// Refactored by https://github.com/BideoWego
// Originally from:
// https://gist.github.com/kottenator/9d936eb3e4e3c3e02598


function pagination(current, max, delta) {

  // Return nada if we got
  // no pages
  if (!max) {
    return [];
  }

  // Find page display range
  let left = current - delta;
  let right = current + delta;

  // Initialize page list
  // with first page
  let pages = [1];

  // Find max between next page `2`
  // and the beginning of the page
  // range around the current page
  // and set it to our start
  let start = Math.max(2, left)

  // Find the min between the right
  // and the max pages
  // and set it to our stop
  let stop = Math.min(right, max);

  // Begin iterating at the min
  // and stop when we get to the right
  // or when we get to max
  for (let i = start; i <= stop; i++) {

    // If the current i is in our display
    // range push it onto the array
    let isInRange = i >= left && i <= right;
    if (isInRange) {
      pages.push(i);
    }
  }

  // If the last element in our array
  // is not the last page number push it
  if (pages[pages.length - 1] !== max) {
    pages.push(max)
  }

  // Initialize last index
  let last = 0;

  // Adding ellipsis
  // Start at 1 since last is set to 0
  for (let i = 1; i < pages.length; i++) {

    // Get previous page number
    let a = pages[last];

    // Get current page number
    let b = pages[i];

    // Get difference between them
    let diff = b - a;

    // If there is a difference
    // larger than 1
    if (diff > 1) {

      // Close the gap if it is
      // a single space
      if (diff === 2) {
        pages.splice(i, 0, b - 1);
      } else {

        // Else and ellipsis
        pages.splice(i, 0, '...');
      }
    }

    // Set last index
    // to current
    last = i;
  }

  // Return page list
  return pages;
}


// Test the algo
if (require.main === module) {
  for (let i = 0; i < 1000; i++) {
    console.log(`Testing max = ${ i }`);
    for (let current = 1, max = i; current <= max; current++) {
      let range = pagination(current, max, 4);
      console.log(`Selected page: ${ current }:`, range);
    }
  }
}