Simple pagination with three dots

simple-pagination-with-three-dots.js

function Pagination(countPages, maxLength) {
  if (maxLength < 5) return;
  if (countPages < maxLength) maxLength = countPages;

  this.countPages = countPages;
  this.maxLength = maxLength;
  this.currentLength = 0;

  this.currentPage = this.firstPage = 1;
  this.lastPage = this.countPages;

  this.pages = [];
}

Pagination.prototype.setCurrentPage = function(number) {
  if (number <= this.countPages)
    this.currentPage = number;

  return this;
};

Pagination.prototype.generate = function() {
  this.pages = (new Array(this.countPages)).fill(null);
  this.currentLength = 0;

  this.stretchWith(this.firstPage);
  this.stretchWith(this.lastPage);
  this.stretchWith(this.currentPage);

  var delta = 1;
  while (true) {
    if (this.stretchWith(this.currentPage - delta)) break;
    if (this.stretchWith(this.currentPage + delta)) break;
    delta++;

    if (this.currentPage - delta <= this.firstPage && this.currentPage + delta >= this.lastPage) break;
  }
  
  return this.pages.join(' ').replace(/\s\s+/g, ' ... ').split(' ');
};

Pagination.prototype.stretchWith = function(pageNumber) {
  if (this.pages[pageNumber - 1] === null) {
    this.pages[pageNumber - 1] = pageNumber;
    this.currentLength++;
  }

  return this.currentLength === this.maxLength;
};

/*
 * Tests:
 * (new Pagination(100, 10)).setCurrentPage(1).generate() -> ["1", "2", "3", "4", "5", "6", "7", "8", "9", "...", "100"]
 * (new Pagination(100, 10)).setCurrentPage(5).generate() -> ["1", "2", "3", "4", "5", "6", "7", "8", "9", "...", "100"]
 * (new Pagination(100, 10)).setCurrentPage(17).generate() -> ["1", "...", "13", "14", "15", "16", "17", "18", "19", "20", "...", "100"]
 * (new Pagination(100, 10)).setCurrentPage(24).generate() -> ["1", "...", "20", "21", "22", "23", "24", "25", "26", "27", "...", "100"]
 * (new Pagination(100, 10)).setCurrentPage(49).generate() -> ["1", "...", "45", "46", "47", "48", "49", "50", "51", "52", "...", "100"]
 * (new Pagination(100, 10)).setCurrentPage(66).generate() -> ["1", "...", "62", "63", "64", "65", "66", "67", "68", "69", "...", "100"]
 * (new Pagination(100, 10)).setCurrentPage(78).generate() -> ["1", "...", "74", "75", "76", "77", "78", "79", "80", "81", "...", "100"]
 * (new Pagination(100, 10)).setCurrentPage(81).generate() -> ["1", "...", "77", "78", "79", "80", "81", "82", "83", "84", "...", "100"]
 * (new Pagination(100, 10)).setCurrentPage(100).generate() -> ["1", "...", "92", "93", "94", "95", "96", "97", "98", "99", "100"]
 */