Pagination algo in a project context

helpers.pagination_helper.js

const _ = require('lodash');


function _createPages(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;
}


const PaginationHelper = {};


PaginationHelper.paginationFor = (options) => {
  let limit = options.limit;
  let page = +options.page || 0;
  let count = options.count;
  let offset = page * limit;
  offset = offset < count ? offset : 0;
  let total = Math.floor(count / limit);
  let prev = page - 1;
  let next = page + 1;

  let pagination = {
    limit: limit,
    current: page,
    url: options.url,
    total: total,
    offset: offset,
    count: count
  };

  if (prev > 0) {
    pagination.prev = prev;
  }

  if (next < total) {
    pagination.next = next;
  }

  pagination.pages = _createPages(
    page,
    total,
    options.delta || 4
  );

  return pagination;
};




module.exports = PaginationHelper;

routers.products.js

const express = require('express');
const router = express.Router();
const h = require('./../helpers').registered;
const models = {
  sequelize: require('./../models/sequelize')
};
const Product = models.sequelize.Product;





// ----------------------------------------
// Index
// ----------------------------------------
router.get(['/', '/products'], (req, res, next) => {
  let pagination;

  Product.count()
    .then((result) => {
      count = result;
      pagination = h.paginationFor({
        limit: 12,
        page: +req.query.page,
        count: count,
        url: h.productsPath()
      });
      return Product.findAll({
        limit: pagination.limit,
        offset: pagination.offset
      });
    })
    .then((result) => {
      let products = result;
      res.render('products/index', {
        products,
        pagination,
        count
      });
    })
    .catch(next);
});




module.exports = router;

shared._pagination.handlebars

{{#with options }}
  <nav aria-label="Page navigation" class="text-center">
    <ul class="pagination">
      {{#if prev }}
        <li>
          <a href="{{ url }}?page={{ prev }}" aria-label="Previous">
            <span aria-hidden="true">&laquo;</span>
          </a>
        </li>
      {{/if }}
      {{#each pages as |i| }}
        <li
          {{#if (eq i ../current) }}
            class="active"
          {{/if }}>
          {{#if (eq i '...') }}
            <a href="#" onclick="return false;">{{ i }}</a>
          {{else }}
            <a href="{{ url }}?page={{ i }}">{{ i }}</a>
          {{/if}}
        </li>
      {{/each }}
      {{#if next }}
        <li>
          <a href="{{ url }}?page={{ next }}" aria-label="Next">
            <span aria-hidden="true">&raquo;</span>
          </a>
        </li>
      {{/if }}
    </ul>
  </nav>
{{/with }}