A function to generate the page number list for page links.

pagination.js

/**
 * This function is used to generate the page list like below:
 * 
 * ```
 * [1] 2 3 4 5 6 » 20
 * 1 « 6 7 [8] 9 10 » 20
 * 1 « 15 16 17 18 19 [20]
 * ```
 * 
 * Note:
 * 
 * In the returned page number array:
 * 
 * 1. The special value `-1` represents a more page **before** the current one.
 * 2. The special value `-2` represents a more page **after** the current one.
 * 
 * This function just returns the page number array (consist of numbers). You
 * can use the returned array to render the page list DOM or something you need.
 * See the function demoRender(current: number, pages: number[]) below.
 *
 * @see {@link https://gist.github.com/john-yuan/11a05426963ed28f9bc43bb2465be3e8}
 * 
 * @author John Yuan <https://github.com/john-yuan>
 * @param {number} current - The current page number
 * @param {number} max - The max page number
 * @returns {number[]} - The page number array
 */
var pagination = function (current, max) {
    'use strict';

    var PREV_MORE = -1;
    var NEXT_MORE = -2;
    var pages = [];
    var prev = current - 1;
    var next = current + 1;

    // if the max is less than 11, there is no `more` button needed
    if (max <= 10) {
        next = 1;
        while (next <= max) {
            pages.push(next);
            next += 1;
        }
    } else {
        // make sure we have 2 pages (if available) before the current page
        if (prev >= 1) {
            pages.unshift(prev);
            prev -= 1;
            if (prev >= 1) {
                pages.unshift(prev);
                prev -= 1;
                if (prev === 1) {
                    pages.unshift(1);
                } else if (prev === 2) {
                    pages.unshift(2);
                    pages.unshift(1);
                } else if (prev > 2) {
                    pages.unshift(PREV_MORE);
                    pages.unshift(1);
                }
            }
        }
    
        pages.push(current);
    
        // make sure that we have at least 6 pages (if available) in the first
        if (pages[1] !== PREV_MORE && pages.length < 4) {
            while (pages.length < 4 && next <= max) {
                pages.push(next);
                next += 1;
            }
        }
    
        // make sure that we have 2 pages (if available) after the current page
        if (next <= max) {
            pages.push(next);
            next += 1;
            if (next <= max) {
                pages.push(next);
                next += 1;
                if (next === max) {
                    pages.push(max);
                } else if (next === (max - 1)) {
                    pages.push(max - 1);
                    pages.push(max);
                } else if (next < (max - 1)) {
                    pages.push(NEXT_MORE);
                    pages.push(max);
                }
            }
        }
    
        // make sure that we have at least 6 pages (if available) in the end
        if (pages[1] === PREV_MORE && pages[pages.length - 2] !== NEXT_MORE) {
            prev = pages[2] - 1;
            pages.shift(); // remove first page
            pages.shift(); // rmeove PREV_MORE
    
            while (pages.length < 6 && prev >= 1) {
                pages.unshift(prev);
                prev -= 1;
            }
    
            // add first page and PREV_MORE back
            if (prev === 1) {
                pages.unshift(1);
            } else if (prev === 2) {
                pages.unshift(2);
                pages.unshift(1);
            } else if (prev > 2) {
                pages.unshift(PREV_MORE);
                pages.unshift(1);
            }
        }
    }

    return pages;
};

//-------------------------------------TEST-------------------------------------

/**
 * A demo render function to render the page list generated by pagination
 * 
 * @param {number} current - The current page number
 * @param {number[]} pages - The page number array
 * @returns {string} - The rendered page list
 */
var demoRender = function (current, pages) {
    var rendered = [];
    
    pages.forEach(function (pageNo) {
        if (pageNo === current) {
            rendered.push('[' + pageNo + ']');
        } else if (pageNo === -1) {
            rendered.push('«')
        } else if (pageNo === -2) {
            rendered.push('»')
        } else {
            rendered.push(pageNo);
        }
    });

    return rendered.join(' ');
};

/**
 * This function is used to test the pagination function.
 * 
 * @param {number} max - The max page number
 */
var testPagination = function (max) {
    for (var i = 1; i <= max; i += 1) {
        var pageList = demoRender(i, pagination(i, max));
        console.log(pageList);
    }
};

testPagination(20);

/*******************************************************************************

The output of testPagination(20):

[1] 2 3 4 5 6 » 20
1 [2] 3 4 5 6 » 20
1 2 [3] 4 5 6 » 20
1 2 3 [4] 5 6 » 20
1 2 3 4 [5] 6 7 » 20
1 « 4 5 [6] 7 8 » 20
1 « 5 6 [7] 8 9 » 20
1 « 6 7 [8] 9 10 » 20
1 « 7 8 [9] 10 11 » 20
1 « 8 9 [10] 11 12 » 20
1 « 9 10 [11] 12 13 » 20
1 « 10 11 [12] 13 14 » 20
1 « 11 12 [13] 14 15 » 20
1 « 12 13 [14] 15 16 » 20
1 « 13 14 [15] 16 17 » 20
1 « 14 15 [16] 17 18 19 20
1 « 15 16 [17] 18 19 20
1 « 15 16 17 [18] 19 20
1 « 15 16 17 18 [19] 20
1 « 15 16 17 18 19 [20]

*******************************************************************************/