getPagination.js

#!/usr/bin/env node


const DEFAULT_PAGINATION_SIZE = 9
const ELLIPSIS = '...'


function getPagination(currentPage, numberOfPages, maxSize) {
    if (numberOfPages === 1) {
        return [1]
    }

    if (currentPage > numberOfPages) {
        throw new Error('Current page must be less than total number of pages.')
    }

    if (typeof maxSize === 'undefined') maxSize = DEFAULT_PAGINATION_SIZE

    const
        result = [currentPage],
        expectedResultLength = Math.min(maxSize, numberOfPages)

    let left = currentPage - 1,
        right = currentPage + 1

    while (result.length < expectedResultLength) {
        if (left > 0) {
            result.unshift(left)
            left--
        } else if (right <= numberOfPages) {
            result.push(right)
            right++
        }

        if (right <= numberOfPages) {
            result.push(right)
            right++
        } else if (left > 0) {
            result.unshift(left)
            left--
        }
    }

    // Fill in left side
    if (result[0] !== 1) result[0] = 1
    if (result[1] !== 2) result[1] = 2

    // Fill in right side
    if (result[result.length - 1] !== numberOfPages) {
        result[result.length - 1] = numberOfPages
    }

    if (result[result.length - 2] !== numberOfPages - 1) {
        result[result.length - 2] = numberOfPages - 1
    }

    return result
}


function ellipsizeGaps(arr) {
    const result = []

    let last = 0,
        ellipsisCount = 0

    for (const index of arr.keys()) {
        result.push(arr[index])

        if (last && last !== arr[index] - 1) {
            const offset = index + ellipsisCount
            result.splice(offset, 0, ELLIPSIS)
            ellipsisCount++
        }

        last = arr[index]
    }

    return result
}


const
    large = getPagination(13, 30, 9),
    smallRightAligned = getPagination(9, 12, 9),
    smallLeftAligned = getPagination(3, 12, 9),
    tiny = getPagination(2, 4, 9),
    single = getPagination(1, 1, 9)


console.assert(large.length === 9, 'large is wrong length')
console.assert(smallRightAligned.length === 9, 'smallRightAligned is wrong length')
console.assert(smallLeftAligned.length === 9, 'smallLeftAligned is wrong length')
console.assert(tiny.length === 4, 'smallLeftAligned is wrong length')
console.assert(single.length === 1, 'single is wrong length')


console.assert(large.join('.') === [1, 2, 11, 12, 13, 14, 15, 29, 30].join('.'), 'large is wrong')
console.assert(smallRightAligned.join('.') === [1, 2, 6, 7, 8, 9, 10, 11, 12].join('.'), 'smallRightAligned is wrong')
console.assert(smallLeftAligned.join('.') === [1, 2, 3, 4, 5, 6, 7, 11, 12].join('.'), 'smallLeftAligned is wrong')
console.assert(tiny.join('.') === [1, 2, 3, 4].join('.'), 'smallLeftAligned is wrong')
console.assert(single.join('.') === [1].join('.'), 'single is wrong')


console.assert(ellipsizeGaps([1, 2, 5, 6, 9, 10]).join('|') === [1, 2, '...', 5, 6, '...', 9, 10].join('|'))
console.assert(ellipsizeGaps([1, 2, 50, 60]).join('|') === [1, 2, '...', 50, '...', 60].join('|'))
console.assert(ellipsizeGaps([1, 2, 50, 51]).join('|') === [1, 2, '...', 50, 51].join('|'))
console.assert(ellipsizeGaps([1, 2, 50]).join('|') === [1, 2, '...', 50].join('|'))