index.js

var pager = require('pager')

for (let i = 1, l = 20; i <= l; i++) {
  console.log(`Selected page ${i}:`, pager(i, l))
}

pager.js

module.exports = pager

function pager (c, m) {
  var current = c
  var last = m
  var delta = 2
  var left = current - delta
  var right = current + delta + 1
  var range = []
  var rangeDotted = []
  var l

  for (let i = 1; i <= last; i++) {
    if (i === 1 || i === last || i >= left && i < right) {
      range.push(i)
    }
  }

  for (let i of range) {
    if (l) {
      if (i - l === 2) {
        rangeDotted.push(l + 1)
      } else if (i - l !== 1) {
        rangeDotted.push('...')
      }
    }
    rangeDotted.push(i)
    l = i
  }

  return rangeDotted
}