parse_log_v2.js

var fs = require('fs')

var parse_log = function (log) {
  var userPages = {}
  var paths = {}
  logLines = log.split(/\r?\n/)
  logLines.forEach(function (logLine) {
    logLineData = logLine.split(/, /)
    var userId = logLineData[1]
    var pageId = logLineData[2]
    if (userId !== undefined && pageId !== undefined) {
      if (!userPages[userId]) {
        userPages[userId] = []
      }
      var userSession = userPages[userId]
      userSession.push(pageId)
      if (userSession.length === 3) {
        var userPathId = userSession[0] + ',' + userSession[1] + ',' + userSession[2]
        if (!paths[userPathId]) {
          paths[userPathId] = 0
        }
        paths[userPathId]++
        userSession.shift()
      }
    }
  })

  return paths
}

var log = fs.readFileSync('./input2.txt', {
  encoding: 'utf8'
})

var paths = parse_log(log)
var best = 0
var bestIds = []
var pathIds = Object.keys(paths)
pathIds.forEach(function (pathId) {
  if (paths[pathId] > best) {
    bestIds = [pathId]
    best = paths[pathId]
  } else if (paths[pathId] === best) {
    bestIds.push(pathId)
  }
})

console.log(bestIds, best)