parse_log.js

var fs = require('fs')

var parse_log = function (log) {
  var userPages = {}
  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] = []
      }
      userPages[userId].push(pageId)
    }
  })

  var paths = {}
  var userIds = Object.keys(userPages)
  userIds.forEach(function (userId) {
    var userSession = userPages[userId]
    for (var i = 0; userSession[i+2] !== undefined; i++) {
      userPathId = userSession[i] + ',' + userSession[i+1] + ',' + userSession[i+2]
      if (!paths[userPathId]) {
        paths[userPathId] = 0
      }
      paths[userPathId]++
    }
  })

  return paths
}

var log = fs.readFileSync('./input1.txt', {
  encoding: 'utf8'
})

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

console.log(bestId, best)