Calculate weekly user retention rate using array arithmetic

WeeklyUserRetentionRate.js

var weeklyRetention = new Array();  // For storing weekly retntion rate

// Unique user sessions grouped by week
var sessions = [{week: 1, ids: [1, 2, 19, 33]}, {week: 2, ids: [5, 2, 19, 33, 55,  88]},....]

// Loop until second to last element of the array
var i = 0;
while(i < sessions.length-1) {
  var usersStartOfPeriod = sessions[i].ids;
  var usersEndOfPeriod = sessions[i+1].ids;
  
  // Find users that dropped out for this week
  var dropouts = _.difference(usersStartOfPeriod, usersEndOfPeriod);
  
  // Calculate the retention rate for this week
  var retention = (usersStartOfPeriod - dropouts)/usersStartOfPeriod
  
  // Store this week's retention rate
  weeklyRetention[sessions[i].week] = retention;
  i+=1;
}