calculateEarnedMerit.js

const calculateEarnedMerit = function(activity, rawMerit){
  
  const meritStartDate = new Date("January 24, 2018")
  
  const today = Date.now()
  
  const startMerits = {
  "Brand new": 0,
  "Newbie":0,
  "Jr Member": 1,
  "Member": 10,
  "Full Member": 100,
  "Sr. Member": 250,
  "Hero Member": 500,
  "Legendary": 1000
  }
  
  const activityRequirements = {
    0: "Brand new",
    1: "Newbie",
    30: "Jr Member",
    60: "Member",
    120: "Full Member",
    240: "Sr. Member",
    480: "Hero Member",
    //average activity of freshly ranked-up Legendary
    902: "Legendary"
  }
  //activity at the day of merit system introduction
  //assuming that activity is maxed out between today and meritStartDate
  let a = Math.max(activity - Math.floor((today - meritStartDate)/(1000*60*60*24)),0)
  
  const k = Object.keys(activityRequirements)
  
  let startingMerit = 1000
  // finding the rank at the day of merit system introduction
 for (let i = 0;i<k.length;i++){
  if (a<k[i]){
    startingMerit = startMerits[activityRequirements[k[i-1]]]
    break;
  }
  }
  
  const earnedMerit = rawMerit - startingMerit
  
  return earnedMerit
}