@jaypatrickm/streak-counter/src/index.ts

a streak counter inspired by Duolingo

import { formattedDate, shouldIncrementOrResetStreakCount, STREAK_KEY, buildStreak } from "./lib";

export interface Streak {
  currentCount: number
  startDate: string
  lastLoginDate: string
}

export function streakCounter(_localStorage: Storage, date: Date): Streak {
  const streakInLocalStorage = _localStorage.getItem(STREAK_KEY);
  if(streakInLocalStorage) {
    try {
      const streak = JSON.parse(streakInLocalStorage || "") as Streak;
      const STATE = shouldIncrementOrResetStreakCount(date, streak.lastLoginDate)
      
      let updatedStreak = <Streak>{}
      
      switch (STATE) {
        case 'increment': {
          updatedStreak = buildStreak(date, {
                currentCount: streak.currentCount += 1
          })
          _localStorage.setItem(STREAK_KEY, JSON.stringify(updatedStreak))
          break;
        }
        case 'reset': {
          updatedStreak = buildStreak(date)
          _localStorage.setItem(STREAK_KEY, JSON.stringify(updatedStreak))
          break;
        }
        case 'none': 
        default: {
          updatedStreak = streak
        }
      }

      return updatedStreak
    } catch (error) {
      console.error("Failed to parse streak from localStorage")
    }
  }
  
  const streak = {
    currentCount: 1,
    startDate: formattedDate(date),
    lastLoginDate: formattedDate(date)
  }

  _localStorage.setItem(STREAK_KEY, JSON.stringify(streak))
  return streak
}