Kakeibo Budget App

Existing budgeting apps were either too complex — mandatory bank connections, account setup, and multi-step onboarding — or too simple to support real spending reflection. Living between Japan and Indonesia added a currency dimension most apps ignore entirely: converting receipts between JPY and IDR manually is friction that kills the habit. I wanted Kakeibo-style category discipline, multi-currency awareness, and a fast-add flow that worked at the register.

Every account has its own currency. Every transaction stores both original amount + currency and the display-currency equivalent. On load, the store fetches live exchange rates from open.er-api.com and runs convertAmount() across all transactions for consistent totals. The display currency is a user setting — switching it re-converts everything in memory without touching stored data.

• Per-account currency: each wallet tracks its own denomination independently
• convertAmount(amount, from, to, rates) runs on every aggregation — dashboard totals, group spending, net worth
• Live rates fetched in background on app init; fallback constants (JPY:1, IDR:107) prevent blank states
• originalAmount + originalCurrency preserved on every transaction for accurate historical records

Expo Router with React Native Web runs one codebase on both native and browser. The Repository interface abstracts the storage layer: expo-sqlite on native, a custom IndexedDB-backed web repository in the browser. Zustand hydrates from whichever repository is active, so the rest of the app never knows the difference.

• Repository interface: getTransactions, addAccount, updateBudget, getReflections — same API everywhere
• Dual storage backend: SQLite on native, IndexedDB-backed web repository for browser
• Zustand store hydrates from the active repository on initialize(); guest mode uses local, auth mode uses Supabase
• Vercel deployment with vercel.json SPA rewrites for clean URL routing

The app launches in guest mode by default — fully offline, no sign-up. Supabase magic link OTP is available for cloud sync but never forced. A billing layer (Supabase user metadata) gates the OCR scan feature behind a credit balance and CSV export behind a paid unlock, while keeping the core tracking loop free.

• Guest mode: Zustand + local repository, zero network dependency, no commitment barrier
• Supabase magic link OTP — no password management, deep-link re-entry preserved post-auth
• consume_scan_credit Supabase RPC deducts credits atomically per OCR scan
• CSV export locked behind billing role check; billing_exempt flag for owner accounts

Beyond core tracking: transfer transactions move money between accounts and update both balances. Duplicate detection (amount + date + merchant) flags accidental double-entries before save. Merchant keyword mapping auto-suggests categories from known merchant names. Monthly reflections (what worked / what overspent / what changes) add a journaling layer. No-spend day tracking surfaces clean days on the dashboard.

• Transfer transactions: income/expense/transfer types with toAccountId for balance updates on both sides
• checkForDuplicate() matches amount, date, and merchant before any add — surfaced as a warning, not a block
• merchantKeywords map in settings auto-suggests category on merchant name input
• Reflection model: whatWorked / whatOverspent / whatWillChange, saved per month
• CSV import with suggestMapping() — auto-maps columns by header name heuristics
• i18n support: English, Indonesian, Japanese — switchable live from settings

A cross-platform budgeting app I use daily across JPY and IDR. The guest mode removed the sign-up barrier. Multi-currency conversion makes mixed-currency months legible at a glance. Kakeibo groups turn monthly review into a 2-minute reflection rather than a spreadsheet audit. The OCR scan flow saves manual entry on konbini and restaurant receipts.