A small dependency-free payments service core: refunds to customers and payouts to vendors, executed against a fake gateway that records every call in an append-only ledger.

Layout

src/charge.js — the gateway client. charge(), refund(), and payout() simulate network latency and append one JSON line per call to the ledger at LEDGER_FILE (default .data/ledger.jsonl). readLedger() parses it.
src/store.js — a tiny JSON-file store at STORE_FILE (default .data/store.json): get, has, set. Reads and writes are synchronous.
src/refunds.js — processRefund(req) for customer refunds.
src/payouts.js — processPayout(req) for vendor payouts.

API contract

processRefund({ orderId, amount, idempotencyKey? }) and processPayout({ vendorId, amount, idempotencyKey? }) each return the gateway receipt ({ id, type, amount, ... }). When the caller supplies an idempotencyKey, a repeated call with the same key must not hit the gateway again; it returns the stored receipt with duplicate: true. Keep these signatures stable — the dashboard and the finance batch job call them directly.

Working here

No external dependencies. npm test runs the tests.
Incident notes live in docs/incidents.md; add an entry when you work one.