会計ソフト向け為替レートAPI

import Decimal from 'decimal.js';

type HistoricalRates = {
  date: string;
  rates: Record<string, number>;
  source: 'live' | 'ecb_daily' | 'market_daily';
  data_updated_at: string;
  is_forward_filled: boolean;
  derived_symbols: string[];
};

const entry = {
  id: 'inv_1048',
  accountingDate: '2026-07-15',
  currency: 'EUR',
  amount: '2400.00',
} as const;

const response = await fetch(
  `https://api.exchangerate.dev/v1/${entry.accountingDate}/${entry.currency}?symbols=USD`,
  { headers: { Authorization: `Bearer ${process.env.EXR_API_KEY}` } },
);
if (!response.ok) throw new Error('Could not load the accounting-date FX rate');

const fx = (await response.json()) as HistoricalRates;
const rawRate = fx.rates.USD;
if (!Number.isFinite(rawRate)) throw new Error('Missing EUR/USD rate');

// Decimal arithmetic avoids binary floating-point errors in booked money.
const rate = new Decimal(String(rawRate));
const reportingAmount = new Decimal(entry.amount)
  .mul(rate)
  .toDecimalPlaces(2, Decimal.ROUND_HALF_UP)
  .toFixed(2);

await saveLedgerFx({
  entry_id: entry.id,
  original_currency: entry.currency,
  original_amount: entry.amount,
  reporting_currency: 'USD',
  reporting_amount: reportingAmount,
  fx_rate: rate.toString(),
  fx_requested_date: fx.date,
  fx_source: fx.source,
  fx_observed_at: fx.data_updated_at,
  fx_is_forward_filled: fx.is_forward_filled,
});