Automatically insert Actual Budget transactions for loan interest. Add "interestRate:0.xx interestDay:yy" to the account note, where 'yy' is the day of month to insert the transaction. Run `npm install` to install packages, then `node apply-interest.js` to run it (I run it in a daily cron).

apply-interest.js

require("dotenv").config();

const api = require('@actual-app/api');

const payeeName = process.env.IMPORTER_PAYEE_NAME || 'Loan Interest';
const url = process.env.ACTUAL_SERVER_URL || '';
const password = process.env.ACTUAL_SERVER_PASSWORD || '';
const sync_id = process.env.ACTUAL_SYNC_ID || '';
const cache = process.env.IMPORTER_CACHE_DIR || './cache';

if (!url || !password || !sync_id) {
  console.error('Required settings for Actual not provided.');
  process.exit(1);
}

const getAccountBalance = async (account, cutoffDate=new Date()) => {
  const data = await api.runQuery(
    api.q('transactions')
      .filter({
        'account': account.id,
        'date': { $lt: cutoffDate },
      })
      .calculate({ $sum: '$amount' })
      .options({ splits: 'grouped' })
  );
  return data.data;
};

const getLastTransactionDate = async (account, cutoffDate=new Date()) => {
  const data = await api.runQuery(
    api.q('transactions')
      .filter({
        'account': account.id,
        'date': { $lt: cutoffDate },
        'amount': { $gt: 0 },
      })
      .select('date')
      .orderBy({ 'date': 'desc' })
      .limit(1)
      .options({ splits: 'grouped' })
  );
  return data.data[0].date;
};

(async () => {
  console.log("connect");
  await api.init({ serverURL: url, password: password, dataDir: cache });

  console.log("open file");
  await api.downloadBudget(sync_id);

  const payees = await api.getPayees();
  const payee = payees.find(p => p.name === payeeName);

  const accounts = await api.getAccounts();
  for (const account of accounts) {
    if (account.closed) {
      continue;
    }

    const notes = await api.runQuery(
      api.q('notes')
        .filter({
          id: `account-${account.id}`,
        })
        .select('*')
      );

    if (notes.data.length && notes.data[0].note) {
      const note = notes.data[0].note;

      if (note.indexOf('interestRate:') > -1 && note.indexOf('interestDay:') > -1) {
        const interestRate = parseFloat(note.split('interestRate:')[1].split(' ')[0]);
        const interestDay = parseInt(note.split('interestDay:')[1].split(' ')[0]);

        const interestTransactionDate = new Date();
        if (interestTransactionDate.getDate() < interestDay) {
          interestTransactionDate.setMonth(interestTransactionDate.getMonth() - 1);
        }
        interestTransactionDate.setDate(interestDay);
        interestTransactionDate.setHours(5, 0, 0, 0);

        const cutoff = new Date(interestTransactionDate);
        cutoff.setMonth(cutoff.getMonth() - 1);
        cutoff.setDate(cutoff.getDate() + 1);

        const lastDate = await getLastTransactionDate(account, cutoff);
        const balance = await getAccountBalance(account, interestTransactionDate);

        const daysPassed = Math.floor((interestTransactionDate - new Date(lastDate)) / 86400000);

        const compoundedInterest = Math.round(balance * (Math.pow(1 + interestRate / 12, 1) - 1));

        console.log(`== ${account.name} ==`);
        console.log(` -> Balance:  ${balance}`);
        console.log(`      as of ${lastDate}`);
        console.log(` -> # days:   ${daysPassed}`);
        console.log(` -> Interest: ${compoundedInterest}`)
        await api.importTransactions(account.id, [{
          date: interestTransactionDate,
          payee: payee.id,
          amount: compoundedInterest,
          notes: `Interest for 1 month at ${interestRate}%`,
        }]);
      }
    }
  }

  console.log("done");
  await api.shutdown();
})();

package.json

{
  "dependencies": {
    "@actual-app/api": "^6.7.0",
    "dotenv": "^16.4.5"
  }
}