async/await function return undefined?

You are mixing the Promise then pattern with that of async/await.

These are two different and incompatible coding patterns. await returns a non-Promise value (only within the context of an async function), but then never returns anything other than another Promise.

Either use async/await or Promises, but not both in the same logic.

Inside of your currencyConversion you are mixing two approaches to handle functions that return promises.

You do:

await Currency.findOne(...params..).then(...params..);

Although you would like to do the following as you are using async/await syntax:

let isRecipientCurrencyId = await Currency.findOne(...params..);
...rest of the code..

async function

convertedAmountMoney was undefined because nothing was returned in currencyConversion. You returned inside a .then inside some other promise, but currencyConversion itself returned nothing.

I've fixed your code below so as to go fully async/await, but there are three elses you'll have to handle yourself, because as of now your you didn't precised what to do then. I added three warnings for this.

async function addAvailableFunds(
  recipientAvailableFunds,
  amountMoney,
  recipientId,
  transferCurrencyId,
) {
  const convertedAmountMoney = await currencyConversion(
    transferCurrencyId,
    recipientCurrencyId,
    amountMoney,
  );

  console.log(
    'convertedAmountMoney',
    convertedAmountMoney,
  ); // undefined
}


async function currencyConversion(
  transferCurrencyId,
  recipientCurrencyId,
  amountMoney,
) {
  console.log('transferCurrencyId', transferCurrencyId);
  console.log('recipientCurrencyId', recipientCurrencyId);
  console.log('amountMoney', amountMoney);

  const isRecipientCurrencyId = await Currency.findOne({
    where: {
      id: recipientCurrencyId,
    },
  })
  if (isRecipientCurrencyId) {
    const mainCurrency = isRecipientCurrencyId.main_currency;
    const recipientCurrencyExchangeRate =
      isRecipientCurrencyId.currency_exchange_rate;

    console.log('mainCurrency', mainCurrency);
    console.log(
      'recipientCurrencyExchangeRate',
      recipientCurrencyExchangeRate,
    );

    const isTransferCurrencyId = await Currency.findOne({
      where: {
        id: transferCurrencyId,
      },
    })

    if (isTransferCurrencyId) {
      const transferCurrencyExchangeRate =
        isTransferCurrencyId.currency_exchange_rate;

      console.log(
        'transferCurrencyExchangeRate',
        transferCurrencyExchangeRate,
      );

      if (mainCurrency) {
        const convertedAmountMoney =
          (amountMoney / transferCurrencyExchangeRate) *
          recipientCurrencyExchangeRate;
        console.log('convertedAmountMoney', convertedAmountMoney);
        return convertedAmountMoney; // return number
      }
      console.warn('Will return undefined');
    }
    console.warn('Will return undefined');
  }
  console.warn('Will return undefined');
}