Calculate optimal amount of ETF shares to sell to take advantage of the german "Sparer-Pauschbetrag" (Node.js)

sparer-pauschbetrag-ausnutzen.js

// Optimal amount of profit (in EUR) to be gained from ETF sale
const optimalProfit = 1144.29

// Sale price (bid) of one share of the ETF in EUR
const bidPrice = 103.74;

// Amount of shares I own in chronologically ascending order.
// (first element = oldest shares)
const shares = [
    // amount, share price at the time in EUR
    [  30.000, 81.67],
    [   6.147, 81.34],
    [ 119.000, 83.99]
];

let fractionedSharesWarning = 'None of the positions will be fractioned.';

const simulateSale = (numSharesSold) => {
    // Iterate through elements of shares array
    let sharesIndex = 0;

    // Total profit (in EUR) earned when numSharesSold is sold at bidPrice.
    let totalProfit = 0;

    // Keep track of how many shares (starting with numSharesSold) we still have to sell.
    let sharesSold = numSharesSold;

    while (sharesSold > 0 && sharesIndex < shares.length) {
        //console.log('--------------------------------------------------------------------');
        //console.log('Shares left to sell:', sharesSold);
    
        const [originalAmount, originalPrice] = shares[sharesIndex];
        let amount = originalAmount;
    
        if (amount > sharesSold) {
            // The position is bigger than the number of shares we still need to sell
            fractionedSharesWarning =
                `Warning: The position with ${amount} ` +
                `shares bought at ${originalPrice} ` +
                `EUR will be reduced to ${Number((amount - sharesSold).toFixed(3))} shares.\n` +
                'Following FIFO, your next sale will start with this position.';
            amount = sharesSold;
        }
    
        const profit = Number(((amount * bidPrice) - (amount * originalPrice)).toFixed(2));

        /*
        console.log(
            'Selling', amount,
            'ETF shares at', bidPrice,
            'EUR per share results in a profit of', profit,
            'EUR'
        );
        */    

        totalProfit += profit;
        sharesSold -= amount;
        sharesIndex += 1;
    }

    totalProfit = Number(totalProfit.toFixed(2));
    //console.log('--------------------------------------------------------------------');
    /*
    console.log(
        'Total profit:', totalProfit,
        'EUR for', numSharesSold,
        'shares sold. Total sale value:', Number((numSharesSold * bidPrice).toFixed(2))
    );
    */
    
    return totalProfit;
};

let numSharesToSell = 0;
let profit = 0;
while (true) {
    numSharesToSell += 1;
    profit = simulateSale(numSharesToSell);
    if (profit >= optimalProfit) {
        numSharesToSell -= 1;
        profit = simulateSale(numSharesToSell);
        break;
    }
}

console.log(
    'You should sell', numSharesToSell,
    'shares to approximate but not exceed the optimal profit of', optimalProfit,
    'EUR with', profit,
    'EUR.'
);

console.log(fractionedSharesWarning);