const taxRates = [
    {
        "country": "BG",
        "rates": [
            {
                "date": "2020-01-01",
                "rate": 10,
            },
            {
                "date": "2020-06-01",
                "rate": 15,
            },
            {
                "date": "2021-01-01",
                "rate": 20,
            }
        ]
    },
    {
        "country": "US",
        "rates": [
            {
                "date": "2020-01-01",
                "rate": 10,
            },
            {
                "date": "2021-01-01",
                "rate": 15,
            }
        ],
    }
];

function findTaxRate(country, taxPointDateString) {
    const taxRate = taxRates.find((rate) => {
        return rate.country === country;
    });

    const taxPointDate = Date.parse(taxPointDateString);
    let resultRate;
    taxRate.rates.forEach(({ date, rate }) => {
        const dateObject = Date.parse(date);
        if (dateObject <= taxPointDate) {
            resultRate = rate;
        }
    });

    if (!resultRate) {
        throw new Error('Cannot determine rate!');
    }

    return resultRate;
}

console.log(findTaxRate('BG', '2020-03-01'));
console.log(findTaxRate('BG', '2020-06-01'));
console.log(findTaxRate('BG', '2020-09-01'));
console.log(findTaxRate('BG', '2021-01-01'));
console.log(findTaxRate('BG', '2021-03-01'));
console.log(findTaxRate('BG', '2021-06-01'));
console.log(findTaxRate('US', '2020-03-01'));
console.log(findTaxRate('US', '2020-06-01'));
console.log(findTaxRate('US', '2021-01-01'));
console.log(findTaxRate('US', '2021-03-01'));
console.log(findTaxRate('US', '2018-03-01'));