Reduce_w_Praktyce_JS_Master_by_Overment.js

const formData = {
    name: 'Adam',
    'billing_address_city': 'Warsaw',
    'billing_address_street': 'ul. Pomorska',
    'shipping_address_city': 'Krakow',
    'shipping_address_street': 'ul. Karmelicka'
}

// own implementation
const mergeAddress = (data, keyParam) => {
    regex = new RegExp(keyParam)
    const filtered = Object.keys(data).filter((name) => regex.test(name));
    result = {};
    filtered.forEach(key => {
        resultKey = key.replace(regex, '').substring(1, key.length);
        result[resultKey] = data[key];
        delete data[key];
    });

    data[keyParam] = result;

    return data;
};

console.log(mergeAddress(formData, 'billing_address'));
console.log(mergeAddress(formData, 'shipping_address'));