let currency = prompt('Укажите валюту:').toLowerCase();
let sale = confirm('Укажите тип операции - продажа или покупка?');
let exchangeRate = null;
let exchangeRateSymbol = '';

let ratios = {
	usd: {
		buy: 39.15,
		sale: 38.45,
	},
	eur: {
		buy: 39.55,
		sale: 38.55,
	}
}

switch (currency) {
	case 'usd':
		exchangeRate = sale ? ratios.usd.buy : ratios.usd.sale;
		exchangeRateSymbol = '$';
		break;
	case 'eur':
		exchangeRate = sale ? ratios.eur.buy : ratios.eur.sale;
		exchangeRateSymbol = '€';
		break;
	default:
		alert('Неверно указанная валюта');
}

if(currency === 'usd' || currency === 'eur' ) {

	let quantity = +prompt('Укажите величину в гривне для обмена');

	alert(`${quantity} грн по курсу ${sale ? 'продажи' : 'покупки'} в ${currency} = ${(quantity / exchangeRate).toFixed(2)}${exchangeRateSymbol}`);
}