使用ajax从json文件返回值

Question

我有以下代码。我认为return语句或赋值均无效。警报的输出为NaN。

let currencyRate = getCurrencyRate();
let output = currencyRate * 22.50;
alert(output);

function getCurrencyRate() {
    let currencyRate = "";
    $.ajax({
        url: "http://free.currencyconverterapi.com/api/v5/convert?q=USD_INR&compact=y",
        dataType: "json",
        success: function(json) {
            currencyRate += json.USD_INR.val;
            return currencyRate;
        }
    })
}