var btcAmount = 10;

var prendi_rate = function (callback) {
    var xmlHttp = new XMLHttpRequest();
    xmlHttp.onreadystatechange = function () {
        if (xmlHttp.readyState == 4 && xmlHttp.status == 200) // quando riceviamo una risposta
            callback(xmlHttp.responseText);
    }
    xmlHttp.open("GET", "https://api.coindesk.com/v1/bpi/currentprice/EUR.json", true); // true per asincrona
    xmlHttp.send(null);
}
prendi_rate(function (Rate) { //function(rate) prende il rate

    console.log(Rate);
    var result = JSON.parse(Rate)['bpi']['EUR']['rate_float'];
    console.log(result);

    /*     var btcResult = btcAmount * result;
         console.log(btcResult);
    */

});