关于nodejs的module导出object时调用问题!

// mymodule.js

var currency = function () {
    var canadianDollar = 0.91;
    var roundTwoDecimals = function (amount) {
        return Math.round(amount * 100) / 100;
    }
    this.canadianToUS = function (canadian) {
        return roundTwoDecimals(canadian * canadianDollar);
    }
    this.USToCanadisn = function (us) {
        return roundTwoDecimals(us / canadianDollar);
    }
}
module.exports = currency;

    // main.js 
var currency = require("./mymodule");

// 50加元等于这个数量的美元:
console.log("50 Canadian dollars equals this amount of US dollars: ");
console.log(currency.canadianToUS(50));

// 30美元加元等于这个数字:
console.log("30 US dollars equals this amount of canadian dollars: ");
console.log(currency.USToCanadisn(30));

这种写法对吗？程序报错了！
还有一种写法：

    // mymodule.js
exports.currency = {
    canadianDollar: 0.91,
    roundTwoDecimals: function (amount) {
        return Math.round(amount * 100) / 100;
    },
    canadianToUS: function (canadian) {
        return this.roundTwoDecimals(canadian * this.canadianDollar);
    },
    USToCanadisn: function (us) {
        return this.roundTwoDecimals(us / this.canadianDollar);
    }
}

    // main.js 
var c = require("./mymodule");

// 50加元等于这个数量的美元:
console.log("50 Canadian dollars equals this amount of US dollars: ");
console.log(c.currency.canadianToUS(50));

// 30美元加元等于这个数字:
console.log("30 US dollars equals this amount of canadian dollars: ");
console.log(c.currency.USToCanadisn(30));

它就可以正常运行了，但是object没法设置私有成员了，此外它也不是书上的例子！

找到问题了！少了个new object