JavaScript：＆＃34; TypeError：无法设置属性＆＃39; monthDay＆＃39;在ES6下未定义

Question

我的代码：

(function(exports) {
   var name = ["January", "Febuary", "March", "April", "May", "June","July",
                "August", "September", "October", "November", "December"];
   exports.month = function(number) {
     return name[number];
   };
   exports.number = function(number) {
     return name.indexOf(number);
   };
})(this.monthDay = {});

console.log(monthDay.number("April"));

我在ES6下遇到以下错误：

  

TypeError：无法设置属性＆＃39; monthDay＆＃39;未定义的

但是，它可以在ES5下运行良好。为什么？