javascript中的函数行为

Question

  var dayName = function() {
  var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
               "Thursday", "Friday", "Saturday"];
  return function(number) {
    return names[number];
  };
}();

console.log(dayName(1));
o/p->Monday

当执行相同的代码而没有paranthesis＆＃39;（）＆＃39;在dayName变量O / P的末尾是不同的......原因是什么？