javascript date object lost its type
<p>For the following code, </p>

<pre><code>$(function() {
    a = new Date("2008-1-2");

    monthDiff = function(now, then) {
        var months;
        months = (now.getFullYear() - then.getFullYear()) * 12;
        months -= then.getMonth() + 1;
        months += now.getMonth();
        return months;
    };

    intervalToDate = function(interval, start, unit) {
        {
            return {
                day: function() {return new Date(start.getTime() + (interval*24*60*60*1000)); },
                week: function() {return new Date(start.getTime() + (interval*7*24*60*60*1000)); },
                month: function() {
                    // the result value below will not return a date object when running (only an object), what is weird is in the debug console, using the line below will totally return a date object.
                    var result = new Date(start.getTime() + interval*4*7*24*60*60*1000);
                    while (monthDiff(result, start) !== interval) {
                        result += 24*60*60*1000;
                    }
                    return result;
                } ,
                year: function() {
                    return start.getFullYear() + interval;
                }
            }[unit]();
        }
    };

    console.log(intervalToDate(20, a, "day"));
    console.log(intervalToDate(20, a, "week"));
    console.log(intervalToDate(20, a, "month"));
    console.log(intervalToDate(20, a, "year"));
})
</code></pre>

<p>this line:</p>

<pre><code>month: function() {
                        // the result value below will not return a date object when running (only an object), what is weird is in the debug console, using the line below will totally return a date object.
                        var result = new Date(start.getTime() + interval*4*7*24*60*60*1000);
</code></pre>

<p>result will be correctly returned in the debug console. But on running, somehow it is no longer a date object so I ran into the "no method error" when I try to call getFullYear function on it.</p>
