## 問題 Problem 你需要找出某月的最后一天，但是又不想去當年每個月的日期表。 You need to find the last day of the month, but don’t want to keep a lookup table of the number of days in each month of the year. ## 方案 Solution 使用JavaScript Date下溢特性來找到該月份的前一（第-1天）天。 Use JavaScript’s Date underflow to find the -1th day of the following month: ~~~ now = new Date lastDayOfTheMonth = new Date(1900+now.getYear(), now.getMonth()+1, 0) ~~~ ## 討論 Discussion JavaScript的Date構造函數樂于處理上溢或者下溢情況，這使得日期計算很容易。讓操作變得很容易，無需擔心給定月份有多少天；主需要在算術上做微調即可。在12月份時，上面這個方法實際上會請求當年第13個月的第0天，結果算出來就是下一年1月第一天的前一天，即當年的12月份的第31天。 JavaScript’s Date constructor cheerfully handles overflow and underflow conditions, which makes date math very easy. Given this ease of manipulation, it doesn’t make sense to worry about how many days are in a given month; just nudge the math around. In December, the solution above will actually ask for the 0th day of the 13th month of the current year, which works out to the day before the 1st day of January of NEXT year, which works out to the 31st day of December of the current year.