如何在Javascript中获取每个月的指定日期？

Question

我需要找到这个月,上个月和特定日期的下个月.

例如,日期设置为每月31日,我期望得到的日期是2018-02-28,2018-03-31和2018-04-30.那些没有31岁的日子比前一天好.

最后生成2期,2018-02-28至2018-03-29,201-03-30至2018-04-31.我不知道如何处理feb和少于31的月份.

var d = new Date();
var tyear = d.getFullYear(); //2018
var tmonth = d.getMonth();  //2  
new Date(2018, tmonth-1, 31);//output 2018-03-02 not what I wanted

Answer 1

我想你需要一个包含 12 个数字的数组。每个数字是每个月的天数，数组中的数字按顺序排列（第一个数字是 31，因为 1 月有 31 天，第二个数字是 28 或 29，表示 2 月），依此类推。然后您将从输入日期并在数组中查找与月份数字 +/- 1 对应的数字。

然后，您需要根据当月的天数构建上个月和下个月的日期。

请参阅内嵌评论：

let daysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

document.getElementById("date").addEventListener("input", function(){
  console.clear();
  
  // Create new Date based on value in date picker
  var selectedDate = new Date(this.value + 'T00:00');

  var year = selectedDate.getYear();

  // Determine if it is a leap year (Feb has 29 days) and update array if so.
  if (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0)) {
    daysInMonths[1] = 29;
  }

  var selectedDateMonth = selectedDate.getMonth();
     
  // Get previous month number (if current month is January, get December)
  let prevMonth = selectedDateMonth > 0 ? selectedDateMonth - 1 : 11;
  
  let prevMonthDate = null;
  
  // If selected date is last day of month...
  if(selectedDate.getDate() === daysInMonths[selectedDateMonth]){
    // Create new date that takes the selected date and subtracts the correct amount of
    // days from it based on a lookup in the array.
    var newDate1 = new Date(selectedDate.getTime());
    prevMonthDate = 
     new Date(newDate1.setDate(selectedDate.getDate() - daysInMonths[selectedDateMonth]));
  } else {
    // Create a new date that is last month and one day earlier
    var newDate2 = new Date(selectedDate.getTime());
    prevMonthDate = 
      new Date(new Date(newDate2.setDate(selectedDate.getDate() - 1))
        .setMonth(selectedDate.getMonth() - 1));
  }
  
  // Get next month (if current month is December, get January
  let nextMonth = selectedDateMonth < 11 ? selectedDateMonth + 1 : 0;  
  
  let nextMonthDate = null;
  
  // Same idea for next month, but add instead of subtract.
  
  // If selected date is last day of month...
  if(selectedDate.getDate() === daysInMonths[selectedDateMonth]){
    var newDate3 = new Date(selectedDate.getTime());
    nextMonthDate = 
     new Date(newDate3.setDate(selectedDate.getDate() + daysInMonths[selectedDateMonth + 1]));
  } else {
    var newDate4 = new Date(selectedDate.getTime());
    nextMonthDate = new Date(new Date(newDate4.setDate(selectedDate.getDate() + 1)).setMonth(selectedDate.getMonth() + 1));
  }  

  console.log("Last month date: " + prevMonthDate.toLocaleDateString());
  console.log("Next month date: " + nextMonthDate.toLocaleDateString());  
});

<p>Pick a date: <input type="date" id="date"></p>