如何在上一年获得月?

I try to get the previous month according to current month. But the problem occurs when "year" is not 2017.

2 .我尽量按月领上月的货。但当“年份”不是2017年时，问题就出现了。

So how can I get the month of the previous year?. The code below will describe what I want, if anybody know how to get it please tell me the method. Thank you :)

那么我怎样才能得到上一年的月份呢?下面的代码将描述我想要的，如果有人知道如何得到它，请告诉我方法。谢谢你:)

var month = new Array();
		month[0] = "January";
		month[1] = "February";
		month[2] = "March";
		month[3] = "April";
		month[4] = "May";
		month[5] = "June";
		month[6] = "July";
		month[7] = "August";
		month[8] = "September";
		month[9] = "October";
		month[10] = "November";
		month[11] = "December";

		var cur_month = new Date();
		var cur_month_now = month[cur_month.getMonth()];
		var pre_month_1 = month[cur_month.getMonth()-1];
		var pre_month_2 = month[cur_month.getMonth()-2];
		var pre_month_3 = month[cur_month.getMonth()-3];
		var pre_month_4 = month[cur_month.getMonth()-4];
		var pre_month_5 = month[cur_month.getMonth()-5];

		document.getElementById("cur_month").innerHTML = cur_month_now;
		document.getElementById("pre_month_1").innerHTML = pre_month_1;
		document.getElementById("pre_month_2").innerHTML = pre_month_2;
		document.getElementById("pre_month_3").innerHTML = pre_month_3;
		document.getElementById("pre_month_4").innerHTML = pre_month_4;
		document.getElementById("pre_month_5").innerHTML = pre_month_5;

<div class="dropdown">
	  <button class="btn btn-danger dropdown-toggle" type="button" data-toggle="dropdown">Select Month
	  <span class="caret"></span></button>
	  <ul class="dropdown-menu">
	    <li><a href="#" id="cur_month"></a></li>
	    <li><a href="#" id="pre_month_1"></a></li>
	    <li><a href="#" id="pre_month_2"></a></li>
	    <li><a href="#" id="pre_month_3"></a></li>
	    <li><a href="#" id="pre_month_4"></a></li>
	    <li><a href="#" id="pre_month_5"></a></li>
	  </ul>
	</div>

4 个解决方案

#1

You are getting this undefined because month[-1] and month[-2] are undefined

因为月[-1]和月[-2]是没有定义的

You need to actually do date manipulation in a date object rather than just getting date from index.

实际上，您需要在一个日期对象中执行日期操作，而不是从索引中获取日期。

Use this method to get last month date

使用这种方法获取上个月的日期

function getPrevMonth(date) {
  date.setMonth(date.getMonth() - 1);
  return date;
}

invoke this method as many times as you need.

根据需要多次调用此方法。

Demo

演示

function getPrevMonth(date) {
  date.setMonth(date.getMonth() - 1);
  return date;
}

var month = new Array();
month[0] = "January";
month[1] = "February";
month[2] = "March";
month[3] = "April";
month[4] = "May";
month[5] = "June";
month[6] = "July";
month[7] = "August";
month[8] = "September";
month[9] = "October";
month[10] = "November";
month[11] = "December";

var cur_month = new Date();
var cur_month_now = month[cur_month.getMonth()];
var pre_month_1 = month[getPrevMonth(cur_month).getMonth()];
var pre_month_2 = month[getPrevMonth(cur_month).getMonth()];
var pre_month_3 = month[getPrevMonth(cur_month).getMonth()];
var pre_month_4 = month[getPrevMonth(cur_month).getMonth()];
var pre_month_5 = month[getPrevMonth(cur_month).getMonth()];

document.getElementById("cur_month").innerHTML = cur_month_now;
document.getElementById("pre_month_1").innerHTML = pre_month_1;
document.getElementById("pre_month_2").innerHTML = pre_month_2;
document.getElementById("pre_month_3").innerHTML = pre_month_3;
document.getElementById("pre_month_4").innerHTML = pre_month_4;
document.getElementById("pre_month_5").innerHTML = pre_month_5;

<div class="dropdown">
  <button class="btn btn-danger dropdown-toggle" type="button" data-toggle="dropdown">Select Month
	  <span class="caret"></span></button>
  <ul class="dropdown-menu">
    <li>
      <a href="#" id="cur_month"></a>
    </li>
    <li>
      <a href="#" id="pre_month_1"></a>
    </li>
    <li>
      <a href="#" id="pre_month_2"></a>
    </li>
    <li>
      <a href="#" id="pre_month_3"></a>
    </li>
    <li>
      <a href="#" id="pre_month_4"></a>
    </li>
    <li>
      <a href="#" id="pre_month_5"></a>
    </li>
  </ul>
</div>

#2

Let Date do the wrap-around for you. There are also a couple of improvements we can make to the code, see comments:

让约会为你做个总结。我们还可以对代码做一些改进，见注释:

// Array initializers are cleaner and less typing
var month = [
    "January",
    "February",
    "March",
    "April",
    "May",
    "June",
    "July",
    "August",
    "September",
    "October",
    "November",
    "December"
];

var dt = new Date();
var cur_month_now = month[dt.getMonth()];
dt.setMonth(dt.getMonth() - 1); // Date handles wrapping to previous year
var pre_month_1 = month[dt.getMonth()];
dt.setMonth(dt.getMonth() - 1);
var pre_month_2 = month[dt.getMonth()];
dt.setMonth(dt.getMonth() - 1);
var pre_month_3 = month[dt.getMonth()];
dt.setMonth(dt.getMonth() - 1);
var pre_month_4 = month[dt.getMonth()];
dt.setMonth(dt.getMonth() - 1);
var pre_month_5 = month[dt.getMonth()];

document.getElementById("cur_month").innerHTML = cur_month_now;
document.getElementById("pre_month_1").innerHTML = pre_month_1;
document.getElementById("pre_month_2").innerHTML = pre_month_2;
document.getElementById("pre_month_3").innerHTML = pre_month_3;
document.getElementById("pre_month_4").innerHTML = pre_month_4;
document.getElementById("pre_month_5").innerHTML = pre_month_5;

<div class="dropdown">
  <button class="btn btn-danger dropdown-toggle" type="button" data-toggle="dropdown">Select Month
	  <span class="caret"></span></button>
  <ul class="dropdown-menu">
    <li>
      <a href="#" id="cur_month"></a>
    </li>
    <li>
      <a href="#" id="pre_month_1"></a>
    </li>
    <li>
      <a href="#" id="pre_month_2"></a>
    </li>
    <li>
      <a href="#" id="pre_month_3"></a>
    </li>
    <li>
      <a href="#" id="pre_month_4"></a>
    </li>
    <li>
      <a href="#" id="pre_month_5"></a>
    </li>
  </ul>
</div>

#3

Nothing really new, but for smaller js code …

没有什么新东西，但对于较小的js代码……

function getPrevMonth(date, m) {
  date.setMonth(date.getMonth() - m);
  return date;
}

//no need for such array
locale = "en-us";//or e.g. navigator.languages[0]
var d = new Date();

for (var i=0; i<6; i++){
  //also worth trying: create element instead of writing to exiting ones only: https://www.w3schools.com/jsref/met_document_createelement.asp
  document.getElementById("pre_month_"+i).innerHTML =  getPrevMonth(d,1).toLocaleString(locale, { month: "long" });
}

<div class="dropdown">
  <button class="btn btn-danger dropdown-toggle" type="button" data-toggle="dropdown">Select Month
	  <span class="caret"></span></button>
  <ul class="dropdown-menu">
    <li>
      <a href="#" id="pre_month_0"></a>
    </li>
    <li>
      <a href="#" id="pre_month_1"></a>
    </li>
    <li>
      <a href="#" id="pre_month_2"></a>
    </li>
    <li>
      <a href="#" id="pre_month_3"></a>
    </li>
    <li>
      <a href="#" id="pre_month_4"></a>
    </li>
    <li>
      <a href="#" id="pre_month_5"></a>
    </li>
  </ul>
</div>

#4

You could do this a bit more concise, and using modulo operator:

你可以做得更简洁一些，使用模运算符:

var month = "January,February,March,April,May,June,July,August,September,October,November,December"
           .split(','),
    monthNum = 12 + new Date().getMonth();
Array.from(document.querySelectorAll('.dropdown-menu a'), function (elem, i) {
    elem.textContent = month[(monthNum-i)%12];
});
              

<div class="dropdown">
  <button class="btn btn-danger dropdown-toggle" type="button" data-toggle="dropdown">Select Month
  <span class="caret"></span></button>
  <ul class="dropdown-menu">
    <li><a href="#" id="cur_month"></a></li>
    <li><a href="#" id="pre_month_1"></a></li>
    <li><a href="#" id="pre_month_2"></a></li>
    <li><a href="#" id="pre_month_3"></a></li>
    <li><a href="#" id="pre_month_4"></a></li>
    <li><a href="#" id="pre_month_5"></a></li>
  </ul>
</div>

NB: It is better practice to use textContent instead of innerHTML, since you want to put text, not HTML.

NB:最好是使用textContent而不是innerHTML，因为你想要的是文本而不是HTML。