如何获得前一年的月份？

你得到这个未定义因为month[-1]而且month[-2]是undefined

您需要在日期对象中实际执行日期操作,而不是仅从索引中获取日期.

使用此方法获取上个月的日期

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

根据需要多次调用此方法.

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;

  Select Month