SOURCE

    getStartEndDate (dayLength) { // 计算开始时间及结束时间,参数为日期长度
      let arr = []
      const startDate = '2018-02-1'
      const startYear = Number(startDate.slice(0, 4))
      const startMonth = Number(startDate.slice(5, 7))
      const startDay = Number(startDate.slice(8, 10))
      // const startDay = 2
      console.log('开始：', startYear, startMonth, startDay)
      // 判断起始月份有多少天
      let thisMonthDay = new Date(startYear, startMonth, 0).getDate()
      // console.log(thisMonthDay)
      // 判断本天离月底还剩多少天
      let thisRemainDay = new Date(startYear, startMonth, 0).getDate() - startDay
      // 判断下月还需锻炼多少天
      let nextMonthDay = 31 - thisRemainDay
      // 生成一个数组
      // 判断开始天是否 > 3，是则数组从本月开始天数 - 3 开始，否则获取上一月份最后3天日期
      if (startDay > 3) {
        // 再做一层判定：当起始月份为1月且日期 > 28，显示1、2、3三个月
        console.log(startDay - 3, thisMonthDay, nextMonthDay)
        if (startMonth === 1 && startDay > 28) {
          let once = true
          for (let i = startDay - 3; i <= thisMonthDay + nextMonthDay; i++) {
            let j = i % thisMonthDay
            if (i % thisMonthDay === 0) {
              j = thisMonthDay
              once = !once
              if (!once) {
                thisMonthDay = new Date(startYear, 2, 0).getDate() // %2月天数
                i = i - 3
              }
            }
            arr.push(j)
          }
        } else {
          for (let i = startDay - 3; i <= thisMonthDay + nextMonthDay; i++) {
            let j = i % thisMonthDay
            if (i % thisMonthDay === 0) {
              j = thisMonthDay
            }
            arr.push(j)
          }
        }
      } else {
        // 获取上月日期
        const beforeMonthDate = new Date(startYear, startMonth - 1, 0).getDate()
        console.log(beforeMonthDate, beforeMonthDate - (4 - startDay))
        for (let i = beforeMonthDate; i > beforeMonthDate - (4 - startDay); i--) {
          arr.unshift(i)
        }
        for (let j = 1; j <= dayLength - (4 - startDay); j++) {
          let i = j % thisMonthDay
          if (j % thisMonthDay === 0) {
            i = thisMonthDay
          }
          arr.push(i)
        }
      }
      console.log(arr)
    }