JavaScript实现日期智能格式化：显示今天、昨天或绝对日期

function transDate() {
    var $time =document.getElementById("share-time");
    var date = $time.innerHTML.trim();
    var tt = new Date(parseInt(date));
    var days = parseInt((new Date().getTime() - date) / 86400000);
    var today = new Date().getDate();
    var year = tt.getFullYear();
    var mouth = tt.getMonth() + 1;
    var day = tt.getDate();
    var time = tt.getHours() < 10 ? "0" + tt.getHours() : tt.getHours();
    var min = tt.getMinutes() < 10 ? "0" + tt.getMinutes() : tt.getMinutes();
    var result, offset;
　　　　　　　offset = Math.abs(today - day);
　　　　if (days < 4&&offset<4) {
　　　　　　　if (offset === 0) {
        result = "今天" + time + ":" + min;
      } else if (offset === 1) {
        result = "昨天" + time + ":" + min;
      } else if (offset === 2) {
        result = "前天" + time + ":" + min;
      }
    } else {
      result = year + "-" + mouth + "-" + day + " " + time + ":" + min;
    }
    $time.innerHTML = result;
  }
  transDate();

  function getDay(day){
    var today = new Date()
    // 获取时间戳(毫秒级)
    /*
      day为1，则是，明天的时间戳
      day为-1，则是，昨天的时间戳
      day为-2，则是，前天的时间戳
    */
    var targetday_milliseconds = today.getTime() + 1000 * 60 * 60 * 24 * day
    // Date.setTime(时间戳)：设置当前日期的时间
    today.setTime(targetday_milliseconds)
    console.log('today=', today) // today= Sun Mar 05 2023 16:14:56 GMT+0800 (中国标准时间)
    var tYear = today.getFullYear() // 年
    var tMonth = today.getMonth() // 月
    var tDate = today.getDate() // 日
    tMonth = this.doHandleMonth(tMonth + 1)
    tDate = this.doHandleMonth(tDate)
    console.log('返回年月日=', tYear + '-' + tMonth + '-' + tDate)
    return tYear + '-' + tMonth + '-' + tDate
  }
  function doHandleMonth(month) {
    var m = month
    if (month.toString().length == 1) {
      m = '0' + month
    }
    return m
  }

/* 
  * @params date 日期
  * @params type 日期 prev/current/next 昨天/今天/明天
  * @params fmt 日期拼接符
*/
function getDays(date, type, fmt) {
    let currentDate = new Date(date)
    let y = currentDate.getFullYear()
    let m = currentDate.getMonth() + 1
    let d = currentDate.getDate()
    function dateFormat(date, fmt) {
        let y = new Date(date).getFullYear()
        let m = new Date(date).getMonth() + 1
        let d = new Date(date).getDate()
        return `${y}${fmt}${m}${fmt}${d}`
    }
    switch (type) {
        case "prev":
            if (d - 1 < 1) {
                if (m - 1 < 1) {
                    y = y - 1
                    m = 12
                } else {
                    m = m - 1
                }
                d = new Date(y, m, 0).getDate()
            } else {
                d = d - 1
            }
            break
        case "current":
            break
        case "next":
            if (d + 1 > new Date(y, m, 0).getDate()) {
                if (m + 1 > 12) {
                    y = y + 1
                    m = 1
                    d = 1
                } else {
                    m = m + 1
                    d = 1
                }
            } else {
                d = d + 1
            }
            break;
    default:
      break;
    }
    return dateFormat(new Date(`${y}-${m}-${d}`), fmt)
}
console.log(getDays(new Date('2023-5-13'), "prev", "-"));
console.log(getDays(new Date('2023-5-30'), "next", "-"));
console.log(getDays(new Date('2023-5-31'), "next", "-"));