js获取昨天今天明天日期

/* 
  * @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('2021-1-1'), "prev", "-"));