js格式化时间

function(date, format = null) {
      if (!date) {
        return ''
      } else {
        if (date instanceof Date) {
          if (!format) {
            format = 'yyyy-MM-dd HH:mm:ss'
          }
          const map = {
            'M+': `${date.getMonth() + 1}`,
            'd+': `${date.getDate()}`,
            'H+': `${date.getHours()}`,
            'm+': `${date.getMinutes()}`,
            's+': `${date.getSeconds()}`,
            'S': date.getMilliseconds()
          }
          if (/(y+)/.test(format)) {
            format = format.replace(RegExp.$1, `${date.getFullYear()}`.substr(4 - RegExp.$1.length))
          }
          for (const key in map) {
            if (new RegExp(`(${key})`).test(format)) {
              format = format.replace(RegExp.$1, (RegExp.$1.length === 1) ? map[key] : (`00${map[key]}`.substr(map[key].length)))
            }
          }
          return format
        } else {
          return ''
        }
      }
    }