yyyy-MM-dd hh:mm:ss => 2017-06-29 09:20:43

function getNowFormatDate() {
    let date = new Date(dateStr);
    let seperator1 = '-';
    let seperator2 = ':';
    let month = date.getMonth() + 1;
    let strDate = date.getDate();
    let hour = date.getHours();
    let minute = date.getMinutes();
    let second = date.getSeconds();
    if (month >= 1 && month <= 9) {
        month = '0' + month;
    }
    if (strDate >= 0 && strDate <= 9) {
        strDate = '0' + strDate;
    }
    if (hour >= 0 && hour <= 9) {
        hour = '0' + hour;
    }
    if (minute >= 0 && minute <= 9) {
        minute = '0' + minute;
    }
    if (second >= 0 && second <= 9) {
        second = '0' + second;
    }
    let currentdate =
        date.getFullYear() +
        seperator1 +
        month +
        seperator1 +
        strDate +
        ' ' +
        hour +
        seperator2 +
        minute +
        seperator2 +
        second;
    return currentdate;
}

setInterval(() => {
  console.log(getNowFormatDate())
}, 1000)

or

let current_datetime = new Date()
let formatted_date = current_datetime.getFullYear() + "-" + (current_datetime.getMonth() + 1) + "-" + current_datetime.getDate() + " " + current_datetime.getHours() + ":" + current_datetime.getMinutes() + ":" + current_datetime.getSeconds() 
console.log(formatted_date) // 2020-5-26 9:51:16