js后端返回一个时间戳，用原生怎么对时间进行格式化？

function fn(time) {
    var date = new Date(time);
    var len = time.toString().length;
    // 时间戳不足13位则在后面加零
    if (len < 13) {
        var sub = 13 - len;
        sub = Math.pow(10, sub);
        date = new Date(time * sub);
    }
    var y = date.getFullYear() + '-';
    var M = date.getMonth() + 1;
    M = (M < 10 ? '0' + M : M) + '-';
    var d = date.getDate();
    d = (d < 10 ? '0' + d : d) + ' ';
    var h = date.getHours();
    h = (h < 10 ? '0' + h : h) + ':';
    var m = date.getMinutes();
    m = (m < 10 ? '0' + m : m) + ':';
    var s = date.getSeconds();
    s = s < 10 ? '0' + s : s;
    return y + M + d + h + m + s;
}
console.log(fn(1557003785));  //2019-05-05 05:03:05