String to Date / Date to String

String.prototype.toDate = function(format){
	var normalized      = this.replace(/[^a-zA-Z0-9]/g, '-');
	var normalizedFormat= format.toLowerCase().replace(/[^a-zA-Z0-9]/g, '-');
	var formatItems     = normalizedFormat.split('-');
	var dateItems       = normalized.split('-');

	var monthIndex  = formatItems.indexOf("mm");
	var dayIndex    = formatItems.indexOf("dd");
	var yearIndex   = formatItems.indexOf("yyyy");
	var hourIndex     = formatItems.indexOf("hh");
	var minutesIndex  = formatItems.indexOf("ii");
	var secondsIndex  = formatItems.indexOf("ss");

	var today = new Date();

	var year  = yearIndex>-1  ? dateItems[yearIndex]    : today.getFullYear();
	var month = monthIndex>-1 ? dateItems[monthIndex]-1 : today.getMonth()-1;
	var day   = dayIndex>-1   ? dateItems[dayIndex]     : today.getDate();

	var hour    = hourIndex>-1      ? dateItems[hourIndex]    : today.getHours();
	var minute  = minutesIndex>-1   ? dateItems[minutesIndex] : today.getMinutes();
	var second  = secondsIndex>-1   ? dateItems[secondsIndex] : today.getSeconds();

	return new Date(year,month,day,hour,minute,second);
};

Date.prototype.format = function(f) {
    if (!this.valueOf()) return " ";
 
    var weekName = ["일요일", "월요일", "화요일", "수요일", "목요일", "금요일", "토요일"];
    var d = this;
     
    return f.replace(/(yyyy|yy|MM|dd|E|hh|mm|ss|a\/p)/gi, function($1) {
        switch ($1) {
            case "yyyy": return d.getFullYear();
            case "yy": return (d.getFullYear() % 1000).zf(2);
            case "MM": return (d.getMonth() + 1).zf(2);
            case "dd": return d.getDate().zf(2);
            case "E": return weekName[d.getDay()];
            case "HH": return d.getHours().zf(2);
            case "hh": return ((h = d.getHours() % 12) ? h : 12).zf(2);
            case "mm": return d.getMinutes().zf(2);
            case "ss": return d.getSeconds().zf(2);
            case "a/p": return d.getHours() < 12 ? "오전" : "오후";
            default: return $1;
        }
    });
};
 
String.prototype.string = function(len){var s = '', i = 0; while (i++ < len) { s += this; } return s;};
String.prototype.zf = function(len){return "0".string(len - this.length) + this;};
Number.prototype.zf = function(len){return this.toString().zf(len);};

 

사용법

const str = "2020-12-17";
const d = str.toDate("yyyy-MM-dd");
const dstr = d.format("yyyy-MM-dd");

 

출처: https://stove99.tistory.com/46

stackoverflow.com/questions/5619202/converting-a-string-to-a-date-in-javascript