Introducing a handy JavaScript date formatting function.

#javascript #dateformat

I created A function to format the date (Date object) with a pattern like yyyy-MM-dd'T'HH:mm:ssXXX in JavaScript.
With this function alone, ISO8601 format, RFC1123 format, time zone RFC822, etc. can be expressed.
Pattern specification is like Java's SimpleDateFormat.

Date formatting function

function formatDate(formatStr, date, opts) {

    if (!date) {
        date = new Date();
    }

    opts = opts || {};

    let _days = opts.days;

    if (!_days) {
        _days = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
    }

    let _months = opts.months;

    if (!_months) {
        _months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];
    }

    const pad = (number, strDigits, isUnpad) => {
        const strNum = Math.abs(number).toString();
        if (!isUnpad && strNum.length > strDigits.length) {
            return strNum;
        } else {
            return ('0000' + strNum).slice(-strDigits.length);
        }
    };

    const timezone = (date, letter) => {
        const chunk = [];
        const offset = -date.getTimezoneOffset();
        chunk.push(offset === 0 ? 'Z' : offset > 0 ? '+' : '-');//add Z or +,-
        if (offset === 0) return chunk;
        chunk.push(pad(Math.floor(offset / 60), '00'));//hour
        if (letter === 'X') return chunk.join('');
        if (letter === 'XXX') chunk.push(':');
        chunk.push(pad((offset % 60), '00'));//min
        return chunk.join('');
    };

    const DELIM = '\0\0';
    const escapeStack = [];

    const escapedFmtStr = formatStr.replace(/'.*?'/g, m => {
        escapeStack.push(m.replace(/'/g, ''));
        return `${DELIM}${escapeStack.length - 1}${DELIM}`;
    });

    const formattedStr = escapedFmtStr
        .replace(/y{4}|y{2}/g, m => pad(date.getFullYear(), m, true))
        .replace(/M{3}/g, m => _months[date.getMonth()])
        .replace(/M{1,2}/g, m => pad(date.getMonth() + 1, m))
        .replace(/M{1,2}/g, m => pad(date.getMonth() + 1, m))
        .replace(/d{1,2}/g, m => pad(date.getDate(), m))
        .replace(/H{1,2}/g, m => pad(date.getHours(), m))
        .replace(/h{1,2}/g, m => {
            const hours = date.getHours();
            return pad(hours === 0 ? 12 : hours > 12 ? hours - 12 : hours, m);
        })
        .replace(/a{1,2}/g, m => date.getHours() >= 12 ? 'PM' : 'AM')
        .replace(/m{1,2}/g, m => pad(date.getMinutes(), m))
        .replace(/s{1,2}/g, m => pad(date.getSeconds(), m))
        .replace(/S{3}/g, m => pad(date.getMilliseconds(), m))
        .replace(/[E]+/g, m => _days[date.getDay()])
        .replace(/[Z]+/g, m => timezone(date, m))
        .replace(/X{1,3}/g, m => timezone(date, m));

    const unescapedStr = formattedStr.replace(new RegExp(`${DELIM}\\d+${DELIM}`, 'g'),
        m => {
            const unescaped = escapeStack.shift();
            return unescaped.length > 0 ? unescaped : '\'';
        });

    return unescapedStr;
}

Call function like as follows

const date = new Date();

//Format with patterns
console.log(formatDate("MMM dd, yyyy",date));
//->Feb 17, 2020

console.log(formatDate("MM/dd/yyyy",date));
//->02/17/2020

console.log(formatDate("EEE, MMM d, ''yy",date));
//->Mon, Feb 17, '20

console.log(formatDate("h:mm a",date));
//->8:46 PM

console.log(formatDate("hh 'o''''clock' a, Z",date));
//->08 o'clock PM, -0600

//ISO8601 format
console.log(formatDate("yyyyMMdd'T'HHmmssXX",date));
//->20200217T204746-0600

//ISO8601 format alt
console.log(formatDate("yyyy-MM-dd'T'HH:mm:ssXXX",date));
//->2020-02-17T20:47:46-06:00

//RFC1123(RFC822) format
console.log(formatDate("E, dd MMM yyyy HH:mm:ss Z",date));
//->Mon, 17 Feb 2020 20:47:46 -0600

The result is as follows:

Feb 17, 2020
02/17/2020
Mon, Feb 17, '20
8:47 PM
08 o'clock PM, -0600
20200217T204746-0600
2020-02-17T20:47:46-06:00
Mon, 17 Feb 2020 20:47:46 -0600
Mon, Feb 17, '20
Hello! It's 20:47:46.

Pattern of the date

The following examples show how date and time patterns are interpreted.
The given date and time are 2018-07-17 12:08:56 local time in Tokyo/Japan time zone.

Letter(s)	Date or Time Component	Examples
yyyy	Year	2018
yy		18
M	Month in year	7
MM		07
MMM	Month name	Jul
d	Day in month	17
dd		17
a	Am/pm marker	PM
H	Hour in day (0-23)	12
HH		12
E	day of week	Tue
h	Hour in am/pm (1-12)	12
hh		12
m	Minute in hour	8
mm		08
s	Second in minute	56
ss		56
S	Millisecond	789
Z	Timezone(RFC822)	+0900
X	Timezone(ISO8601)	+09
XX		+0900
XXX		+09:00
' '	Strings(Escaped)
''	Single Quote

Pattern examples

Format String	Example
yyyyMMdd'T'HHmmssXX (ISO 8601)	20180717T120856+0900
yyyy-MM-dd'T'HH:mm:ssXXX (ISO 8601)	2018-07-17T12:08:56+09:00
E, dd MMM yyyy HH:mm:ss Z	Tue, 17 Jul 2018 12:08:56 +0900
yyyy.MM.dd 'at' hh:mm:ss Z	2018.07.17 at 12:08:56 +0900
EEE, MMM d, ''yy	Tue, Jul 17, '18
h:mm a	12:08 PM
hh 'o''''clock' a, X	12 o'clock PM, +09
yyyyMMddHHmmssSSS	20180717120856789