strftime.js

/* Port of strftime(). Compatibility notes:
 *
 * %c - formatted string is slightly different
 * %D - not implemented (use "%m/%d/%y" or "%d/%m/%y")
 * %e - space is not added
 * %E - not implemented
 * %h - not implemented (use "%b")
 * %k - space is not added
 * %n - not implemented (use "\n")
 * %O - not implemented
 * %r - not implemented (use "%I:%M:%S %p")
 * %R - not implemented (use "%H:%M")
 * %t - not implemented (use "\t")
 * %T - not implemented (use "%H:%M:%S")
 * %U - not implemented
 * %W - not implemented
 * %+ - not implemented
 * %% - not implemented (use "%")
 *
 * strftime() reference:
 * http://man7.org/linux/man-pages/man3/strftime.3.html
 *
 * Day of year (%j) code based on Joe Orost's answer:
 * http://stackoverflow.com/questions/8619879/javascript-calculate-the-day-of-the-year-1-366
 *
 * Week number (%V) code based on Taco van den Broek's prototype:
 * http://techblog.procurios.nl/k/news/view/33796/14863/calculate-iso-8601-week-and-year-in-javascript.html
 */
export default function strftime(sFormat, date) {
    if (!(date instanceof Date)) date = new Date();
    let nDay = date.getDay(),
        nDate = date.getDate(),
        nMonth = date.getMonth(),
        nYear = date.getFullYear(),
        nHour = date.getHours(),
        aDays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'],
        aMonths = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'],
        aDayCount = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334],
        isLeapYear = function () {
            return (nYear % 4 === 0 && nYear % 100 !== 0) || nYear % 400 === 0;
        },
        getThursday = function () {
            let target = new Date(date);
            target.setDate(nDate - ((nDay + 6) % 7) + 3);
            return target;
        },
        zeroPad = function (nNum, nPad) {
            return ('' + (Math.pow(10, nPad) + nNum)).slice(1);
        };
    return sFormat.replace(/%[a-z]/gi, function (sMatch) {
        return {
            '%a': aDays[nDay].slice(0, 3),
            '%A': aDays[nDay],
            '%b': aMonths[nMonth].slice(0, 3),
            '%B': aMonths[nMonth],
            '%c': date.toUTCString(),
            '%C': Math.floor(nYear / 100),
            '%d': zeroPad(nDate, 2),
            '%e': nDate,
            '%F': date.toISOString().slice(0, 10),
            '%G': getThursday().getFullYear(),
            '%g': ('' + getThursday().getFullYear()).slice(2),
            '%H': zeroPad(nHour, 2),
            '%I': zeroPad((nHour + 11) % 12 + 1, 2),
            '%j': zeroPad(aDayCount[nMonth] + nDate + ((nMonth > 1 && isLeapYear()) ? 1 : 0), 3),
            '%k': '' + nHour,
            '%l': (nHour + 11) % 12 + 1,
            '%m': zeroPad(nMonth + 1, 2),
            '%M': zeroPad(date.getMinutes(), 2),
            '%p': (nHour < 12) ? 'AM' : 'PM',
            '%P': (nHour < 12) ? 'am' : 'pm',
            '%s': Math.round(date.getTime() / 1000),
            '%S': zeroPad(date.getSeconds(), 2),
            '%u': nDay || 7,
            '%V': (function () {
                let target = getThursday(),
                    n1stThu = target.valueOf();
                target.setMonth(0, 1);
                let nJan1 = target.getDay();
                if (nJan1 !== 4) target.setMonth(0, 1 + ((4 - nJan1) + 7) % 7);
                return zeroPad(1 + Math.ceil((n1stThu - target) / 604800000), 2);
            })(),
            '%w': '' + nDay,
            '%x': date.toLocaleDateString(),
            '%X': date.toLocaleTimeString(),
            '%y': ('' + nYear).slice(2),
            '%Y': nYear,
            '%z': date.toTimeString().replace(/.+GMT([+-]\d+).+/, '$1'),
            '%Z': date.toTimeString().replace(/.+\((.+?)\)$/, '$1')
        }[sMatch] || sMatch;
    });
}