Replicate PHP's native date() formatting in JavaScript for many common format types

date.js

/**
 * Return a formated string from a date Object mimicking PHP's date() functionality
 *
 * @param {string} format "Y-m-d H:i:s" or similar PHP-style date format string
 * @param {* | null} date Date Object, Datestring, or milliseconds 
 *
 */
function date(format, date) {
    if (!date || date === "") {
        date = new Date();
    } else if (typeof date !== 'object') {
        // attempt to convert string/number to date object
        try {
            if (typeof date === 'number') date = new Date(date);
            else date = new Date(date.replace(/-/g, "/"));
        } catch (e) {
            date = new Date();
        }
    }

    let string = '',
        mo = date.getMonth(), // month (0-11)
        m1 = mo + 1, // month (1-12)
        dow = date.getDay(), // day of week (0-6)
        d = date.getDate(), // day of the month (1-31)
        y = date.getFullYear(), // 1999 or 2003
        h = date.getHours(), // hour (0-23)
        mi = date.getMinutes(), // minute (0-59)
        s = date.getSeconds(); // seconds (0-59)

    for (let i of format.match(/(\\)*./g)) {
        switch (i) {
            case 'j': // Day of the month without leading zeros  (1 to 31)
                string += d;
                break;

            case 'd': // Day of the month, 2 digits with leading zeros (01 to 31)
                string += (d < 10) ? "0" + d : d;
                break;

            case 'l': // (lowercase 'L') A full textual representation of the day of the week
                var days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
                string += days[dow];
                break;

            case 'w': // Numeric representation of the day of the week (0=Sunday,1=Monday,...6=Saturday)
                string += dow;
                break;

            case 'D': // A textual representation of a day, three letters
                var days = ["Sun", "Mon", "Tue", "Wed", "Thr", "Fri", "Sat"];
                string += days[dow];
                break;

            case 'm': // Numeric representation of a month, with leading zeros (01 to 12)
                string += (m1 < 10) ? "0" + m1 : m1;
                break;

            case 'n': // Numeric representation of a month, without leading zeros (1 to 12)
                string += m1;
                break;

            case 'F': // A full textual representation of a month, such as January or March 
                var months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
                string += months[mo];
                break;

            case 'M': // A short textual representation of a month, three letters (Jan - Dec)
                var months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
                string += months[mo];
                break;

            case 'Y': // A full numeric representation of a year, 4 digits (1999 OR 2003)	
                string += y;
                break;

            case 'y': // A two digit representation of a year (99 OR 03)
                string += y.toString().slice(-2);
                break;

            case 'H': // 24-hour format of an hour with leading zeros (00 to 23)
                string += (h < 10) ? "0" + h : h;
                break;

            case 'g': // 12-hour format of an hour without leading zeros (1 to 12)
                var hour = (h === 0) ? 12 : h;
                string += (hour > 12) ? hour - 12 : hour;
                break;

            case 'h': // 12-hour format of an hour with leading zeros (01 to 12)
                var hour = (h === 0) ? 12 : h;
                hour = (hour > 12) ? hour - 12 : hour;
                string += (hour < 10) ? "0" + hour : hour;
                break;

            case 'a': // Lowercase Ante meridiem and Post meridiem (am or pm)
                string += (h < 12) ? "am" : "pm";
                break;

            case 'i': // Minutes with leading zeros (00 to 59)
                string += (mi < 10) ? "0" + mi : mi;
                break;

            case 's': // Seconds, with leading zeros (00 to 59)
                string += (s < 10) ? "0" + s : s;
                break;

            case 'c': // ISO 8601 date (eg: 2012-11-20T18:05:54.944Z)
                string += date.toISOString();
                break;

            default:
                if (i.startsWith("\\")) i = i.substr(1);
                string += i;
        }
    }
    return string;
}