JavaScript strftime() implementation

strftime.js

function pad(s, len, c, pr) {
	var rest;

 	/* prevent arithmetic issues */
	s = s.toString();
	c = c.toString();
	rest = len - s.length;
	if(rest <= 0)
		return s;
	[...Array(rest)].forEach(() => s=pr ? s+c : c+s);
	return s;
}

/* An attempt to have a consistent strftime().
 * Not everything is implemented, only what makes sense.
 * Reference: http://php.net/manual/en/function.strftime.php */
function strftime(fmt, d) {
	var ret = "", i, len, c;
	var txt = (d, o) => {
		d = d.toLocaleString(navigator.language, o).toString();
		/* not always capitalized (see it-IT) */
		d = d[0].toUpperCase() + d.substring(1);
		return d;
	};

	for(i = 0, len = fmt.length; i < len; ++i) {
		c = fmt[i];
		if(c != '%' || fmt[++i] == '%') {
			ret += c;
			continue;
		}
		c = fmt[i];
		switch(c) {
		case 'a': ret += txt(d, {weekday:"short"}); break;
		case 'A': ret += txt(d, {weekday:"long"}); break;
		case 'b': ret += txt(d, {month:"short"}); break;
		case 'B': ret += txt(d, {month:"long"}); break;
		case 'd': ret += pad(d.getDate(), 2, 0); break;
		case 'e': ret += pad(d.getDate(), 2, ' '); break;
		case 'm': ret += pad(d.getMonth()+1, 2, 0); break;
		case 'H': ret += pad(d.getHours(), 2, 0); break;
		case 'M': ret += pad(d.getMinutes(), 2, 0); break;
		case 'S': ret += pad(d.getSeconds(), 2, 0); break;
		case 'Y': ret += d.getFullYear(); break;
		case 'y':
			/* recycle c */
			c = d.getFullYear().toString();
			ret += c.substring(c.length - 2);
			break;
		default: console.warn(c+": invalid format operator"); break;
		}
	}
	return ret;
}