date_formatter.ts

export class DateFormatter {
    private regex = /%-?[a-zA-Z]/gm;
    private translations: { [directive: string]: (date: Date, locale: string) => string; } = {
        'a': (date: Date, locale: string) => date.toLocaleDateString(locale, {weekday: 'short'}),
        'A': (date: Date, locale: string) => date.toLocaleDateString(locale, {weekday: 'long'}),
        'w': (date: Date) => date.getDay().toString(),
        'y': (date: Date, locale: string) => date.toLocaleDateString(locale, {year: '2-digit'}),
        'Y': (date: Date, locale: string) => date.toLocaleDateString(locale, {year: 'numeric'}),
        'b': (date: Date, locale: string) => date.toLocaleDateString(locale, {month: 'short'}),
        'B': (date: Date, locale: string) => date.toLocaleDateString(locale, {month: 'long'}),
        'm': (date: Date, locale: string) => date.toLocaleDateString(locale, {month: '2-digit'}),
        '-m': (date: Date, locale: string) => date.toLocaleDateString(locale, {month: 'numeric'}),
        'd': (date: Date, locale: string) => date.toLocaleDateString(locale, {day: '2-digit'}),
        '-d': (date: Date, locale: string) => date.toLocaleDateString(locale, {day: 'numeric'}),
        'H': (date: Date) => DateFormatter.leftPad(date.getHours().toString()),
        '-H': (date: Date) => date.getHours().toString(),
        'I': (date: Date) => DateFormatter.leftPad(DateFormatter.twelveHours(date)),
        '-I': (date: Date) => DateFormatter.twelveHours(date),
        'M': (date: Date) => DateFormatter.leftPad(date.getMinutes().toString()),
        '-M': (date: Date) => date.getMinutes().toString(),
        'S': (date: Date) => DateFormatter.leftPad(date.getSeconds().toString()),
        '-S': (date: Date) => date.getSeconds().toString(),
        'p': (date, locale) => DateFormatter.amPm(date, locale),
        'z': (date: Date) => DateFormatter.timezoneOffset(date),
        'Z': (date: Date, locale: string) => DateFormatter.timezone(date, locale),
        'j': (date: Date) => DateFormatter.leftPad(DateFormatter.dayOfTheYear(date).toString(), 3),
        '-j': (date: Date) => DateFormatter.dayOfTheYear(date).toString(),
        'c': (date: Date, locale: string) => date.toLocaleString(locale),
        'x': (date: Date, locale: string) => date.toLocaleDateString(locale),
        'X': (date: Date, locale: string) => date.toLocaleTimeString(locale),
        '%': () => '%'
    };

    private static leftPad(text: string, length: number = 2, padString: string = '0'): string {
        let paddedText = text;
        while (paddedText.length < length) {
            paddedText = padString + paddedText;
        }
        return paddedText;
    }

    private static twelveHours(date: Date): string {
        let hours: number = date.getHours();
        if (hours > 12) {
            hours = date.getHours() - 12;
        }
        return hours.toString();
    }

    private static dayOfTheYear(date: Date): number {
        const start = new Date(date.getFullYear(), 0, 0);
        const diff = (date.getTime() - start.getTime()) + ((start.getTimezoneOffset() - date.getTimezoneOffset()) * 60 * 1000);
        const oneDay = 1000 * 60 * 60 * 24;
        return Math.floor(diff / oneDay);
    }

    private static amPm(date: Date, locale: string): string {
        const timeString = date.toLocaleTimeString(locale, {hour12: true});
        return timeString.substr(timeString.length - 2);
    }

    private static timezone(date: Date, locale: string): string {
        const withTimezone = date.toLocaleString(locale, {timeZoneName: 'short'});
        const withoutTimezone = date.toLocaleString(locale);
        return withTimezone.replace(withoutTimezone, '').trim();
    }

    private static timezoneOffset(date: Date): string {
        const offset = date.getTimezoneOffset() * -1;
        const hours = Math.abs(Math.floor(offset / 60));
        const minutes = offset % 60;
        const sign = offset >= 0 ? '+' : '-';
        return `${sign}${this.leftPad(hours.toString())}${this.leftPad(minutes.toString())}`;
    }

    constructor(private readonly locale: string = 'en-US') {

    }

    format(date: Date, format: string) {
        let formattedString = format;
        let m;

        const regex = this.regex;
        while ((m = regex.exec(format)) !== null) {
            if (m.index === regex.lastIndex) {
                regex.lastIndex++;
            }

            m.forEach((match) => {
                const matched = match.substr(1);
                formattedString = formattedString.replace(match, this.translations[matched](date, this.locale) || '');
            });
        }
        return formattedString;
    }
}

export const displayDateFormatter = new DateFormatter();