Date format function similar to date command(https://man7.org/linux/man-pages/man1/date.1.html) in unix, Note: this code haven't been tested even once yet, may contain tons of bugs

Date.prototype.format.ts

export default function format(formatString: string): string {
    const day = this.getDate();
    const month = this.getMonth();
    const year = this.getFullYear();
    const isLeapYear = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
    
    const data = {
        weekdays: ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'],
        months: ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"],
        dayCount: [31, Number(isLeapYear) + 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],
        get cumulativeDayCount() {
            // return this.dayCount.map(function p(c, i, a): number { return i ? a[i - 1] + p(c, i - 1, a): 0 });
            const cumulated: number[] = [];
            let sum = 0;
            for(const count of this.dayCount){
                cumulated.push(sum);
                sum += count;
            }
            return cumulated;
        }
    } as const;

    const dayOfYear = data.cumulativeDayCount[month] + day;

    const abbreviate = (value: string) => value.slice(0, 3);
    const removeFraction = (value: number): number => Number(value.toString().split('.')[0]);
    const padWith = (filler?: string) => (value: number | string, digits: number = 2) => value.toString().padStart(digits, filler);
    const padZero = padWith("0");
    const padSpace = padWith(" ");
    // const removePadding = (value: string, filler: string = "[0 ]") => value.replace(new RegExp('^' + filler + '+'), '');

    const dayOfWeek = this.getDay();
    const hour = this.getHours();
    const minutes = this.getMinutes();
    const seconds = this.getSeconds();
    const twelveHour = ((hour + 11) % 12) + 1;
    const weekDay = data.weekdays[dayOfWeek];
    const monthName = data.months[month];
    const abbrMonth = abbreviate(monthName);
    const weekNumber = removeFraction((data.cumulativeDayCount[month] + day) / 7);
    const timezoneDiff = this.getTimezoneOffset() * -1;
    const timezone = {
        sign: timezoneDiff >= 0 ? '+' : '-',
        hour: removeFraction(timezoneDiff / 60),
        minute: timezoneDiff % 60,
        name: this.toTimeString().replace(/^.+\(|\).*$/g, ''),
        get abbr() { return this.name.replace(/[a-z áéä'-]/g, ''); }
    } as const;

    const specifiers: { readonly [_: string]: string | number } = {
        "a": abbreviate(weekDay),
        "A": weekDay,
        "b": abbrMonth,
        "B": monthName,
        "c": this.format("%a %b %-d %R:%S %Y"),
        "C": removeFraction(year / 100),
       "-d": day,
        "d": padZero(day),
        "D": this.format("%m/%d/%y"),
       "-e": day,
        "e": padSpace(day),
        "F": this.format("%4Y-%m-%d"),
        "h": abbrMonth, // same as "b"
       "-H": hour,
        "H": padZero(hour),
       "-I": twelveHour,
        "I": padZero(twelveHour),
       "-j": dayOfYear,
        "j": padZero(dayOfYear, 3),
       "-k": hour,
        "k": padSpace(hour),
       "-l": twelveHour,
        "l": padSpace(twelveHour),
       "-m": month + 1,
        "m": padZero(month + 1),
       "-M": minutes,
        "M": padZero(minutes),
        "n": `\n`,
        "N": this.getMilliseconds() * 1000000,
       "#o": timezone.name.toLowerCase(), // not in unix date command
       "^o": timezone.name, // not in unix date command
        "o": timezone.name, // not in unix date command
       "^p": hour < 12 ? 'AM' : 'PM',
       "#p": hour < 12 ? 'AM' : 'PM',
        "p": hour < 12 ? 'am' : 'pm',
       "^P": hour < 12 ? 'AM' : 'PM',
       "#P": hour < 12 ? 'am' : 'pm',
        "P": hour < 12 ? 'AM' : 'PM',
        "q": removeFraction((month / 3) + 1),
        "r": this.format("%I:%M:%S %P"),
        "R": this.format("%H:%M"),
        "s": removeFraction(Date.parse(this.toJSON()) / 1000),
       "-S": seconds,
        "S": padZero(seconds),
        "t": `\t`,
        "T": this.format("%H:%M:%S"),
        "u": ((dayOfWeek + 6) % 7) + 1,
       "-U": weekNumber,
        "U": padZero(weekNumber),
        "w": dayOfWeek,
       "-W": weekNumber - Number(dayOfWeek == 0),
        "W": padZero(weekNumber - Number(dayOfWeek == 0)),
        "x": this.format("%m/%d/%y"), // same as D
        "X": this.format("%H:%M:%S"),
       "-y": year % 100,
        "y": padZero(year % 100),
        "Y": year,
       "-z": timezone.hour + padZero(timezone.minute),
        "z": padZero(timezone.hour) + padZero(timezone.minute),
      "-:z": `${timezone.hour}:${padZero(timezone.minute)}`,
       ":z": `${padZero(timezone.hour)}:${padZero(timezone.minute)}`,
     "-::z": `${timezone.hour}:${padZero(timezone.minute)}:00`,
      "::z": `${padZero(timezone.hour)}:${padZero(timezone.minute)}:00`,
    "-:::z": timezone.hour + (timezone.minute ? `:${padZero(timezone.minute)}`: ""),
     ":::z": padZero(timezone.hour) + (timezone.minute ? `:${padZero(timezone.minute)}`: ""),
       "#Z": timezone.abbr.toLowerCase(),
       "^Z": timezone.abbr,
        "Z": timezone.abbr
    } as const;

    return formatString.replace(/%([-_0^#]?)(\d*)(:*z|[A-DFHIMNPR-UW-Za-ehj-uwxy])/g, (match: string, flag: string, width: string, identifier: string) => {
        if(flag == '-') width = '';
        else if(['_', '0'].includes(flag)) {
            identifier = `-${identifier}`;
            if(!width){
                switch(identifier) {
                    case 'N': width = '9'; break;
                    case 'j': width = '3'; break;
                    case 'z': width = '4'; break;
                    case ':z': width = '5'; break;
                    case '::z': width = '8'; break;
                    case ':::z': width = timezone.minute == 0 ? '2' : '5'; break;
                    default: width = [..."deHIklmMSUWy"].includes(identifier.slice(-1)) ? '2' : '';
                }
            }
        }
        
        let output: string = match;
        if(width) {
            if(flag == '_') {
                if(identifier.slice(-1) == 'z')
                output = padSpace(timezone.sign + specifiers[identifier], Number(width));
            } else if(flag == '0') output = padZero(specifiers[identifier], Number(width));
            else if([...'aAbBcDFhopPrRTxXZ'].includes(identifier.slice(-1))) output = padSpace(specifiers[identifier], Number(width));
            else output = padZero(specifiers[identifier], Number(width));
        } else output = specifiers[identifier].toString();

        return (identifier.slice(-1) == 'z' && flag != '_' ? timezone.sign : "") + output;
    });
}

Download

Download a copy of this into your project as:

project/
├── src/
│   ├── scripts/
│   │   ├── Date.prototype.format.js
│   │   └── ...
│   └── ...
└── ...

Usage

import format from '/src/scripts/Date.prototype.format.js'
Date.prototype.format = format;

...

const date = new Date();
console.log(date.format("%H:%M:%S"));   // "23:59:59"