A TypeScript format extension for dates. Quick & easy C# like date formatting📅

date.extensions.ts

interface Date {
    /**
     * Extends the Date object with a `format` method to format dates as strings.
     *
     * @param {string} format - The format string specifying the desired date/time format.
     * @returns {string} The formatted date string.
     *
     * @example
     * ```typescript
     * const myDate: Date = new Date();
     * console.log(myDate.format("yyyy/MM/dd HH:mm:ss.fff"));  // Output: "2023/10/09 15:30:45.123"
     * ```
     *
     * @remarks
     * This method supports various format options, such as:
     * - `d`, `dd`: Day of the month (1 through 31).
     * - `ddd`, `dddd`: Abbreviated and full names of the day (Mon, Monday, etc).
     * - `H`, `HH`, `h`, `hh`: 24-hour and 12-hour clock hours.
     * - `m`, `mm`: Minutes.
     * - `M`, `MM`, `MMM`, `MMMM`: Month in various formats.
     * - `s`, `ss`: Seconds.
     * - `t`, `tt`: AM/PM.
     * - `y`, `yy`, `yyy`, `yyyy`: Year in various formats.
     * - `K`: Time zone offset.
     * - `z`, `zz`, `zzz`: Time zone offset in various formats.
     * - `f`, `ff`, `fff`: Milliseconds and fractions of a second.
     *
     *  
     * Date formatting extension inspired by and adapted from the work of OpenAI"s ChatGPT, created by OpenAI.
     *
     * @see {@link https://www.openai.com/}
     * @see {@link https://platform.openai.com/docs/api-reference/introduction}
     *
     * Original implementation: OpenAI"s ChatGPT
     * Curated and adapted for TypeScript: Andre Schoultz
     */
    format(format: string): string;
}

Date.prototype.format = function (format: string): string {
    const daysOfWeek: string[] = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
    const months: string[] = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
    
    return format.replace(/(d{1,4}|M{1,4}|y{1,4}|HH|H|hh|h|mm|ss|t{1,2}|K|z{1,3}|f{1,3})/g, (match: string): string => {
        switch (match) {
            case "d":
                return this.getDate().toString();
            case "dd":
                return ("0" + this.getDate()).slice(-2);
            case "ddd":
                return daysOfWeek[this.getDay()].slice(0, 3);
            case "dddd":
                return daysOfWeek[this.getDay()];
            case "H":
                return this.getHours().toString();
            case "HH":
                return ("0" + this.getHours()).slice(-2);
            case "h":
                return (this.getHours() % 12 || 12).toString();
            case "hh":
                return ("0" + (this.getHours() % 12 || 12)).slice(-2);
            case "m":
                return this.getMinutes().toString();
            case "mm":
                return ("0" + this.getMinutes()).slice(-2);
            case "M":
                return (this.getMonth() + 1).toString();
            case "MM":
                return ("0" + (this.getMonth() + 1)).slice(-2);
            case "MMM":
                return months[this.getMonth()].slice(0, 3);
            case "MMMM":
                return months[this.getMonth()];
            case "s":
                return this.getSeconds().toString();
            case "ss":
                return ("0" + this.getSeconds()).slice(-2);
            case "t":
                return this.getHours() < 12 ? "A" : "P";
            case "tt":
                return this.getHours() < 12 ? "AM" : "PM";
            case "y":
                return (this.getFullYear() % 100).toString();
            case "yy":
                return ("0" + (this.getFullYear() % 100)).slice(-2);
            case "yyy":
                return this.getFullYear().toString().padStart(3, "0");
            case "yyyy":
                return this.getFullYear().toString().padStart(4, "0");
            case "K":
                const offsetMinutes: number = this.getTimezoneOffset();
                const offsetHours: number = Math.abs(offsetMinutes / 60);
                const offsetSign: string = offsetMinutes > 0 ? "-" : "+";
                return offsetSign + ("0" + offsetHours).slice(-2) + ":" + ("0" + Math.abs(offsetMinutes % 60)).slice(-2);
            case "z":
                return (this.getTimezoneOffset() / 60).toString();
            case "zz":
                const timezoneOffset: number = this.getTimezoneOffset() / 60;
                return (timezoneOffset > 0 ? "+" : "-") + ("0" + Math.abs(timezoneOffset)).slice(-2);
            case "zzz":
                const tzOffset: number = this.getTimezoneOffset();
                const tzHours: number = Math.abs(tzOffset / 60);
                const tzSign: string = tzOffset > 0 ? "-" : "+";
                return tzSign + ("0" + tzHours).slice(-2) + ":" + ("0" + Math.abs(tzOffset % 60)).slice(-2);
            case "f":
                return Math.floor(this.getMilliseconds() / 100).toString();
            case "ff":
                return ("0" + Math.floor(this.getMilliseconds() / 10)).slice(-2);
            case "fff":
                return this.getMilliseconds().toString().padEnd(3, "0");
            default:
                return match;
        }
    });
};

dateExtenstionTests.ts

const runFromatTest = () => {
  const testDate = new Date("2015-05-29T05:50:06.7199222");

  const expectedResults: any = {
    "MM/dd/yyyy": "05/29/2015",
    "dddd, dd MMMM yyyy": "Friday, 29 May 2015",
    "dddd, dd MMMM yyyy HH:mm": "Friday, 29 May 2015 05:50",
    "dddd, dd MMMM yyyy hh:mm tt": "Friday, 29 May 2015 05:50 AM",
    "dddd, dd MMMM yyyy H:mm": "Friday, 29 May 2015 5:50",
    "dddd, dd MMMM yyyy h:mm tt": "Friday, 29 May 2015 5:50 AM",
    "dddd, dd MMMM yyyy HH:mm:ss": "Friday, 29 May 2015 05:50:06",
    "MM/dd/yyyy HH:mm": "05/29/2015 05:50",
    "MM/dd/yyyy hh:mm tt": "05/29/2015 05:50 AM",
    "MM/dd/yyyy H:mm": "05/29/2015 5:50",
    "MM/dd/yyyy h:mm tt": "05/29/2015 5:50 AM",
    "MM/dd/yyyy HH:mm:ss": "05/29/2015 05:50:06",
    "MMMM dd": "May 29",
    "yyyy-MM-ddTHH:mm:ss.fffK": "2015-05-29T05:50:06.719+02:00", // Adapt to own timezone when testing
    "ddd, dd MMM yyy HH:mm:ss": "Fri, 29 May 2015 05:50:06",
    "yyyy-MM-ddTHH:mm:ss": "2015-05-29T05:50:06",
    "HH:mm": "05:50",
    "hh:mm tt": "05:50 AM",
    "H:mm": "5:50",
    "h:mm tt": "5:50 AM",
    "HH:mm:ss": "05:50:06",
    "yyyy MMMM": "2015 May",
  };

  Object.keys(expectedResults).forEach((format) => {
    const result = testDate.format(format);
    const expected = expectedResults[format];

    console.log(`${format}:`, result);

    if (result === expected) {
      console.log("   Test Passed");
    } else {
      console.error(`   Test Failed! Expected: ${expected}, Actual: ${result}`);
    }
  });
};