Determine date and time formats in JavaScript

getDateTimeFormats.js

// https://en.wikipedia.org/wiki/Date_format_by_country
// MM/DD/YYYY  (en-US) USA, Panama, Puerto Rico
// DD/MM/YYYY  (en-GB) Great Britain, Australia, New Zeland, France, Italy, Spain, Brazil
// DD.MM.YYYY  (de-DE) Germany, Poland, Turkey, Ukraine
// DD-MM-YYYY  (nl-NL) Netherlands, Chile
// YYYY-MM-DD  (en-CA) Canada, Sweden
// YYYY/MM/DD  (en-ZA) South Africa, Japan, China
// YYYY.MM.DD  (hu-HU) Hungary, South Korea (actually, it should be YYYY. MM. DD.)

function getDateTimeFormats(/** @type {string | undefined} */ locale) {

    let dateTime = new Date(1999, 9, 23, 18, 0, 0);
    let dateText = dateTime.toLocaleDateString(locale);

    const yIndex = dateText.indexOf('99');
    const mIndex = dateText.indexOf('10');
    const dIndex = dateText.indexOf('23');

    let /** @type {string | undefined} */ separator;
    if (yIndex >= 0 && mIndex >= 0 && dIndex >= 0) {
        separator = dateText.match(/[\/\-\.]/)?.[0];
    }

    let dateFormat;
    if (yIndex < mIndex || yIndex < dIndex) {
        dateFormat = ['YYYY', 'MM', 'DD'].join(separator ?? '-');
    } else if (mIndex < dIndex) {
        dateFormat = 'MM/DD/YYYY';
    } else {
        dateFormat = ['DD', 'MM', 'YYYY'].join(separator ?? '/');
    }

    let timeFormat = 'H:mm';
    if (dateTime.toLocaleTimeString(locale).indexOf('6') >= 0) {
        timeFormat = 'h:mm a';
    }
    return { dateFormat, timeFormat }
}

let { dateFormat, timeFormat } = getDateTimeFormats();

const expect = (/** @type {any} */actual) => ({
    toBe: (/** @type {any} */expected) => {
        if (actual !== expected) {
            console.error(actual + ' should be ' + expected)
        }
    }
});

console.clear();
expect(getDateTimeFormats('en-US').dateFormat).toBe('MM/DD/YYYY');
expect(getDateTimeFormats('en-GB').dateFormat).toBe('DD/MM/YYYY');
expect(getDateTimeFormats('en-CA').dateFormat).toBe('YYYY-MM-DD');
expect(getDateTimeFormats('en-ZA').dateFormat).toBe('YYYY/MM/DD');
expect(getDateTimeFormats('ja-JP').dateFormat).toBe('YYYY/MM/DD');
expect(getDateTimeFormats('de-DE').dateFormat).toBe('DD.MM.YYYY');
expect(getDateTimeFormats('cs-CZ').dateFormat).toBe('DD.MM.YYYY');
expect(getDateTimeFormats('bg-BG').dateFormat).toBe('DD.MM.YYYY');
expect(getDateTimeFormats('sr-RS').dateFormat).toBe('DD.MM.YYYY');
expect(getDateTimeFormats('lv-LV').dateFormat).toBe('DD.MM.YYYY');
expect(getDateTimeFormats('nl-NL').dateFormat).toBe('DD-MM-YYYY');
expect(getDateTimeFormats('hu-HU').dateFormat).toBe('YYYY.MM.DD');
expect(getDateTimeFormats('th-TH').dateFormat).toBe('YYYY-MM-DD');
expect(getDateTimeFormats('zh-SG').dateFormat).toBe('YYYY-MM-DD');

expect(getDateTimeFormats('en-US').timeFormat).toBe('h:mm a');
expect(getDateTimeFormats('en-GB').timeFormat).toBe('H:mm');

console.log('Tests finished');