momentjs has its own specific format symbols which was confusing, whereas other libraries like date-fns are following the LDML standard. I ended up using date-fns as Java is having the same standard.

https://date-fns.org/v2.26.0/docs/Unicode-Tokens

https://www.unicode.org/reports/tr35/tr35-dates.html#Date_Field_Symbol_Table

This will be a pretty hand-held operation. Also you may not be able to cover all cases, but certainly the most common ones. The following should get you started.

function replacer(match, offset, string) {
  switch (match) {
    case 'M': return 'M';
    case 'MM': return 'MM';
    case 'MMM': return 'MMM';
    case 'MMMM': return 'MMMM';
    case 'd': return 'D';
    case 'dd': return 'DD';
    case 'D': return 'DDD';
    case 'DDD': return 'DDDD';
    case 'yy': return 'YY';
    case 'yyyy': return 'YYYY';
    default: return '(not supported)';
  }
}

var javaPattern = 'yyyy-MM-dd';
var regEx = /([a-z]+)/g;
var javaScriptPattern = javaPattern.replace(regEx, replacer);
console.log(javaScriptPattern);

Please go through the Java pattern letters and the JavaScript pattern letters in the two links below and expand the translations in my code as needed.

Links

• Java format pattern letters are in the documetnation of DateTimeFormatter
• momentjs format pattern letters are in Format - momentjs.com