TL;DR, The requirement is to be able to take a Persian (Jalali) date (also known as Persian Solar Hijri Calendar) like Esfand 19, 1400 (i.e. "12/19/1400" and convert it to other calendars (Gregorian, Islamic, Chinese, Hebrew, etc.) without using external libraries or complex Astronomical Equations. And without using the pending implementation of the new date Temporal API into Javascript.

Javascript built-in method Intl.DateTimeFormat() converts Gregorian Dates into various calendars' dates (18 World Calendars) including the formatting of the output string.

However, as of today (March 2022), Javascript does not provide a built-in method for the reverse operation, i.e. converting the Persian Dates (and other calendars' dates) back into Gregorian Dates or into other calendars. For such purposes, you will need to use External Date Libraries to do the conversion such as 'moment.js' and many others.

My method of doing the date conversion follows as an answer to this question as recommended by StackOverflow here: Can I answer my own question?