a javascript getter of Japanese nengo system (which i kinda hate, PLEASE USE ISO 8601!!!)

nengoCheck.js

const nengos = [{ "name":"reiwa", "baseYear":2019 }, {"name":"heisei", "baseYear":1989}];

// Could you guys just use ISO-8601? PLEASE?
function getNengo(date) {
  
  // First, Convert to JST
  const jpDate = new Date(date.setMinutes(date.getMinutes() + (date.getTimezoneOffset() - -540)));
  
  // get Year and Months
  let year = jpDate.getFullYear();
  const month = jpDate.getMonth()+1; // counting from 0
  
  if (month > 4) {
    // new Nengo System is used. Dafaq
  } else {
    // prev Nengo System is used.
    year--;
  }
  
  function inNengoRange(baseYear, year, years) {
    const workYear = (typeof years === 'undefined') ? 30 : years;
    return (baseYear <= year && year < baseYear+workYear)
  }
  
  
  
  for (const nengo of nengos) {
    if (inNengoRange(nengo.baseYear, year, nengo.years)) {
      return nengo.name;
    }
  }
  
  // Why am i doing this
  return undefined;
}

// Due to their ignorant against ISO 8601, It makes me mad that they wrote every single year system as NENGO system.
// Which Literally gives me a FUCK
function nengo2ISOCompliant(fuck) {
  const theNengo = fuck.nengo;
  const theYear = fuck.year;
  let theNengoBaseYear = 0;
  
  for (const nengo of nengos) {
    if (nengo.name.toLowerCase() === theNengo.toLowerCase()) {
      theNengoBaseYear = nengo.baseYear;
    }
  }
  
  return (theYear - 1) + theNengoBaseYear;
}

If you are using C#, Just use System.Globalization.JapaneseCalendar. Not this non-sense 🤦

Oh, if you have browser or Javascript runtime modern enough, Don't use this. Try Intl.Locale.getCalendars().

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Locale/getCalendars