Thi is the javascript library to convert date to lunar date and vice versa.
npm install lunardate
convert from solar date to lunar date
import {convertSolar2Lunar} from "lunardate"
const lunarDate = convertSolar2Lunar(20, 5, 1990);
console.log(lunarDate)
// output will give below result
// { year: 1990, month: 4, day: 26, isLeafYear: false }
This library using the source from https://www.informatik.uni-leipzig.de/~duc/amlich/ with few small modification