Jalaali-js is a JavaScript library for converting between Jalaali (Persian) and Gregorian calendar systems. It provides functions to convert dates between the two calendars, validate Jalaali dates, check for leap years, and determine the number of days in a given month. The library is useful for developers working on applications that need to handle both calendar systems, particularly for Persian-speaking users or applications that require accurate date conversions between the two calendars.