Errors

Invalid input throws a typed error you can check with instanceof. Validation helpers like isValidBsDate return false instead of throwing.

Error	Extends	Thrown when	go-bs equivalent
`InvalidBSDateError`	`RangeError`	A BS year, month or day is out of range or not an integer (including `NaN`, `Infinity` and fractions). `field` says which: `"year"`, `"month"` or `"day"`.	`ErrInvalidYear` / `ErrInvalidMonth` / `ErrInvalidDay`
`DateOutOfRangeError`	`RangeError`	An AD date, or the result of `addBsDays`/`subtractBsDays`, falls outside the supported range.	`ErrOutOfRange`
`BSDateFormatError`	`SyntaxError`	`parseBsDate` gets a string not shaped like `"YYYY-MM-DD"`. `input` holds the string.	`ErrInvalidFormat`
`InvalidDateOrderError`	`RangeError`	`getBsAge`'s `birth` is after its `today`.	`ErrInvalidDateOrder`
`RangeError`	—	`addBsDays`/`subtractBsDays` gets a day count that isn't a safe integer, or a weekday-name function gets a weekday outside 0–6.	— / `ErrInvalidWeekday`
`TypeError`	—	An argument has the wrong type: not a `Date`, an Invalid Date, not an object, not a string.	—

Checking for a specific error

import { InvalidBSDateError, bsToAd } from "bikram-sambat-ts";
 
try {
  bsToAd({ year: 2083, month: 13, day: 1 });
} catch (error) {
  if (error instanceof InvalidBSDateError) {
    error.field; // "month"
    error.message; // "bs: invalid month: 13"
  }
}

Because every error extends RangeError, SyntaxError or TypeError, code that only checks the built-in classes still works. If your app somehow loads both the ESM and the CommonJS build of the package, their error classes are distinct and instanceof across them fails; checking error.name (e.g. "InvalidBSDateError") works either way.