I am writing a script in JavaScript (sorry should have had mentioned that earlier) and the backend is in Motoko.
Found something in the playground and candid, so trying the following:
import { IDL } from "@dfinity/candid";
const arg = IDL.encode([IDL.Principal], owner);
but I get an error
Cannot mix BigInt and other types, use explicit conversions
owner is a principal
console.log(JSON.stringify(owner))
// {"_arr":{"0":154,"1":1 .... "28":2},"_isPrincipal":true}