Do not set an expiration claim if it has already been defined by the user

[pingvinen]

I find it a little confusing that I have to do the following in order to set an expiration - am I just missing something?

const claims = {
	exp: expirationTimestampInSeconds
}
const jwt = njwt.create(claims, signingKey, 'HS256');
jwt.setExpiration(claims.exp * 1000); // this accepts milliseconds for some reason

return jwt.compact();

In create, why not skip the expiration setting part, if the claims already contains the exp claim?