refreshtoken not generated

i have the below code that gives me access token but no refresh token. please advise.

let callbackUrl = 'https://XXX.XX.ai'
let oauth = OAuth.createInstance('MY APP CLIENT KEY','', callbackUrl);
oauth.login().then(
async (oauthResult) => {
DataService.createInstance(oauthResult);
console.log("Logged Into Salesforce Successfully:::" + JSON.stringify(oauthResult));
});