nodejs - how to refresh the token with the SDK

I'm new to Box node sdk and wrote a simple nodejs app using box sdk (oAuth) to upload files. Everything works but the tokens expire after an hour.

{ accessToken: 'xxx',
  refreshToken: 'YPQxTLtCRpOPMnKrt31cuisdFrtHILdxxxxxxxhkelhR2uSbJxo',
  accessTokenTTLMS: 3980000,
  acquiredAtMS: 152***phone number removed for privacy*** }

when I call the SDK functions after an hour, I get

get Error =  undefined
uncaughtException: Unexpected API Response [401 Unauthorized]

Looks like I need to refresh the token every hour. I'm using getPersistentClient and thought that the SDK will auto refresh the tokens.

const client = sdk.getPersistentClient(boxtoken);

Can someone give me pointers on how to auto refresh the tokens without the end user intervention of re-authenticating my app ?

Thank you