Cannot authenticate with xero-node integration

I'm trying to integrate Xero API into my node app, using the official xero-node library.

Following the docs, I registered a public app, supplied consumerKey and consumerSecret to config, tried both the sample app and built my own, but getting the same error:

{ oauth_problem: 'token_rejected',
     oauth_problem_advice: 'Token has not been provided' }

Any idea?

via Stanley Luo