I have followed the flow described here https://developer.xero.com/documentation/oauth2/auth-flow and can then get a tokenset which works with api requests.
However, looking at https://github.com/XeroAPI/xero-node-oauth2-app/blob/master/src/app.ts I don't see how/where the authorisation code provided to the callback is used to obtain the tokenset. (compare with Steps 2 & 3 of the auth-flow.)
Looking at https://github.com/XeroAPI/xero-node/blob/master/src/XeroClient.ts I think that apiCallback()
looks like it should be the place - but nowhere any mention of the authorisation code.