Xero API Integration into nODEJS EXPRESS APP

I'm looking for someone to integrate Xero API into my web application, the application is build with nodejs. So far i have written the controllers, routes and service needed to connect to the api, but for some reason when testing authentication to the API, it fails.
I need that to be sorted out so that i can connect to xero and be able to get a response from there when i fetch accounts and reports. Also I want the developer to be very experienced with Xero integration because i need the integration done immediately