How do i create authprovider in exisitance .node js project

tejaswini1010 , You need to follow Step 1 to Step 4 form the given link for get Access Token from sharepoint app.

http://pratapreddypilaka.blogspot.com/2018/05/sharepoint-online-authentication-for.html

Once you get the Access Token, you can call the GET API for List with the given access token. Here is the link for calling Share point API.

https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/working-with-lists-and-list-items-with-rest

These samples are in c#, you can use the same logic in node.js.

Hope it helps.