If you want to have a complete character list of a signed-in user you need the Authorization Code flow. Now if you only want to query a character by its name and realm, you can use the Client Credentials Flow.
Here is a code snippet for generating the access_token: OAuth2 client credentials implementations - #5 by Schiller-1822
And in this post you can find a list of open source projects made by the community using different ways to integrate a NodeJS app.