Need to create an api doc for an existing application written with nodejs/express

There's a wide array of node.js tools to integrate Swagger with your application, and I assume they offer different ways of doing so. You can find a list of such integrations here - https://github.com/webron/swagger-spec/#nodejs - but I can tell you that there are additional tools out there that are not listed there. You can try searching github for swagger and node/express.

As for the manual spec and the basePath - Swagger 2.0 actually solves that for you. You can use the online editor - http://editor.swagger.io - to write your specs in a more human-friendly YAML form, which then you can export to JSON. Unlike Swagger 1.2 and previous versions, the basePath is now split into three properties - schemes (http, https), host (domain, port) and basePath (the root context of the application). None of these properties are mandatory, and they all default to whatever is serving the swagger.json file (the spec itself). schemes defaults to the scheme service the swagger.json, host defaults to the host used for serving the swagger.json and basePath will be \ unless explicitly specified. I believe this should solve your concerns regarding the basePath.