My vote is definitely for swagger. Besides being a great way to write API spec, it has tons of tooling to take a given spec to the next level. See the list at https://github.com/swagger-api/swagger-spec. My favorite is Swagger Editor which allows you to write a spec in YAML and shows you consumable API in real time.
There also exist an opens source project that has Swagger at its core called a127 (https://github.com/apigee-127/a127-documentation/wiki) that allows you to build enterprise-class APIs in Node.js locally and deploy to any PaaS where you can run a node app.
There also exist an opens source project that has Swagger at its core called a127 (https://github.com/apigee-127/a127-documentation/wiki) that allows you to build enterprise-class APIs in Node.js locally and deploy to any PaaS where you can run a node app.
Disclaimer: I work with a127 project.