This tool demo paper brings forward a new CLI tool called ExpressO for developers who need to analyze a Web API implemented using the Express.js framework and automatically extract a specification written in the standard OpenAPI interface description language. The specification includes all of the implemented endpoints along with their response status codes and path and query parameters. Developers can use it to automatically determine whether the interface of a Web API matches its implementation based on the Express.js framework. The tool has been released on the npm component registry as ‘expresso-api’.