Creating an OpenAPI contract from an existing Express API can be a valuable way to document your API, improve collaboration, and generate client SDKs. This guide outlines the steps to generate an OpenAPI specification from your existing Express routes.

Inspect Your Existing Express Routes

Before generating an OpenAPI contract, carefully inspect your existing Express routes. Pay attention to:

• Endpoints: List all the API endpoints (e.g., /users, /products/{id}).
• HTTP Methods: Identify the HTTP methods used for each endpoint (e.g., GET, POST, PUT, DELETE).
• Request Parameters: Note any path parameters, query parameters, or request body structures.
• Response Structures: Document the expected response structures for each endpoint.
• Headers and Cookies: Identify any custom headers or cookies used in requests or responses.

Choose an OpenAPI Generation Tool

Several tools can help generate OpenAPI contracts from existing Express APIs. Popular options include:

• swagger-jsdoc: Generates OpenAPI specifications from JSDoc annotations in your code.
• express-openapi: Allows you to define your API using a YAML or JSON file and maps it to your Express routes.
• ts-node-dev and openapi-typescript: When combined with properly typed code, you can generate an openapi spec from your typescript code.
• openapi-express: Generates an openapi spec from the running express server.