Expose swagger json from API Gateway lambda

handler.js

module.exports.getDocs = function(event, context, callback) {
    let options = {
    swaggerDefinition: {
        info: {                         // API informations (required)
          title: 'API Gateway project', // Title (required)
          version: '1.0.0',             // Version (required)
          description: 'Set of API endpoints exposed via API Gateway', // Description (optional)
        },
        host: 'localhost', // Host (optional)
        basePath: '/', // Base path (optional),
        schemes:['https','http'],
        definitions: {}
      },
    // Path to the API docs
    apis: ['handlers/*.js'],
    };
    
    // Initialize swagger-jsdoc -> returns validated swagger spec in json format
    let swaggerdoc = swaggerJSDoc(options);
    let stage = '';
    if (!process.env.IS_OFFLINE) // if you are using serverless-offline and dev/prod stage
    {
        if (event.requestContext) stage=event.requestContext.stage;
        if (stage && stage.length && stage.length > 0) swaggerdoc.basePath += stage + '/'; 
    }
    swaggerdoc.host = event.headers.Host;
    swaggerdoc.definitions=dbTools.getSwaggerDefinitions();
    // API Gateway response should have specific format (JSON.stringify below is mandatory!)
    // need to add CORS headers to overwrite API Gateway default CORS settings
    const response = {
        statusCode: 200,
        headers: {
        "x-custom-header" : "Inspired by Acclaim Consulting",
        "Access-Control-Allow-Origin" : "*",
        "Access-Control-Allow-Credentials" : true
        },
        body: JSON.stringify(swaggerdoc)
    };
    callback(null, response);
};

Most examples of using swagger specs for REST APIs are based on ExpressJs/Hapi/Restify and other similar frameworks. However, if your REST API is deployed as API Gateway lambdas, there are no web server to expose API docs.
Gist above provide you with a solution. It is just another service endpoint which dynamically generates Swagger specs from your comments (code above assumes that your methods located in 'handlers/*.js' files).

In your serverless.yml you can add next :

 getApiDocs:
   handler: handlers/handler.getDocs
   events:
     - http:
         path: api-docs
         method: get
         private: false
         cors: true