Express OpenAPI

A middleware for generating and validating OpenAPI documentation from an Express app.

This middleware will look at the routes defined in your app and fill in as much as it can about them into an OpenAPI document. Optionally you can also flesh out request and response schemas, parameters, and other parts of your api spec with path specific middleware. The final document will be exposed as json served by the main middleware (along with component specific documents).

Philosophy

It is common in the OpenAPI community to talk about generating code from documentation. There is value in this approach, as often it is easier for devs to let someone else make the implementation decisions for them. For me, I feel the opposite. I am an engineer whose job it is to make good decisions about writing quality code. I want control of my application, and I want to write code. With this module I can both write great code, as well as have great documentation!

Installation

$ npm install --save @wesleytodd/openapi

Usage

const openapi = require('@wesleytodd/openapi')
const app = require('express')()

const oapi = openapi({
  openapi: '3.0.0',
  info: {
    title: 'Express Application',
    description: 'Generated docs from an Express api',
    version: '1.0.0',
  }
})

// This will serve the generated json document(s)
// (as well as the swagger-ui if configured)
app.use(oapi)

// To add path specific schema you can use the .path middleware
app.get('/', oapi.path({
  responses: {
    200: {
      description: 'Successful response',
      content: {
        'application/json': {
          schema: {
            type: 'object',
            properties: {
              hello: { type: 'string' }
            }
          }
        }
      }
    }
  }
}), (req, res) => {
  res.json({
    hello: 'world'
  })
})

app.listen(8080)

In the above example you can see the output of the OpenAPI spec by requesting /openapi.json.

$ curl -s http://localhost:8080/openapi.json | jq .
{
  "openapi": "3.0.0",
  "info": {
    "title": "Express Application",
    "version": "1.0.0",
    "description": "Generated docs from an Express api"
  },
  "paths": {
    "/": {
      "get": {
        "responses": {
          "200": {
            "description": "Successful response",
            "content": {
              "application/json": {
                "schema": {
                  "type": "object",
                  "properties": {
                    "hello": {
                      "type": "string"
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

Api Docs

openapi([route [, document[, options]]])

Creates an instance of the documentation middleware. The function that is returned is a middleware function decorated with helper methods for setting up the api documentation.

Options:

• route <string>: A route for which the documentation will be served at
• document <object>: Base document on top of which the paths will be added
• options <object>: Options object
  • options.coerce: Enable data type coercion
  • options.htmlui: Turn on serving swagger-ui html ui
  • options.basePath: When set, will strip the value of basePath from the start of every path.

Coerce

By default coerceTypes is set to true for AJV, but a copy of the req data is passed to prevent modifying the req in an unexpected way. This is because the coerceTypes option in (AJV modifies the input)[https://github.com/epoberezkin/ajv/issues/549]. If this is the behavior you want, you can pass true for this and a copy will not be made. This will result in params in the path or query with type number will be converted to numbers based on the rules from AJV.

OpenApiMiddleware.path([definition])

Registers a path with the OpenAPI document. The path definition is an OperationObject with all of the information about the requests and responses on that route. It returns a middleware function which can be used in an express app.

Example:

app.get('/:foo', oapi.path({
  description: 'Get a foo',
  responses: {
    200: {
      content: {
        'application/json': {
          schema: {
            type: 'object',
            properties: {
              foo: { type: 'string' }
            }
          }
        }
      }
    }
  }
}), (req, res) => {
  res.json({
    foo: req.params.foo
  })
})

OpenApiMiddleware.validPath([definition [, pathOpts]])

Registers a path with the OpenAPI document, also ensures incoming requests are valid against the schema. The path definition is an OperationObject with all of the information about the requests and responses on that route. It returns a middleware function which can be used in an express app and will call `next(err) if the incoming request is invalid.

The error is created with (http-errors)[https://www.npmjs.com/package/http-errors], and then is augmented with information about the schema and validation errors. Validation uses (avj)[https://www.npmjs.com/package/ajv], and err.validationErrors is the format exposed by that package. Pass { keywords: [] } as pathOpts to support custom validation based on ajv-keywords.

Example:

app.get('/:foo', oapi.validPath({
  description: 'Get a foo',
  responses: {
    200: {
      content: {
        'application/json': {
          schema: {
            type: 'object',
            properties: {
              foo: { type: 'string' }
            }
          }
        }
      }
    },
    400: {
      content: {
        'application/json': {
          schema: {
            type: 'object',
            properties: {
              error: { type: 'string' }
            }
          }
        }
      }
    }
  }
}), (err, req, res, next) => {
  res.status(err.status).json({
    error: err.message,
    validation: err.validationErrors,
    schema: err.validationSchema
  })
})

app.get('/zoom', oapi.validPath({
  ...
  requestBody: {
    required: true,
    content: {
      'application/json': {
        schema: {
          type: 'object',
          properties: {
            name: { type: 'string', not: { regexp: '/^[A-Z]/' } }
          }
        }
      }
    }
  },
  ...
}, { keywords: ['regexp'] }), (err, req, res, next) => {
  res.status(err.status).json({
    error: err.message,
    validation: err.validationErrors,
    schema: err.validationSchema
  })
})

OpenApiMiddleware.component(type[, name[, definition]])

Defines a new Component on the document.

Example:

oapi.component('examples', 'FooExample', {
  summary: 'An example of foo',
  value: 'bar'
})

If neither definition nor name are passed, the function will return the full components json.

Example:

oapi.component('examples', FooExample)
// { '$ref': '#/components/examples/FooExample' }

If name is defined but definition is not, it will return a Reference Object pointing to the component by that name.

Example:

oapi.component('examples')
// { summary: 'An example of foo', value: 'bar' }

OpenApiMiddleware.schema(name[, definition])

OpenApiMiddleware.response(name[, definition])

OpenApiMiddleware.parameters(name[, definition])

OpenApiMiddleware.examples(name[, definition])

OpenApiMiddleware.requestBodies(name[, definition])

OpenApiMiddleware.headers(name[, definition])

OpenApiMiddleware.securitySchemes(name[, definition])

OpenApiMiddleware.links(name[, definition])

OpenApiMiddleware.callbacks(name[, definition])

There are special component middleware for all of the types of component defined in the OpenAPI spec. Each of which is just the component method with a bound type, and behave with the same variadic behavior.

OpenApiMiddleware.swaggerui()

Serve an interactive UI for exploring the OpenAPI document.

SwaggerUI is one of the most popular tools for viewing OpenAPI documents and are bundled with the middleware. The UI is not turned on by default but can be with the option mentioned above or by using one of these middleware. Both interactive UIs also accept an optional object as a function argument which accepts configuration parameters for Swagger and Redoc. The full list of Swagger and Redoc configuration options can be found here: https://swagger.io/docs/open-source-tools/swagger-ui/usage/configuration/ and here: https://redocly.com/docs/redoc/config/ respectively.

Example:

app.use('/swaggerui', oapi.swaggerui())