Generate OpenAPI spec from typera routes

Usage no npm install needed!

<script type="module">
  import typeraOpenapi from 'https://cdn.skypack.dev/typera-openapi';


typera-openapi - OpenAPI generator for typera


typera-openapi is an tool that automatically creates OpenAPI v3 definitions for projects that use typera for routes.

Table of Contents

Getting started

Install typera-openapi:

npm install typera-openapi

Your route files must have a single default export that exports a typera router. JSDoc comments serve as additional documentation:

// src/my-routes.ts

import { Response, Route, route, router } from 'typera-express'

interface MyResult {
  /** The JSDoc text is used as a description for object properties */
  field: number

const bodyCodec = t.type({
  /** Descriptions are also supported in io-ts codecs */
  name: t.string

 * Routes can also have a description. Note that descriptions are entirely optional.
 * @summary You can also set a short summary
 * @tags Tag1, Tag2
 * @response 200 Success response description.
 * @response 400 Another description for a response. This one
 * spans multile lines.
const myRoute: Route<Response.Ok<MyResult> | Response.BadRequest<string>> =

// ...

export default router(myRoute, ...)

Run the typera-openapi tool giving paths to your route files as command line arguments:

npx typera-openapi src/my-routes.ts

This creates src/my-routes.openapi.ts which contains the OpenAPI definitions.

Use the definitions in your app to serve documentation:

// src/app.ts

import * as express from 'express'
import { OpenAPIV3 } from 'openapi-types'
import * as swaggerUi from 'swagger-ui-express'
import { prefix } from 'typera-openapi'

import myRoutes from './my-routes'
import myRouteDefs from './my-routes.openapi'

const openapiDoc: OpenAPIV3.Document = {
  openapi: '3.0.0',
  info: {
    title: 'My cool API',
    version: '0.1.0',
  paths: {
    ...prefix('/api', myRouteDefs.paths),

const app = express()
app.use('/api', myRoutes.handler())
app.use('/api-docs', swaggerUi.serve, swaggerUi.setup(openapiDoc))

The prefix function is used to move OpenAPI path definitions to a different prefix, because the myRoutes are served from the /api prefix.


typera-openapi [options] FILE...

Generate OpenAPI definitions for routes found in the given files.

For each input file file.ts, writes a file.openapi.ts or file.openapi.json, depending on --format.



Output file format. Either ts or json. Default: ts.

--prettify, -p

Apply prettier formatting to output files.

--check, -c

Check that generated files are up-to-date without actually generating them. If any file is outdated, print an error and exit with status 1. Useful for CI.

How it works?

typera-openapi uses TypeScript type information to generate OpenAPI definitions. Methods, paths, route params, query params, request and response bodies, and request and response headers are determined by looking up the type information provided by the TypeScript compiler API.

For example, assume we have this route:

 * @response 200 User created successfully
 * @response 400 Validation error
const createUser: Route<Response.Ok<User> | Response.BadRequest<string>> = route
  .handler(async (request) => { ... })
  • Response descriptions are looked up from the JSDoc comment. Other tags are also available.
  • Response body and header types are available in the type of the createUser variable. The explicit type annotation would not even be needed, because the compiler can infer the type.
  • Method and path are available in the .post('/users') call.
  • The rest are looked up by inspecting the type of the request parameter of the route handler function.

This has one caveat: The request type must match the input you expect your user to send. Assume the createUserBody codec is like this:

const createUserBody = t.type({
  name: t.string,
  shoeSize: t.number,
  allowMarketing: BooleanFromString, // from io-ts-types

The name and shoeSize fields are fine, since both t.string and t.number map their input type to the same output type. But the allowMarketing field is problematic, since BooleanFromString takes a string as an input, but converts it to boolean.

So the JSON input this route expects is:

type Input = {
  name: string
  shoeSize: number
  allowMarketing: string

But the type of request.body that the compiler sees is:

type Body = {
  name: string
  shoeSize: number
  allowMarketing: boolean

For this reason, you shouldn't use decoders that change the type of the input directly. If needed, you should instead add another step for converting the data from input to the format you expect. This can be achieved with a custom middleware, for example.

The Date type

JSON doesn't have a native Date type. As explained above, when typera-openapi encounters a Date type, it doesn't know from which input type it is parsed, so it uses string and sets format to date-time, because this is how JSON.stringify encodes Date objects by default. In the future we may add a way to override this behavior.


$ yarn version --new-version <major|minor|patch>
$ yarn publish
$ git push origin main --tags

Open https://github.com/akheron/typera-openapi/releases, edit the draft release, select the newest version tag, adjust the description as needed.