express-openapi-typer

Caution! Alpha-level software ahead. Use at your own peril

Code-generation-free conversion of OpenAPI v3.1 schema into type-checked Express request handlers.

Derive Express handler types from an OpenAPI schema to get

• type errors when a handler doesn't match the schema, and
• auto-completion on handler path, req.param, req.query, req.body, res.send(), res.json() etc.

Note that the library does not perform runtime validation against the OpenAPI schema: add something like https://github.com/Hilzu/express-openapi-validate for that purpose.

Requires OpenAPI v3.1. This library relies heavily on existing JSON Schema tooling whereas earlier OpenAPI versions use the OpenAPI Schema Object instead of pure JSON Schema. OpenAPI v3.1 is yet unpublished; track progress here. Read more about the OpenAPI/JSON Schema divergence at https://apisyouwonthate.com/blog/openapi-and-json-schema-divergence-part-1 and how v3.1 solves it at https://phil.tech/2019/09/07/update-openapi-json-schema/.

Install

yarn add express-openapi-typer

Usage

First define your OpenAPI schema as a TypeScript type:

interface PetStoreSchema {
  openapi: '3.1.0'
  info: { ... }
  paths: {
    '/pets': {
        get: { ...}
    },
    ...
  }
}

Then override your Express router's type from

const router = express.Router()

into the following:

import { OpenAPIRouter } from 'express-openapi-typer'

const router = (express.Router() as unknown) as OpenAPIRouter<PetStoreSchema>

Handler functions in router now get type-checked as per PetStoreSchema! For example when using the full sample PetStore schema we end up with the following:

Access OpenAPI schema type from a runtime value

It can be useful to instantiate the OpenAPI schema as a runtime value instead of just defining a type. For example when serving the schema as documentation or handling validation we need to access the schema at runtime. In cases like these combine typeof and as const to access the schema value's type:

const petStoreSchema = {
  openapi: '3.1.0',
  info: { ... },
  paths: {
    '/pets': {
        get: { ... }
    },
    ...
  }
} as const // <-- important!

type PetStoreSchema = typeof petStoreSchema

Allow additional paths

By default OpenAPIRouter doesn't allow any additional handlers not defined in the OpenAPI schema. To loosen this restriction you can expand the type as follows:

import * as express from 'express'

const router = express.Router() as OpenAPIRouter<PetStoreSchema> & express.Router

You can also select a subset of express.Router with Pick/Omit when allowing additional methods only for a specific HTTP method, for example.

TODO

• All the limitations from json-schema-type-mapper apply here as well
• Handle request header parameters?
• API client type checking, using Axios?
• Figure a way out of the unfortunate as unknown cast
• Support path-based $refs, not just $id-based ones
  • requires some sort of manual mapping as we can't take "#/components/schemas/NewUser" apart at type-level

Related projects

• Check out restyped and rest.ts for different approaches to type-safe REST APIs