Zod to OpenAPI

A library that uses zod schemas to generate an Open API Swagger documentation.

1. Purpose and quick example
2. Usage
   1. Installation
   2. The openapi method
   3. The Registry
   4. Defining schemas
   5. Defining routes
   6. Defining custom components
   7. A full example
   8. Adding it as part of your build
3. Zod schema types
   1. Supported types
   2. Unsupported types
4. Technologies

We keep a changelog as part of the GitHub releases.

Purpose and quick example

We at Astea Solutions made this library because we use zod for validation in our APIs and are tired of the duplication to also support a separate OpenAPI definition that must be kept in sync. Using zod-to-openapi, we generate OpenAPI definitions directly from our zod schemas, this having single source of truth.

Simply put, it turns this:

const UserSchema = registry.register(
  'User',
  z.object({
    id: z.string().openapi({ example: '1212121' }),
    name: z.string().openapi({ example: 'John Doe' }),
    age: z.number().openapi({ example: 42 }),
  })
);

registry.registerPath({
  method: 'get',
  path: '/users/{id}',
  summary: 'Get a single user',
  request: {
    params: z.object({ id: z.string() }),
  },

  responses: {
    200: {
      description: 'Object with user data.',
      content: {
        'application/json': {
          schema: UserSchema,
        },
      },
    },
  },
});

into this:

components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: string
          example: '1212121'
        name:
          type: string
          example: John Doe
        age:
          type: number
          example: 42
      required:
        - id
        - name
        - age

/users/{id}:
  get:
    summary: Get a single user
    parameters:
      - in: path
        name: id
        schema:
          type: string
        required: true
    responses:
      '200':
        description: Object with user data
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/User'

and you can still use UserSchema and the request.params object to validate the input of your API.

Usage

Installation

npm install @asteasolutions/zod-to-openapi
# or
yarn add @asteasolutions/zod-to-openapi

The openapi method

To keep openapi definitions natural, we add an openapi method to all Zod objects. For this to work, you need to call extendZodWithOpenApi once in your project.

Note: This should be done only once in a common-entrypoint file of your project (for example an index.ts/app.ts). If you're using tree-shaking with Webpack, mark that file as having side-effects.

import { extendZodWithOpenApi } from '@asteasolutions/zod-to-openapi';
import { z } from 'zod';

extendZodWithOpenApi(z);

// We can now use `.openapi()` to specify OpenAPI metadata
z.string().openapi({ description: 'Some string' });

The Registry

The OpenAPIRegistry is used to track definitions which are later generated using the OpenAPIGenerator class.

import {
  OpenAPIRegistry,
  OpenAPIGenerator,
} from '@asteasolutions/zod-to-openapi';

const registry = new OpenAPIRegistry();

// Register definitions here

const generator = new OpenAPIGenerator(registry.definitions);

return generator.generateComponents();

generateComponents will generate only the /components section of an OpenAPI document (e.g. only schemas and parameters), not generating actual routes.

Defining schemas

An OpenAPI schema should be registered using the register method of an OpenAPIRegistry instance.

const UserSchema = registry.register(
  'User',
  z.object({
    id: z.string().openapi({ example: '1212121' }),
    name: z.string().openapi({ example: 'John Doe' }),
    age: z.number().openapi({ example: 42 }),
  })
);

If run now, generateComponents will generate the following structure:

components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: string
          example: '1212121'
        name:
          type: string
          example: John Doe
        age:
          type: number
          example: 42
      required:
        - id
        - name
        - age

The key for the schema in the output is the first argument passed to .register (in this case - User).

Note that generateComponents does not return YAML but a JS object - you can then serialize that object into YAML or JSON depending on your use-case.

The resulting schema can then be referenced by using $ref: #/components/schemas/User in an existing OpenAPI JSON. This will be done automatically for Routes defined through the registry.

Defining routes

Registering a path

An OpenAPI path is registered using the registerPath method of an OpenAPIRegistry instance.

registry.registerPath({
  method: 'get',
  path: '/users/{id}',
  description: 'Get user data by its id',
  summary: 'Get a single user',
  request: {
    params: z.object({
      id: z.string().openapi({ example: '1212121' }),
    }),
  },
  responses: {
    200: {
      description: 'Object with user data.',
      content: {
        'application/json': {
          schema: UserSchema,
        },
      },
    },
    204: {
      description: 'No content - successful operation',
    },
  },
});

The YAML equivalent of the schema above would be:

'/users/{id}':
  get:
    description: Get user data by its id
    summary: Get a single user
    parameters:
      - in: path
        name: id
        schema:
          type: string
          example: '1212121'
        required: true
    responses:
      '200':
        description: Object with user data.
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/User'
      '204':
        description: No content - successful operation

The library specific properties for registerPath are method, path, request and responses. Everything else gets directly appended to the path definition.

• method - One of get, post, put, delete and patch;
• path - a string - being the path of the endpoint;
• request - an optional object with optional body, params, query and headers keys,
  • query, params - being instances of ZodObject
  • body - an object with a description and a content record where:
    • the key is a mediaType string like application/json
    • and the value is an object with a schema of any zod type
  • headers - an array of zod instances
• responses - an object where the key is the status code or default and the value is an object with a description and a content record where:
  • the key is a mediaType string like application/json
  • and the value is an object with a schema of any zod type

Defining route parameters

If you don't want to inline all parameter definitions, you can define them separately with registerParameter and then reference them:

const UserIdParam = registry.registerParameter(
  'UserId',
  z.string().openapi({
    param: {
      name: 'id',
      in: 'path',
    },
    example: '1212121',
  })
);

registry.registerPath({
  ...
  request: {
    params: z.object({
      id: UserIdParam
    }),
  },
  responses: ...
});

The YAML equivalent would be:

components:
  parameters:
    UserId:
      in: path
      name: id
      schema:
        type: string
        example: '1212121'
      required: true

'/users/{id}':
  get:
    ...
    parameters:
      - $ref: '#/components/parameters/UserId'
    responses: ...

Note: In order to define properties that apply to the parameter itself, use the param property of .openapi. Any properties provided outside of param would be applied to the schema for this parameter.

Generating the full document

A full OpenAPI document can be generated using the generateDocument method of an OpenAPIGenerator instance. It takes one argument - the document config. It may look something like this:

return generator.generateDocument({
  openapi: '3.0.0',
  info: {
    version: '1.0.0',
    title: 'My API',
    description: 'This is the API',
  },
  servers: [{ url: 'v1' }],
});

Defining custom components

You can define components that are not OpenAPI schemas, including security schemes, response headers and others. See this test file for examples.

A full example

A full example code can be found here. And the YAML representation of its result - here

Adding it as part of your build

In a file inside your project you can have a file like so:

export const registry = new OpenAPIRegistry();

export function generateOpenAPI() {
  const config = {...}; // your config comes here

  return new OpenAPIGenerator(schemas.definitions).generateDocument(config);
}

You then use the exported registry object to register all schemas, parameters and routes where appropriate.

Then you can create a script that executes the exported generateOpenAPI function. This script can be executed as a part of your build step so that it can write the result to some file like openapi-docs.json.

Zod schema types

Supported types

The list of all supported types as of now is:

• ZodString
  • adding format for .uuid(), .email() and .url() and pattern for .regex() is also supported
• ZodNumber
  • including z.number().int() being inferred as type: 'integer'
• ZodBoolean
• ZodDefault
• ZodEffects - only for .refine()
• ZodLiteral
• ZodEnum
• ZodNativeEnum
• ZodObject
• ZodArray
• ZodUnion
• ZodIntersection
• ZodRecord
• ZodUnknown

Extending an instance of ZodObject is also supported and results in an OpenApi definition with allOf

Unsupported types

In case you try to create an OpenAPI schema from a zod schema that is not one of the aforementioned types then you'd receive an UnknownZodTypeError.

You can still register such schemas on your own by providing a type via the .openapi method. In case you think that the desired behavior can be achieved automatically do not hesitate to reach out to us by describing your case via Github Issues.

Note: The ZodEffects schema from the .transform method is an example for a zod schema that cannot be automatically generated since the result of the transformation resides only as a type definition and is not an actual zod specific object.

Technologies

• Typescript
• Zod 3.x
• OpenAPI 3.x TS