express-zod-api

Express Zod API

Start your API server with I/O schema validation and custom middlewares in minutes.

1. Technologies
2. Concept
3. Installation
4. Basic usage
   1. Set up config
   2. Create an endpoints factory
   3. Create your first endpoint
   4. Set up routing
   5. Start your server
5. Advanced usage
   1. Create a middleware
   2. Refinements
   3. Transformations
   4. ResultHandler
   5. Non-object response
   6. Your custom logger
   7. Your custom server
   8. Multiple schemas for a single route
6. Disclosing API specifications
   1. Reusing endpoint types on your frontend
   2. Swagger / OpenAPI Specification
7. Known issues
   1. Excess property check of endpoint output
8. Your input to my output

If you're upgrading from v1 please check out the information in Changelog.

Technologies

• Typescript first
• Schema validation — Zod 3.x.
• Webserver — Express.js.
• Logger — Winston.
• Swagger - OpenAPI 3.x

Concept

The API operates object schemas for input and output, including unions and intersections of object schemas (.or(), .and()), but in general the API can respond with any data type (see advanced example below).

The object being validated is the request.query for GET request, the request.body for PUT, PATCH and POST requests, or their merging for DELETE requests.

Middlewares can handle validated inputs and the original request, for example, to perform the authentication or provide the endpoint's handler with some request properties like the actual method. The returns of middlewares are combined into the options parameter available to the next middlewares and the endpoint's handler.

The handler's parameter input combines the validated inputs of all connected middlewares along with the handler's one. The result that the handler returns goes to the ResultHandler which is responsible for transmission of the final response or possible error.

All inputs and outputs are validated and there are also advanced powerful features like transformations and refinements. The diagram below can give you a better idea of the dataflow.

Installation

yarn add express-zod-api
# or
npm install express-zod-api

Add the following option to your tsconfig.json file in order to make it work as expected:

{
  "compilerOptions": {
    "strict": true
  }
}

Basic usage

See the full implementation example here.

Set up config

import {ConfigType} from 'express-zod-api';

const config: ConfigType = {
  server: {
    listen: 8090,
  },
  cors: true,
  logger: {
    level: 'debug',
    color: true
  }
};

See all available options here.

Create an endpoints factory

import {defaultEndpointsFactory} from './endpoints-factory';
// same as: new EndpointsFactory(defaultResultHandler)
const endpointsFactory = defaultEndpointsFactory;

You can also instantly add middlewares to it using .addMiddleware() method.

Create your first endpoint

import {z} from 'express-zod-api';

const setUserEndpoint = endpointsFactory.build({
  method: 'post',
  input: z.object({
    id: z.number(),
    name: z.string(),
  }),
  output: z.object({
    timestamp: z.number(),
  }),
  handler: async ({input: {id, name}, options, logger}) => {
    logger.debug(`Requested id: ${id}`);
    logger.debug('Options:', options); // provided by middlewares
    return { timestamp: Date.now() };
  }
});

The endpoint can also handle multiple types of requests, this feature is available by using methods property that accepts an array. You can also add middlewares to the endpoint by using .addMiddleware() before .build().

Set up routing

import {Routing} from 'express-zod-api';

const routing: Routing = {
  v1: {
    setUser: setUserEndpoint
  }
};

This implementation sets up setUserEndpoint to handle requests to the /v1/setUser route.

Start your server

import {createServer} from 'express-zod-api';

createServer(config, routing);

Advanced usage

Create a middleware

You can create middlewares separately using createMiddleware() function and connect them later. All returns of the connected middlewares are combined into the options argument of the endpoint's handler. The inputs of middlewares are combined with the inputs of the endpoint's handler.

import {
  createMiddleware, z, Method, createHttpError
} from 'express-zod-api';

// This one provides the method of the request to your 
// endpoint. It's useful for the ones that handle 
// multiple types of request (GET, POST, ...)
const methodProviderMiddleware = createMiddleware({
  input: z.object({}).nonstrict(),
  middleware: async ({request}) => ({
    method: request.method.toLowerCase() as Method,
  })
});

// This one performs the authentication using a key from 
// the input and a token from headers. It supplies the
// endpoint with a user from a database.
const authMiddleware = createMiddleware({
  input: z.object({
    key: z.string().nonempty()
  }),
  middleware: async ({input: {key}, request, logger}) => {
    logger.debug('Checking the key and token...');
    const user = await db.Users.findOne({key});
    if (!user) {
      throw createHttpError(401, 'Invalid key');
    }
    if (request.headers['token'] !== user.token) {
      throw createHttpError(401, 'Invalid token');
    }
    return { user };
  }
});

Refinements

You can implement additional validation inside the schema:

import {createMiddleware, z} from 'express-zod-api';

const authMiddleware = createMiddleware({
  input: z.object({
    key: z.string().nonempty()
      .refine((key) => key === '123', 'Invalid key')
  }),
  ...
})

Transformations

Since parameters of GET requests come in the form of strings, there is often a need to transform them into numbers or arrays of numbers.

import {z} from 'express-zod-api';

const getUserEndpoint = endpointsFactory.build({
  method: 'get',
  input: z.object({
    id: z.string().transform((id) => parseInt(id, 10)),
    ids: z.string().transform(
      (ids) => ids.split(',').map((id) => parseInt(id, 10))
    )
  }),
  output: z.object({...}),
  handler: async ({input: {id, ids}, logger}) => {
    logger.debug('id', id); // type: number
    logger.debug('ids', ids); // type: number[]
  }
});

ResultHandler

ResultHandler is the type of function that is responsible for transmission of the final response or possible error. ResultHandlerDefinition contains this handler and additional methods defining the schema of the positive and negative responses as well as their MIME types for the further disclosing to consumers and documentation. Positive schema is the schema of successful response. Negative schema is the one that describes the response in case of error. The defaultResultHandler sets the HTTP status code and ensures the following type of the response:

type DefaultResponse<OUT> = {
  status: 'success',
  data: OUT
} | {
  status: 'error',
  error: {
    message: string;
  }
};

In order to customize the result handler you need to use createResultHandler() first wrapping the response schema in createApiResponse() optionally specifying its mime types, and wrapping the endpoint output schema in markOutput(). Here is an example you can use as a template:

import {createResultHandler, IOSchema, createApiResponse, markOutput, z} from 'express-zod-api';

const myResultHandler = createResultHandler({
  getPositiveResponse: <OUT extends IOSchema>(output: OUT) => createApiResponse(
    z.object({
      ...,
      someProperty: markOutput(output)
    }), 
    ['mime/type1', 'mime/type2'] // optional, default: application/json
  ),
  getNegativeResponse: () => createApiResponse(z.object({
     error: z.string()
  })),
  handler: ({error, input, output, request, response, logger}) => {
    // your implementation
  }
});

Then you need to use it as an argument for EndpointsFactory instance creation:

import {EndpointsFactory} from 'express-zod-api';

const endpointsFactory = new EndpointsFactory(myResultHandler);

Non-object response

Starting from the version 2.0.0, ResultHandler also supports non-object response types, for example, sending an image file including its MIME type in Content-type header.

You can find two approaches to EndpointsFactory and ResultHandler implementation in this example. One of them implements file streaming, in this case the endpoint just has to provide the filename. The response schema generally may be just z.string(), but there is also a specific one: z.file() that also supports .binary() and .base64() refinements which are reflected in the generated documentation.

const fileStreamingEndpointsFactory = new EndpointsFactory(createResultHandler({
  getPositiveResponse: () => createApiResponse(z.file().binary(), 'image/*'),
  getNegativeResponse: () => createApiResponse(z.string(), 'text/plain'),
  handler: ({response, error, output}) => {
    if (error) {
      response.status(400).send(error.message);
      return;
    }
    if ('filename' in output) {
      fs.createReadStream(output.filename)
        .pipe(response.type(output.filename));
    } else {
      response.status(400).send('Filename is missing');
    }
  }
}));

Your custom logger

You can specify your custom Winston logger in config:

import * as winston from 'winston';
import {ConfigType, createServer} from 'express-zod-api';

const config: ConfigType = {
  logger: winston.createLogger(),
  ...
};
createServer(config, routing);

Your custom server

You can instantiate your own express app and connect your endpoints the following way.

import * as express from 'express';
import {ConfigType, attachRouting} from 'express-zod-api';

const app = express();
const config: ConfigType = {app, ...};
const routing = {...};

attachRouting(config, routing);
app.listen();

Please note that in this case you probably need to: parse request.body, call app.listen() and handle 404 errors yourself;

Multiple schemas for a single route

Thanks to the DependsOnMethod class a route may have multiple Endpoints attached depending on different methods. It can also be the same Endpoint that handles multiple methods as well.

import {DependsOnMethod} from 'express-zod-api';

// the route /v1/user has two Endpoints 
// which handle a couple of methods each
const routing: Routing = {
  v1: {
    user: new DependsOnMethod({
      get: myEndpointForGetAndDelete,
      delete: myEndpointForGetAndDelete,
      post: myEndpointForPostAndPatch,
      patch: myEndpointForPostAndPatch,
    })
  }
};

Disclosing API specifications

Reusing endpoint types on your frontend

You can export only the types of your endpoints for your front-end:

export type MyEndpointType = typeof endpoint;

Then use provided helpers to obtain their input and response types:

import {EndpointInput, EndpointResponse} from 'express-zod-api';
import {MyEndpointType} from '../your/backend';

type MyEndpointInput = EndpointInput<MyEndpointType>;
type MyEndpointResponse = EndpointResponse<MyEndpointType>; // unites positive and negative schemas

Swagger / OpenAPI Specification

You can generate the specification of your API the following way and write it to a .yaml file, that can be used as documentation:

import {OpenAPI} from 'express-zod-api';

const yamlString = new OpenAPI({
  routing, 
  version: '1.2.3',
  title: 'Example API',
  serverUrl: 'http://example.com'
}).getSpecAsYaml();

Known issues

Excess property check of endpoint output

Unfortunately Typescript does not perform excess property check for objects resolved in Promise, so there is no error during development of endpoint's output.

import {z} from 'express-zod-api';

endpointsFactory.build({
  methods, input,
  output: z.object({
    anything: z.number()
  }),
  handler: async () => ({
    anything: 123,
    excessive: 'something' // no type error
  })
});

You can achieve this check by assigning the output schema to a constant and reusing it in additional definition of handler's return type:

import {z} from 'express-zod-api';

const output = z.object({
  anything: z.number()
});

endpointsFactory.build({
  methods, input, output,
  handler: async (): Promise<z.input<typeof output>> => ({
    anything: 123,
    excessive: 'something' // error TS2322, ok!
  })
});

Your input to my output

Do you have a question or idea? Your feedback is highly appreciated in Discussions section.

Found a bug? Please let me know in Issues section.

Found a vulnerability or other security issue? Please refer to Security policy.

Changelog

v2.1.0

• Zod version is 3.7.1.
• New response schema type ZodFile can be created using z.file(). It has two refinements: .binary() and .base64() which also reflected in the generated Swagger / OpenAPI documentation. You can use it instead of z.string() with createApiResponse():

// before
const fileStreamingEndpointsFactoryBefore = new EndpointsFactory(
  createResultHandler({
    getPositiveResponse: () => createApiResponse(z.string(), "image/*"),
    // ...,
  }),
);

// after
const fileStreamingEndpointsFactoryAfter = new EndpointsFactory(
  createResultHandler({
    getPositiveResponse: () => createApiResponse(z.file().binary(), "image/*"),
    // ...,
  }),
);

• Please do NOT use z.file() within the Endpoint input / output object schemas.