express-zod-api

Express Zod API

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

1. Tech
2. Installation
3. Basic usage
   1. Set up config
   2. Create an endpoints factory
   3. Set up routing
   4. Start your server
4. Advanced usage
   1. Create a middleware
   2. Refinements
   3. Your custom server
5. Disclosing API specifications
   1. Reusing endpoint types on your frontend
   2. Swagger / OpenAPI Specification
6. Known issues
   1. Excess property check of endpoint output

Tech

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

Installation

yarn install express-zod-api

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

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true
  }
}

Basic usage

Full example in ./example. You can clone the repo and run yarn start to check it out in action.

Set up config

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

See ./src/config-type.ts for all available options.

Create an endpoints factory

const endpointsFactory = new EndpointsFactory();

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

Create your first endpoint

const getUserEndpoint = endpointsFactory
  .build({
    methods: ['get'],
    input: z.object({
      id: z.string().transform((id) => parseInt(id, 10))
    }),
    output: z.object({
      name: z.string(),
    }),
    handler: async ({input: {id}, options, logger}) => {
      logger.debug(`Requested id: ${id}`);
      logger.debug('Options:', options);
      const name = 'John Doe';
      return { name: 'John Doe' };
    }
  });

Note: options come from the output of middlewares. You can add middlewares by using .addMiddleware() method before .build(). All inputs and outputs are validated.

Set up routing

const routing: Routing = {
  v1: {
    getUser: getUserEndpoint
  }
};

This sets up getUserEndpoint to handle requests to the /v1/getUser path.

Start your server

createServer(config, routing);

Advanced usage

Create a middleware

You can create middlewares separately using createMiddleware() function and connect them later. All outputs of connected middlewares are put in options argument of the endpoint handler. All middleware inputs are also available as the endpoint inputs.

// This one provides the method of the request
const methodProviderMiddleware = createMiddleware({
  input: z.object({}).nonstrict(),
  middleware: async ({request}) => ({
    method: request.method.toLowerCase() as Method,
  })
});

// This one performs the authentication 
// using key from the input and token from headers
const authMiddleware = createMiddleware({
  input: z.object({
    key: z.string().nonempty()
  }),
  middleware: async ({input: {key}, request, logger}) => {
    logger.debug('Checking the key and token...');
    if (key !== '123') {
      throw createHttpError(401, 'Invalid key');
    }
    if (request.headers['token'] !== '456') {
      throw createHttpError(401, 'Invalid token');
    }
    return {token: request.headers['token']};
  }
});

Refinements

You can also implement the validation inside the input schema:

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

Your custom server

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

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

initRouting({app, logger, config, routing});

Disclosing API specifications

Reusing endpoint types on your frontend

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

export type GetUserEndpoint = typeof getUserEndpoint;

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

type GetUserEndpointInput = EndpointInput<GetUserEndpoint>;
type GetUserEndpointOutput = EndpointOutput<GetUserEndpoint>;

Swagger / OpenAPI Specification

You can generate the specification of your API the following way and write it to a .yaml file:

const yamlString = generateOpenApi({
  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 proprety check for objects resolved in Promise, so there is no error during development of endpoint's output.

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:

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