Express Zod API

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

1. Why and what is it for
2. How it works
   1. Technologies
   2. Concept
3. Quick start — Fast Track
   1. Installation
   2. Set up config
   3. Create an endpoints factory
   4. Create your first endpoint
   5. Set up routing
   6. Start your server
   7. Try it
4. Fascinating features
   1. Middlewares
   2. Options
   3. Refinements
   4. Transformations
   5. Dealing with dates
   6. Route path params
   7. Response customization
   8. Non-object response including file downloads
   9. Using native express middlewares
   10. File uploads
   11. Customizing logger
   12. Connect to your own express app
   13. Multiple schemas for one route
   14. Serving static files
   15. Customizing input sources
   16. Enabling compression
   17. Enabling HTTPS
   18. Informing the frontend about the API
   19. Creating a documentation
5. Additional hints
   1. How to test endpoints
   2. Excessive properties in endpoint output
6. Your input to my output

You can find the release notes in Changelog. Along with recommendations for migrating from v4, v3, v2 and v1.

Why and what is it for

I made this library because of the often repetitive tasks of starting a web server APIs with the need to validate input data. It integrates and provides the capabilities of popular web server, logger, validation and documenting solutions. Therefore, many basic tasks can be accomplished faster and easier, in particular:

• You can describe web server routes as a hierarchical object.
• You can keep the endpoint's input and output type declarations right next to its handler.
• All input and output data types are validated, so it ensures you won't have an empty string, null or undefined where you expect a number.
• Variables within an endpoint handler have types according to the declared schema, so your IDE and Typescript will provide you with necessary hints to focus on bringing your vision to life.
• All of your endpoints can respond in a consistent way.
• The expected endpoint input and response types can be exported to the frontend, so you don't get confused about the field names when you implement the client for your API.
• You can generate your API documentation in a Swagger / OpenAPI compatible format.

How it works

Technologies

• Typescript first.
• Web server — Express.js.
• Schema validation — Zod 3.x.
• Logger — Winston.
• Documenting — OpenAPI 3.x (formerly known as the Swagger Specification).
• File uploads — Express-FileUpload (based on Busboy)

Concept

The API operates object schemas for input and output validation. The object being validated is the combination of certain request properties. It is available to the endpoint handler as the input parameter. Middlewares have access to all request properties, they can provide endpoints with options. The object returned by the endpoint handler is called output. It goes to the ResultHandler which is responsible for transmitting consistent responses containing the output or possible error. Much can be customized to fit your needs.

Quick start

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
  }
}

Set up config

import { createConfig } from "express-zod-api";

const config = createConfig({
  server: {
    listen: 8090, // port or socket
  },
  cors: true,
  logger: {
    level: "debug",
    color: true,
  },
});

See all available options here.

Create an endpoints factory

In the basic case, you can just import and use the default factory:

import { defaultEndpointsFactory } from "express-zod-api";

In case you need a global middleware, see Middlewares. In case you need to customize the response, see Response customization.

Create your first endpoint

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

const helloWorldEndpoint = defaultEndpointsFactory.build({
  method: "get",
  input: z.object({
    // for empty input use z.object({})
    name: z.string().optional(),
  }),
  output: z.object({
    greetings: z.string(),
  }),
  handler: async ({ input: { name }, options, logger }) => {
    logger.debug("Options:", options); // middlewares provide options
    return { greetings: `Hello, ${name || "World"}. Happy coding!` };
  },
});

In case you want it to handle multiple methods use methods property instead of method.

Set up routing

Connect your endpoint to the /v1/hello route:

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

const routing: Routing = {
  v1: {
    hello: helloWorldEndpoint,
  },
};

Start your server

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

createServer(config, routing);

You can disable startup logo using startupLogo entry of your config. See the full implementation example here.

Try it

Execute the following command:

curl -L -X GET 'localhost:8090/v1/hello?name=Rick'

You should receive the following response:

{ "status": "success", "data": { "greetings": "Hello, Rick. Happy coding!" } }

Fascinating features

Middlewares

Middleware can authenticate using input or request headers, and can provide endpoint handlers with options. Inputs of middlewares are also available to endpoint handlers within input.

Here is an example on how to provide headers of the request.

import { createMiddleware } from "express-zod-api";

const headersProviderMiddleware = createMiddleware({
  input: z.object({}), // means no inputs
  middleware: async ({ request }) => ({
    headers: request.headers,
  }),
});

By using .addMiddleware() method before .build() you can connect it to the endpoint:

const yourEndpoint = defaultEndpointsFactory
  .addMiddleware(headersProviderMiddleware)
  .build({
    // ...,
    handler: async ({ options }) => {
      // options.headers === request.headers
    },
  });

Here is an example of the authentication middleware, that checks a key from input and token from headers:

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

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 }; // provides endpoints with options.user
  },
});

You can connect the middleware to endpoints factory right away, making it kind of global:

import { defaultEndpointsFactory } from "express-zod-api";

const endpointsFactory = defaultEndpointsFactory.addMiddleware(authMiddleware);

You can connect as many middlewares as you want, they will be executed in order.

Options

In case you'd like to provide your endpoints with options that do not depend on Request, like database connection instance, consider shorthand method addOptions.

import { defaultEndpointsFactory } from "express-zod-api";

const endpointsFactory = defaultEndpointsFactory.addOptions({
  db: mongoose.connect("mongodb://connection.string"),
  privateKey: fs.readFileSync("private-key.pem", "utf-8"),
});

Refinements

By the way, you can implement additional validation within schema. Validation errors are reported in a response with a status code 400.

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

const nicknameConstraintMiddleware = createMiddleware({
  input: z.object({
    nickname: z
      .string()
      .nonempty()
      .refine(
        (nick) => !/^\d.*$/.test(nick),
        "Nickname cannot start with a digit"
      ),
  }),
  // ...,
});

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[]
  },
});

Dealing with dates

Dates in Javascript are one of the most troublesome entities. In addition, Date cannot be passed directly in JSON format. Therefore, attempting to return Date from the endpoint handler results in it being converted to an ISO string in actual response by calling toJSON(), which in turn calls toISOString(). It is also impossible to transmit the Date in its original form to your endpoints within JSON. Therefore, there is confusion with original method z.date() that should not be used within IO schemas of your API.

In order to solve this problem, the library provides two custom methods for dealing with dates: z.dateIn() and z.dateOut() for using within input and output schemas accordingly.

z.dateIn() is a transforming schema that accepts an ISO string representation of a Date, validates it, and provides your endpoint handler or middleware with a Date. It supports the following formats:

2021-12-31T23:59:59.000Z
2021-12-31T23:59:59Z
2021-12-31T23:59:59
2021-12-31

z.dateOut(), on the contrary, accepts a Date and provides ResultHanlder with a string representation in ISO format for the response transmission. Consider the following simplified example for better understanding:

import { z, defaultEndpointsFactory } from "express-zod-api";

const updateUserEndpoint = defaultEndpointsFactory.build({
  method: "post",
  input: z.object({
    userId: z.string(),
    birthday: z.dateIn(), // string -> Date
  }),
  output: z.object({
    createdAt: z.dateOut(), // Date -> string
  }),
  handler: async ({ input }) => {
    // input.birthday is Date
    return {
      // transmitted as "2022-01-22T00:00:00.000Z"
      createdAt: new Date("2022-01-22"),
    };
  },
});

Route path params

You can describe the route of the endpoint using parameters:

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

const routing: Routing = {
  v1: {
    user: {
      // route path /v1/user/:id, where :id is the path param
      ":id": getUserEndpoint,
    },
  },
};

You then need to specify these parameters in the endpoint input schema in the usual way:

const getUserEndpoint = endpointsFactory.build({
  method: "get",
  input: z.object({
    // id is the route path param, always string
    id: z.string().transform((value) => parseInt(value, 10)),
    // other inputs (in query):
    withExtendedInformation: z.boolean().optional(),
  }),
  output: z.object({
    /* ... */
  }),
  handler: async ({ input: { id } }) => {
    // id is the route path param, number
  },
});

Response customization

ResultHandler is responsible for transmitting consistent responses containing the endpoint output or an error. The defaultResultHandler sets the HTTP status code and ensures the following type of the response:

type DefaultResponse<OUT> =
  | {
      // Positive response
      status: "success";
      data: OUT;
    }
  | {
      // or Negative response
      status: "error";
      error: {
        message: string;
      };
    };

You can create your own result handler by using this example as a template:

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

export const yourResultHandler = createResultHandler({
  getPositiveResponse: <OUT extends IOSchema>(output: OUT) =>
    createApiResponse(
      z.object({
        data: markOutput(output),
      }),
      "application/json" // optional, or array of mime types
    ),
  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(yourResultHandler);

Please note: ResultHandler must handle any errors and not throw its own. Otherwise, the case will be passed to the LastResortHandler, which will set the status code to 500 and send the error message as plain text.

Non-object response

Thus, you can configure non-object responses too, for example, to send an image file.

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 I made more specific 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");
      }
    },
  })
);

Using native express middlewares

You can connect any native express middleware that can be supplied to express method app.use(). For this purpose the EndpointsFactory provides method addExpressMiddleware() and its alias use(). There are also two optional features available: a provider of options and an error transformer for ResultHandler. In case the error in middleware is not a HttpError, the ResultHandler will send the status 500.

import { defaultEndpointsFactory, createHttpError } from "express-zod-api";
import cors from "cors";
import { auth } from "express-oauth2-jwt-bearer";

const simpleUsage = defaultEndpointsFactory.addExpressMiddleware(
  cors({ credentials: true })
);

const advancedUsage = defaultEndpointsFactory.use(auth(), {
  provider: (req) => ({ auth: req.auth }), // optional, can be async
  transformer: (err) => createHttpError(401, err.message), // optional
});

File uploads

You can switch the Endpoint to handle requests with the multipart/form-data content type instead of JSON by using z.upload() schema. Together with a corresponding configuration option, this makes it possible to handle file uploads. Here is a simplified example:

import { createConfig, z, defaultEndpointsFactory } from "express-zod-api";

const config = createConfig({
  server: {
    upload: true, // <- required
    // ...,
  },
});

const fileUploadEndpoint = defaultEndpointsFactory.build({
  method: "post",
  input: z.object({
    avatar: z.upload(), // <--
  }),
  output: z.object({
    /* ... */
  }),
  handler: async ({ input: { avatar } }) => {
    // avatar: {name, mv(), mimetype, data, size, etc}
    // avatar.truncated is true on failure
  },
});

You can still send other data and specify additional input parameters, including arrays and objects.

Customizing logger

You can specify your custom Winston logger in config:

import winston from "winston";
import { createConfig } from "express-zod-api";

const logger = winston.createLogger({
  /* ... */
});
const config = createConfig({ logger /* ..., */ });

Connect to your own express app

If you already have your own configured express application, or you find the library settings not enough, you can connect your routing to the app instead of using createServer().

import express from "express";
import { createConfig, attachRouting } from "express-zod-api";

const app = express();
const config = createConfig({ app /* ..., */ });
const routing = {
  /* ... */
};

const { notFoundHandler, logger } = attachRouting(config, routing);

app.use(notFoundHandler); // optional
app.listen();

logger.info("Glory to science!");

Please note that in this case you probably need to parse request.body, call app.listen() and handle 404 errors yourself. In this regard attachRouting() provides you with notFoundHandler which you can optionally connect to your custom express app.

Multiple schemas for one 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: yourEndpointA,
      delete: yourEndpointA,
      post: yourEndpointB,
      patch: yourEndpointB,
    }),
  },
};

Serving static files

In case you want your server to serve static files, you can use new ServeStatic() in Routing using the arguments similar to express.static(). The documentation on these arguments you may find here.

import { Routing, ServeStatic } from "express-zod-api";
import path from "path";

const routing: Routing = {
  // path /public serves static files from ./assets
  public: new ServeStatic(path.join(__dirname, "assets"), {
    dotfiles: "deny",
    index: false,
    redirect: false,
  }),
};

Customizing input sources

You can customize the list of request properties that are combined into input that is being validated and available to your endpoints and middlewares.

import { createConfig } from "express-zod-api";

createConfig({
  // ...,
  inputSources: {
    // the default value is:
    get: ["query"],
    post: ["body", "files"],
    put: ["body"],
    patch: ["body"],
    delete: ["query", "body"],
  },
});

Enabling compression

According to Express JS best practices guide it might be a good idea to enable GZIP compression of your API responses. You can achieve and customize it by using the corresponding configuration option when using the createServer() method.

In order to receive the compressed response the client should include the following header in the request: Accept-Encoding: gzip, deflate. Only responses with compressible content types are subject to compression. There is also a default threshold of 1KB that can be configured.

import { createConfig } from "express-zod-api";

const config = createConfig({
  server: {
    // compression: true, or:
    compression: {
      // @see https://www.npmjs.com/package/compression#options
      threshold: "100b",
    },
    // ... other options
  },
  // ... other options
});

Enabling HTTPS

The modern API standard often assumes the use of a secure data transfer protocol, confirmed by a TLS certificate, also often called an SSL certificate in habit. When using the createServer() method, you can additionally configure and run the HTTPS server.

import { createConfig, createServer } from "express-zod-api";

const config = createConfig({
  server: {
    listen: 80,
  },
  https: {
    options: {
      cert: fs.readFileSync("fullchain.pem", "utf-8"),
      key: fs.readFileSync("privkey.pem", "utf-8"),
    },
    listen: 443, // port or socket
  },
  // ... cors, logger, etc
});

const { app, httpServer, httpsServer, logger } = createServer(config, routing);

At least you need to specify the port or socket (usually it is 443), certificate and the key, issued by the certifying authority. For example, you can acquire a free TLS certificate for your API at Let's Encrypt.

Informing the frontend about the API

You can inform your frontend about the I/O types of your endpoints by exporting them to .d.ts files (they only contain types without any executable code). To achieve that you are going to need an additional tsconfig.dts.json file with the following content:

{
  "extends": "./tsconfig.json",
  "compilerOptions": {
    "outDir": "dts",
    "declaration": true,
    "emitDeclarationOnly": true
  }
}

Most likely you have a file with all the configured routing, in which you can do the following:

import { EndpointInput, EndpointResponse } from "express-zod-api";

export type YourEndpointInput = EndpointInput<typeof yourEndpoint>;
export type YourEndpointResponse = EndpointResponse<typeof yourEndpoint>;

By executing the following command you'll get the compiled /dts/routing.d.ts file.

yarn tsc -p tsconfig.dts.json

The command might become a part of your CI/CD. Then import the I/O type of your endpoint from the compiled file using import type syntax on the frontend.

import type {
  YourEndpointInput,
  YourEndpointResponse,
} from "../your_backend/dts/routing";

Creating a documentation

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

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

const yamlString = new OpenAPI({
  routing, // the same routing and config that you use to start the server
  config,
  version: "1.2.3",
  title: "Example API",
  serverUrl: "https://example.com",
}).getSpecAsYaml();

You can add descriptions and examples to any I/O schema or its properties, and they will be included into the generated documentation of your API. Consider the following example:

import { defaultEndpointsFactory, withMeta } from "express-zod-api";

const exampleEndpoint = defaultEndpointsFactory.build({
  input: withMeta(
    z.object({
      id: z.number().describe("the ID of the user"),
    })
  ).example({
    id: 123,
  }),
  // ..., // similarly for output and middlewares
});

See the example of the generated documentation here

Additional hints

How to test endpoints

The way to test endpoints is to mock the request, response, and logger objects, invoke the execute() method, and assert the expectations for calls of certain mocked methods. The library provides a special method that makes mocking easier, it requires jest (and optionally @types/jest) to be installed, so the test might look the following way:

import { testEndpoint } from "express-zod-api";

test("should respond successfully", async () => {
  const { responseMock, loggerMock } = await testEndpoint({
    endpoint: yourEndpoint,
    requestProps: {
      method: "POST", // default: GET
      body: { ... },
    },
    // responseProps, configProps, loggerProps
  });
  expect(loggerMock.error).toBeCalledTimes(0);
  expect(responseMock.status).toBeCalledWith(200);
  expect(responseMock.json).toBeCalledWith({
    status: "success",
    data: { ... },
  });
});

This method is optimized for the standard result handler. With the flexibility to customize, you can add additional properties as needed.

Excessive properties in endpoint output

The schema validator removes excessive properties by default. However, Typescript does not yet display errors in this case during development. You can achieve this verification by assigning the output schema to a constant and reusing it in forced type of the output:

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.