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    Express Zod API


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    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. Cross-Origin Resource Sharing (CORS)
      2. Middlewares
      3. Options
      4. Refinements
      5. Transformations
      6. Dealing with dates
      7. Route path params
      8. Response customization
      9. Non-object response including file downloads
      10. Using native express middlewares
      11. File uploads
      12. Customizing logger
      13. Connect to your own express app
      14. Multiple schemas for one route
      15. Serving static files
      16. Customizing input sources
      17. Enabling compression
      18. Enabling HTTPS
      19. Generating a Frontend Client
      20. 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 and migration guides in Changelog.

    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



    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


    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 ={
      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

    Cross-Origin Resource Sharing

    You can enable your API for other domains using the corresponding configuration option cors. It's not optional to draw your attention to making the appropriate decision, however, it's enabled in the Quick start example above, assuming that in most cases you will want to enable this feature. See MDN article for more information.

    In addition to being a boolean, cors can also be assigned a function that provides custom headers. That function has several parameters and can be asynchronous.

    import { createConfig } from "express-zod-api";
    const config = createConfig({
      // ... other options
      cors: ({ defaultHeaders, request, endpoint, logger }) => ({
        "Access-Control-Max-Age": "5000",

    Please note: If you only want to send specific headers on requests to a specific endpoint, consider the Middlewares or response customization approach.


    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
        // ...,
        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({
      security: {
        // this information is optional and used for the generated documentation (OpenAPI)
        and: [
          { type: "input", name: "key" },
          { type: "header", name: "token" },
      input: z.object({
        key: z.string().min(1),
      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.


    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"),


    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
            (nick) => !/^\d.*$/.test(nick),
            "Nickname cannot start with a digit"
      // ...,


    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 ={
      method: "get",
      input: z.object({
        id: z.string().transform((id) => parseInt(id, 10)),
        ids: z
          .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 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:


    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 ={
      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,
          // use the empty string to represent /v1/user if needed:
          // "": listAllUsersEndpoint,

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

    const getUserEndpoint ={
      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 {
    } from "express-zod-api";
    export const yourResultHandler = createResultHandler({
      getPositiveResponse: (output: IOSchema) =>
          z.object({ data: 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(
        getPositiveResponse: () => createApiResponse(z.file().binary(), "image/*"),
        getNegativeResponse: () => createApiResponse(z.string(), "text/plain"),
        handler: ({ response, error, output }) => {
          if (error) {
          if ("filename" in output) {
          } 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 ={
      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();"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";
      // ...,
      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
          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.

    Generating a Frontend Client

    There is a new way of informing the frontend about the I/O types of your endpoints starting the version 6.1.0. The new approach offers automatic generation of a client based on routing to a typescript file. The generated client is flexibly configurable on the frontend side using an implementation function that directly makes requests to an endpoint using the libraries and methods of your choice. The client asserts the type of request parameters and response. The feature requires Typescript version 4.1 or higher.

    // example client-generator.ts
    import fs from "fs";
    import { Client } from "express-zod-api";
    fs.writeFileSync("./frontend/client.ts", new Client(routing).print(), "utf-8");
    // example frontend, simple implementation based on fetch()
    import { ExpressZodAPIClient } from "./client.ts";
    const client = new ExpressZodAPIClient(async (method, path, params) => {
      const searchParams =
        method === "get" ? `?${new URLSearchParams(params)}` : "";
      const response = await fetch(`${path}${searchParams}`, {
        method: method.toUpperCase(),
          method === "get" ? undefined : { "Content-Type": "application/json" },
        body: method === "get" ? undefined : JSON.stringify(params),
      return response.json();
    client.provide("get", "/v1/user/retrieve", { id: "10" });
    client.provide("post", "/v1/user/:id", { id: "10" }); // it also substitues path params

    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
      version: "1.2.3",
      title: "Example API",
      serverUrl: "",

    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 ={
      input: withMeta(
          id: z.number().describe("the ID of the user"),
        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
        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(),
      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.


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