openapi-fetch

What is openapi-fetch?

The openapi-fetch npm package is a tool designed to simplify the process of making HTTP requests to APIs that are defined using the OpenAPI specification. It provides a type-safe way to interact with APIs, ensuring that the requests and responses conform to the defined API schema.

What are openapi-fetch's main functionalities?

Type-safe API requests

This feature allows you to make type-safe API requests based on the OpenAPI definition. The `createClient` function initializes the client with the API definition, and you can then use methods like `get` to make requests. The types are inferred from the OpenAPI schema, ensuring that your requests and responses are type-checked.

const { createClient } = require('openapi-fetch');
const api = createClient({ definition: 'https://api.example.com/openapi.json' });

async function fetchUser() {
  const response = await api.get('/users/{id}', { params: { id: 1 } });
  console.log(response.data);
}

fetchUser();

Automatic request validation

This feature ensures that the requests you make are validated against the OpenAPI schema. If the request does not conform to the schema, an error will be thrown. This helps in catching errors early in the development process.

const { createClient } = require('openapi-fetch');
const api = createClient({ definition: 'https://api.example.com/openapi.json' });

async function createUser() {
  const response = await api.post('/users', { body: { name: 'John Doe', email: '[e-mail]' } });
  console.log(response.data);
}

createUser();

Response validation

This feature validates the responses from the API against the OpenAPI schema. It ensures that the data you receive is in the expected format, reducing the chances of runtime errors due to unexpected data structures.

const { createClient } = require('openapi-fetch');
const api = createClient({ definition: 'https://api.example.com/openapi.json' });

async function fetchUser() {
  const response = await api.get('/users/{id}', { params: { id: 1 } });
  if (response.status === 200) {
    console.log('User data:', response.data);
  } else {
    console.error('Failed to fetch user:', response.error);
  }
}

fetchUser();

Other packages similar to openapi-fetch

axios

Axios is a popular HTTP client for making requests to APIs. While it does not provide built-in support for OpenAPI specifications, it is highly configurable and can be used with additional libraries to achieve similar functionality. Compared to openapi-fetch, axios requires more manual setup for type safety and validation.

swagger-client

Swagger Client is a JavaScript client for connecting to APIs that use the OpenAPI (Swagger) specification. It provides similar functionality to openapi-fetch, including type-safe requests and response validation. However, Swagger Client is more tightly integrated with the Swagger ecosystem and may offer more features for users already using Swagger tools.

openapi-client-axios

openapi-client-axios is a library that combines the features of OpenAPI and axios. It generates an axios client based on an OpenAPI definition, providing type-safe requests and response validation. It is similar to openapi-fetch in terms of functionality but leverages axios for the underlying HTTP requests.

README

🎾 openapi-fetch

Ultra-fast fetching for TypeScript generated automatically from your OpenAPI schema. Weighs in at 1 kb and has virtually zero runtime. Works with React, Vue, Svelte, or vanilla JS.

Library	Size (min)
openapi-fetch	1 kB
openapi-typescript-fetch	4 kB
openapi-typescript-codegen	345 kB*

* Note: the larger your schema, the larger your codegen size. This is the actual weight of GitHub’s REST API.

The syntax is inspired by popular libraries like react-query or Apollo client, but without all the bells and whistles and in a 1 kb package.

import createClient from 'openapi-fetch';
import { paths } from './v1'; // (generated from openapi-typescript)

const { get, post } = createClient<paths>({ baseUrl: 'https://myapi.dev/v1/' });

// Type-checked request
await post('/create-post', {
  body: {
    title: 'My New Post',
    // ❌ Property 'publish_date' is missing in type …
  },
});

// Type-checked response
const { data, error } = await get('/post/my-blog-post');

console.log(data.title); // ❌ 'data' is possibly 'undefined'
console.log(error.message); // ❌ 'error' is possibly 'undefined'
console.log(data?.foo); // ❌ Property 'foo' does not exist on type …

Notice there are no generics, and no manual typing. Your endpoint’s exact request & response was inferred automatically off the URL. This makes a big difference in the type safety of your endpoints! This eliminates all of the following:

• ✅ No typos in URLs or params
• ✅ All parameters, request bodies, and responses are type-checked and 100% match your schema
• ✅ No manual typing of your API
• ✅ Eliminates any types that hide bugs
• ✅ Also eliminates as type overrides that can also hide bugs
• ✅ All of this in a 1 kB client package 🎉

🔧 Setup

First, install this package and openapi-typescript from npm:

npm i openapi-fetch
npm i -D openapi-typescript

Next, generate TypeScript types from your OpenAPI schema using openapi-typescript:

npx openapi-typescript ./path/to/api/v1.yaml -o ./src/lib/api/v1.d.ts

Note: be sure to validate your schema first! openapi-typescript will err on invalid schemas.

Lastly, create the client while configuring default options:

import createClient from 'openapi-fetch';
import { paths } from './v1'; // (generated from openapi-typescript)

const { get, post, put, patch, del } = createClient<paths>({
  baseUrl: 'https://myserver.com/api/v1/',
  headers: {
    Authorization: `Bearer ${myAuthToken}`,
  },
});

🏓 Usage

Using openapi-fetch is as easy as reading your schema! For example, given the following schema:

Here’s how you’d fetch both endpoints:

import createClient from 'openapi-fetch';
import { paths } from './v1';

const { get, post } = createClient<paths>({ baseUrl: 'https://myapi.dev/v1/' });

// GET /post/{post_id}
const { data, error } = await get('/post/{post_id}', {
  params: {
    path: { post_id: 'my-post' },
    query: { version: 2 },
  },
});

// POST /create-post
const { data, error } = await post('/create-post', {
  body: {
    title: 'New Post',
    body: '<p>New post body</p>',
    publish_date: new Date('2023-03-01T12:00:00Z').getTime(),
  },
});

• The URL must match the actual schema (/post/{post_id}). This library replaces all path params for you (so they can be typechecked).
• The params object will contain your path and query parameters, enforcing the correct types.
• The request body will only be required if the endpoint needs it.
• The endpoint will then respond with data, error, and response.
  • data will contain your typechecked successful response if the request succeeded (2xx); otherwise it will be undefined
  • error likewise contains your typechecked error response if the request failed (4xx / 5xx); otherwise it will be undefined
  • response has other information about the request such as status, headers, etc. It is not typechecked.

🔀 Parameter Serialization

In the spirit of being lightweight, this library only uses URLSearchParams to serialize parameters. So for complex query param types (e.g. arrays) you’ll need to provide your own querySerializer() method that transforms query params into a URL-safe string:

import createClient from 'openapi-fetch';
import { paths } from './v1';

const { get, post } = createClient<paths>({ baseUrl: 'https://myapi.dev/v1/' });

const { data, error } = await get('/post/{post_id}', {
  params: {
    path: { post_id: 'my-post' },
    query: { version: 2 },
  },
  querySerializer: (q) => `v=${q.version}`, // ✅ Still typechecked based on the URL!
});

🔒 Handling Auth

Authentication often requires some reactivity dependent on a token. Since this library is so low-level, there are myriad ways to handle it:

Nano Stores

Here’s how it can be handled using nanostores, a tiny (334 b), universal signals store:

// src/lib/api/index.ts
import { atom, computed } from 'nanostores';
import createClient from 'openapi-fetch';
import { paths } from './v1';

export const authToken = atom<string | undefined>();
someAuthMethod().then((newToken) => authToken.set(newToken));

export const client = computed(authToken, (currentToken) =>
  createClient<paths>({
    headers: currentToken ? { Authorization: `Bearer ${currentToken}` } : {},
    baseUrl: 'https://myapi.dev/v1/',
  })
);

// src/some-other-file.ts
import { client } from './lib/api';

const { get, post } = client.get();

get('/some-authenticated-url', {
  /* … */
});

Vanilla JS Proxies

You can also use proxies which are now supported in all modern browsers:

// src/lib/api/index.ts
import createClient from 'openapi-fetch';
import { paths } from './v1';

let authToken: string | undefined = undefined;
someAuthMethod().then((newToken) => (authToken = newToken));

const baseClient = createClient<paths>({ baseUrl: 'https://myapi.dev/v1/' });
export default new Proxy(baseClient, {
  get(_, key: keyof typeof baseClient) {
    const newClient = createClient<paths>({ headers: authToken ? { Authorization: `Bearer ${authToken}` } : {}, baseUrl: 'https://myapi.dev/v1/' });
    return newClient[key];
  },
});

// src/some-other-file.ts
import client from './lib/api';

client.get('/some-authenticated-url', {
  /* … */
});

🎛️ Config

createClient accepts the following options, which set the default settings for all subsequent fetch calls.

createClient<paths>(options);

Name	Type	Description
baseUrl	string	Prefix all fetch URLs with this option (e.g. "https://myapi.dev/v1/").
fetch	fetch	Fetch function used for requests (defaults to globalThis.fetch)
(Fetch options)		Any valid fetch option (headers, mode, cache, signal …) (docs)

🎯 Project Goals

1. Infer types automatically from OpenAPI schemas without generics (or, only the absolute minimum needed)
2. Respect the native fetch() API while reducing boilerplate (such as await res.json())
3. Be as small and light as possible

🧙‍♀️ Advanced

Caching

By default, this library does NO caching of any kind (it’s 1 kb, remember?). However, this library can be easily wrapped using any method of your choice, while still providing strong typechecking for endpoints.

Differences from openapi-typescript-fetch

This library is identical in purpose to openapi-typescript-fetch, but has the following differences:

• This library has a built-in error type for 3xx/4xx/5xx errors whereas openapi-typescript-fetch throws exceptions (requiring you to wrap things in try/catch)
• This library has a more terse syntax (get(…)) wheras openapi-typescript-fetch requires chaining (.path(…).method(…).create())
• openapi-typescript-fetch supports middleware whereas this library doesn’t