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3.6.0-alpha.1 • Public • Published

🍻 oazapfts!

Generate TypeScript clients to tap into OpenAPI servers.


  • AST-based: Unlike other code generators oazapfts does not use templates to generate code but uses TypeScript's built-in API to generate and pretty-print an abstract syntax tree.
  • Fast: The cli does not use any of the common Java-based tooling, so the code generation is super fast.
  • Tree-shakeable: Individually exported functions allow you to bundle only the ones you actually use.
  • Human friendly signatures: The generated api methods don't leak an HTTP-specific implementation details. For example, all optional parameters are grouped together in one object, no matter whether they end up in the headers, path or query-string.


npm install oazapfts

NOTE: With version 3.0.0 oazapfts has become a runtime dependency and the generated code does no longer include all the fetch logic.


oazapfts <spec> [filename]

--exclude, -e tag to exclude
--include, -i tag to include

Where <spec> is the URL or local path of an OpenAPI or Swagger spec (in either json or yml) and <filename> is the location of the .ts file to be generated. If the filename is omitted, the code is written to stdout.

Overriding the defaults

The generated file exports a defaults constant that can be used to override the basePath, provide a custom fetch implementation or to send additional headers with each request:

import * as api from "./api.ts";
import nodeFetch from "node-fetch";

api.default.basePath = "https://example.com/api";

api.defaults.headers = {
  access_token: "secret",

api.defaults.fetch = nodeFetch;

Consuming the generated API

For each operation defined in the spec the generated API will export a function with a name matching the operationId. If no id is specified, a reasonable name is generated from the HTTP verb and the path.

The last argument of each function is an optional RequestOpts object that can be used to pass options to the fetch call, for example to pass additional headers or an AbortSignal to cancel the request later on.

Each function returns a Promise for an ApiResponse which is an object with a status and a data property, holding the HTTP status code and the properly typed data from the response body. Since an operation can return different types depending on the status code, the actual return type is a union of all possible responses, discriminated by their status.

Consider the following code generated from the petstore.json example:

export function getPetById(petId: number, opts?: RequestOpts) {
  return fetchJson<
    | {
        status: 200;
        data: Pet;
    | {
        status: 400;
        data: string;
    | {
        status: 404;
        data: string;
  >(`/pet/${petId}`, {

In this case the data property is typed as Pet|string. We can use a type guard to narrow down the type to Pet:

const res = await api.getPetById(1);
if (res.status === 200) {
  const pet = res.data;
  // pet is properly typed as Pet
if (res.status === 404) {
  const message = res.data;
  // message is a string
} else {
  // handle the error

The above code can be simplified by using the handle helper:

import { handle } from "oazapfts";

await handle(api.getPetById(1), {
  200(pet) {
    // pet is properly typed as Pet
  404(message) {
    // message is as string

The helper will throw an HttpError error for any unhanled status code unless you add a default handler:

await handle(api.getPetById(1), {
  200(pet) {
    // ...
  default(status, data) {
    // handle error

Optimistic APIs

Instead of handling errors right in place we can also use the ok helper:

import { ok } from "oazapfts";

const pet = await ok(api.getPetById(1));

With this pattern pet will be typed as Pet and a HttpError will be thrown in case of an error.

You can even turn your whole API into an optimistic one:

import { optimistic } from "oazapfts";
import * as rawApi from "./api.ts";

const api = optimistic(rawApi);
const pet = await api.getPetById(1);


Since version 3.1.0 you can also use the --optimistic flag on the command line to generate an optimistic API by default.

About the name

The name comes from a combination of syllables oa (OpenAPI) and ts (TypeScript) and is pronounced 🗣 like the Bavarian O'zapt'is! (it's tapped), the famous words that mark the beginning of the Oktoberfest.





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  • alerizzo
  • codacy-ci