OpenAPI to TypeScript generator and mock server

This project is maintained by pmcelhaney

Coverage Status Mutation testing badge MIT License


Counterfact is two complimentary tools in one:

Hello World Pet Store ๐Ÿ‘‹

The easiest way to start is to copy and paste this command into your terminal.

npx counterfact@latest api

This will generate TypeScript code for the Swagger Pet Store and start the server. We're using the pet store example because it's well known and convenient. If you have your own OpenAPI document handy, you can point to that instead. You can also change api to wherever you'd like to output the code.

Here are the full details on CLI usage
Usage: counterfact [options] [openapi.yaml] [destination]

Counterfact is a tool for mocking REST APIs in development. See for more info.

  openapi.yaml          path or URL to OpenAPI document or "_" to run without OpenAPI (default: "_")
  destination           path to generated code (default: ".")

  --port <number>       server port number (default: 3100)
  --swagger             include swagger-ui
  -o, --open            open a browser
  -g, --generate        generate all code for both routes and types
  --generate-types      generate types
  --generate-routes     generate routes
  -w, --watch           generate + watch all code for changes
  --watch-types         generate + watch types for changes
  --watch-routes        generate + watch routes for changes
  -s, --serve           start the mock server
  -r, --repl            start the REPL
  --proxy-url <string>  proxy URL
  --prefix <string>     base path from which routes will be served (e.g. /api/v1)
  -h, --help            display help for command

Using with npm or yarn ๐Ÿงถ

If you prefer not to use npx against the @latest version, you can install Counterfact as a dependency with a specific version in npm or yarn. The following example adds a start script to your package.json file and adds Counterfact as a dev dependency.

"scripts": {
  "start": "npx counterfact api"
"devDependencies": {
  "counterfact": "^0.38.3",

This will let your team use the same version of Counterfact across all environments. You can also use npm run start or yarn start to start the server.

Generated Code ๐Ÿ–

After generating code you should have three directories:

When you launch Counterfact with no command line options, the code under components and path-types is regenerated every time you run Counterfact, so that the types can stay in sync with any OpenAPI changes. The code under paths is minimal boilerplate that you're meant to edit by hand. Counterfact will not overwrite your changes in the paths directory, but it will add new files when necessary. If you use any of the command line options then it will only regenerate the code when you tell it to via the --watch or --generate options.

See also Generated Code FAQ

[!TIP] You don't have to use the code generator. It wasn't even part of Counterfact originally. You can also create the files under paths by hand. The main benefit of generating code is all the type information that's managed for you and kept in sync with OpenAPI. See What if I don't have an OpenAPI document?

Routing is where it's at ๐Ÿ”€

In the paths directory, you should find TypeScript files with code like the following.

export const GET: HTTP_GET = ($) => {
  return $.response[200].random();

export const POST: HTTP_POST = ($) => {
  return $.response[200].random();

The TypeScript file's path corresponds to the endpoint's URL. Each of the exported functions implements an HTTP request method (GET, POST, PUT, etc.). Each of these functions takes one argument -- $ -- which is used to access request information, build a response, and interact with the server's state.

[!TIP] If you're familiar with Express, $ is sort of a combination of req and res with type safety and extra super powers.

The $.response object

The $.response object is used to build a valid response for the URL and request method. This object is designed to work with your IDE's autocomplete feature and help you build a valid response without consulting the docs. Try typing $.response. in your IDE. You should see a list of numbers corresponding to HTTP response codes (200, 404, etc). Select one and then type another .. At this point the IDE should present you with one or more of the following methods.

You can build a response by chaining one or more of these functions, e.g.

return $.response[200].header("x-coolness-level", 10).text("This is cool!")`.

[!TIP] Your IDE can help you build a valid response via autocomplete. It can also help ensure the response matches the requirements in the OpenAPI document. For example, if you leave out a required header, the function won't type check. (That's particularly useful when there are API changes. Update the OpenAPI document, regenerate the types, and let TypeScript tell you where to update the code.)

Request parameters: $.path, $.query, $.header, and $.body

Most of the time, the server's response depends on input from various parts of the request: the path, the query string, the headers, or the body. We can use them like so:

export const GET: HTTP_GET = ($) => {
    if ($.header['x-token'] !== 'super-secret') {
       return $.response[401].text('unauthorized');

    const content = `TODO: output the results for "${query.keyword}"`
      + `in ${$.path.groupName}`
      + `that have the following tags: ${$.body.tags.join(',')}.`.

    return $.response[200].text(content);

Note that each of these objects is typed so you can use autocomplete to identify the parameters. For example, if you type $.query. you'll be presented with a list of expected query string parameters.

The $.path parameters are identified by dynamic sections of the path, i.e. /groups/{groupName}/user/{userId}.ts.

Working with state: the $.context object and _.context.ts

There's one more parameter we need to explore, the $.context object. It stands in for microservices, databases, and other entities with which the real API interacts. It looks something like this:

// pet.ts
export const POST: HTTP_POST = ($) => {
  return $.response.json($.context.addPet($.body));
// pet/{id}.ts
 export const GET: HTTP_GET ($) => {
    const pet = $.context.getPetById(body);
    if (pet === undefined) return response[404].text(`Pet ${$} not found.`);
    return response.json(context.getPetById($.body));

The context object is defined in _.context.ts in the same directory as the file that uses it. It's up to you to define the API for a context object. For example, your _.context.ts file might look like this.

export class Context {
  pets: Pet[] = [];

  addPet(pet: Pet) {
    const id = this.pets.length;
    this.pets.push({, id });
    return this.getPetById(id);

  getPetById(id: number) {
    return this.pets[id];

By default, each _.context.ts delegates to its parent directory, so you can define one context object in the root _.context.ts and use it everywhere.

[!TIP] You can make the context objects do whatever you want, including things like writing to databases. But remember that Counterfact is meant for testing, so holding on to data between sessions is an anti-pattern. Keeping everything in memory also makes it fast.

Security: the $.auth object

If a username and password are sent via basic authentication, the username and password can be found via $.auth.username and $.auth.password respectively.

Support for other security schemes ("apiKey", "mutualTLS", "oauth2", "openIdConnect") are coming. You can speed things along by opening an issue.

x-scape Hatch

Counterfact does a good job translating an OpenAPI description into TypeScript types. But if your OpenAPI documentation is incorrect or incomplete, or you want to try something that's not documented yet, the type safety can get in your way.

To work around that problem, Counterfact provides a "loose" types mode in the form of the $.x object. The $.x object is an alias of $ in which all of the types are wider.

The best way to explain is with a couple of examples.

export function GET($): HTTP_GET {
  // There are no headers specified in OpenAPI
  $.headers["my-undocumented-header"]; // TypeScript error
  $.x.headers["my-undocumented-header"]; // ok

  // There is no 500 response type specified in OpenAPI
  return $.response[500].text("Error!"); // TypeScript error
  return $.x.response[500].text("Error!"); // ok

Reloading is So Hot Right Now ๐Ÿ”ฅ

Changes you make will be picked up by the running server immediately. There's no need to restart!

Hot reloading supports one of Counterfact's key design goals. While developing and testing, we want to explore counterfactuals, such as

In such cases, we want to be sure the front end code responds appropriately. Getting a real server to do what we need to test front end code is usually difficult if not impossible. Counterfact is optimized to make bending the server's behavior to suit a test case as painless as possible, in both manual and automated tests.

REPL without a Pause โฏ

Another way to explore counterfactuals in real time is to interact with the running server via the read-eval-print loop (REPL), in the same way that you interact with running UI code in your browser's developer tools console. If you look in the terminal after starting Counterfact you should see a prompt like this:

____ ____ _  _ _ _ ___ ____ ____ ____ ____ ____ ___
|___ [__] |__| |\|  |  |=== |--< |--- |--| |___  |
       High code, low effort mock REST APIs

| API Base URL  ==> http://localhost:3100
| Admin Console ==> http://localhost:3100/counterfact/
| Instructions  ==>

Starting REPL, type .help for more info


At the ๐Ÿค–> prompt, you can enter JavaScript code to interact with the live context object. For example, here's a quick way to add a pet to the store.

context.addPet({ name: "Fluffy", photoUrls: [] });

Or add 100 pets:

for (i = 0; i < 100; i++) context.addPet({ name: `Pet ${i}`, photoUrls: [] });

Or get a list of pets whose names start with "F"

context.pets.find((pet) =>"F"));

Using the REPL is a lot faster (and more fun) than wrangling config files and SQL and whatever else it takes to get a real back end into the states you need to test your UI flows.

Proxy Peek-a-boo ๐Ÿซฃ

At some point you're going to want to test your code against a real server. Or maybe you want to use Counterfact for newer endpoints that don't exist in the real server yet and a real server for everything else. Counterfact has a couple of facilities that allow you to proxy to the real server on a case-by-case basis.

To proxy an individual endpoint, you can use the $.proxy() function.

// pet/{id}.ts
 export const GET: HTTP_GET ($) => {
    return $.proxy("")

To set up a proxy for the entire API, add --proxy <url> in the CLI.

From there, you can switch back and forth between the proxy and mocks by typing .proxy [on|off] <path-prefix>. Type .proxy help for detailed information on using the .proxy command.

No Cap Recap ๐Ÿงข

Using convention over configuration and automatically generated types, Counterfact allows front-end developers to quickly build fake REST APIs for prototype and testing purposes.

We're Just Getting Started ๐Ÿฃ

More features are coming soon:

Please send feedback / questions to or create a new issue.

And yes, contributions are welcome!