• Supports JavaScript and TypeScript
• Runs on Node, Deno, and Bun
• Uses Swagger / OpenAPI if you have it
• File system based routing
• Hot reload server-side code
• Manipulate test data with a REPL
• Toggle between the mock and real backend at runtime
• Works with any “front end” that calls RESTful services (a React app, a native iOS app, a web service, etc.)
• Emulates the real back end, but 100X faster

• Rapidly iterate on the front end without waiting for back end features or changes
• Reproduce bugs when the conditions on the server side are hard to replicate
• Test against a private, local stack that you fully control
• Write contract tests for APIs

Unlike similar tools, Counterfact is not limited to canned or randomly generated responses. It can mimic as much or as little of the business logic in your n-tier / cloud-native back end as you want. Test end-to-end scenarios in your front end code as if a lighting-fast implementation of the real backend is running on your machine. Because it is.

As a front end dev, when you have complete and granular control over the back end, and you can modify anything on a whim, it enhances your developer experience in ways that are hard to describe.

// ./paths/hello/world.js
export const GET = () => "Hello World!";

// ./paths/pet/{petId}.js
export const GET = ({ context, response, path }) => {
  const pet = context.getPetById(path.petId);

  if (!pet) {
    return response[404].text(`Pet with ID ${path.petID} not found.`);
  }

  return response[200].json(pet);
};

// ./paths/pets.js
export const POST = ({ context, response, body }) => {
  const pet = context.addPet(body);

  return response[200].json(pet);
};

// ./paths/$.context.ts
class PetStore () {
    pets = {};

    getPetById(petId) {
        return pets[id];
    }

    addPet(pet) {
        this.pets[pet.id] = pet;
    }
}

export default new PetStore();

For example, run the following command to generate code for the Swagger Petstore.

npx counterfact@latest https://petstore3.swagger.io/api/v3/openapi.json api --open

That command generates and starts a TypeScript implementation of the Swagger Petstore which returns random, valid responses. Not too shabby for a few seconds of work!

Edit the code under ./api/paths to implement real behavior, as we did in the JavaScript examples above. Store and retrieve data, perform calculations, filter / sort / paginate, whatever it takes the make the API real enough to support the development and testing of your front end.

• Use autocomplete as documentation. For example, when you're working on an operation that takes query parameters, type "$.query." to list all of the available parameters.
• Stay in sync with changes in your OpenAPI / Swagger document. Counterfact will keep the type definitions up to date without overwriting your work.
• Get immediate feedback. Modules are hot reloaded so changes apply immediately without restarting the server or resetting its state.
• Interact with the running server using a REPL. For example, type context.addPet({id: 2, name: "Fido"}) to add a pet to the store. Then view the pet you just added at http://localhost:3100/pet/2.
• Much more!

See the Usage Guide for details.

Counterfact is brand new as of November 30, 2022. Please send feedback / questions to [email protected] or create a new issue. If you like what you see, please give this project a star!