Counterfact is a free and open source mock server designed to hit the sweet spot every front-end engineer craves: real enough to be useful but fake enough to be usable. It stands in for the back-end code that doesn't exist yet or is too complex / rigid to suit your front-end development and testing workflow.

Like your favorite pair of sweatpants, Counterfact is lightweight, flexible, and comfortable; it stretches and shrinks to fit your project's unique contours. Best of all, it makes your ass boss look good. Go ahead, try it on.

Why Use Counterfact?

• 🏝️ Effortless API Mocking: Say goodbye to back-end hassles. Counterfact allows you to build and test your front-end code independently of the rest of the stack.
• 👌 Flexibility at Your Fingertips: Effortlessly toggle between mock and real services, change behavior without losing state, simulate complex use cases and error conditions, etc.
• 🤩 Instant Gratification: If you have Node installed and an OpenAPI document handy, you're one command away from a dependency-free workflow.
• 🛠️ Dev Tools on the server: That's what it feels like. Change code in a running server and see the effect immediately. It even has a REPL, like the JS console in your browser.
• 🔄 High code, low effort: Wouldn't you love the simplicity of a "low code" / "no code" tool without giving up the flexibility and power you get from knowing how to write TypeScript? Inconceivable, you say? Don't knock it 'til you try it.
• 🏄 Fluid workflow: Optionally use existing OpenAPI/Swagger documentation to auto-generate TypeScript types. When your documentation changes, the types update automatically.
• 🛝 Plays well with others: Counterfact works with anything that depends on a REST API, including web apps, mobile apps, desktop apps, and microservices. It requires zero changes to your front-end framework or code.

Yeah but...

• 🎭 There are already a bazillion tools for mocking APIs!
  And they all fall short in one way or another. Counterfact is a novel approach designed to address their shortcomings. Sometimes a low-fidelity prototype that returns mock data is sufficient. Sometimes we wish the mocks were stateful, i.e. after POSTing an item in the shopping cart we can GET that same item back out. Sometimes we want to test against the real server, but override the responses on one or two endpoints. Counterfact makes all of these use cases as simple as possible, but no simpler.
• ⛮ I don't like code generators!
  Then you came to the right place! The code generator is optional. If you have an OpenAPI spec (which is highly recommended in any case), you can use it to automatically generate TypeScript types in a split second. As the spec changes, the types are automatically kept in sync. Most of the generated code is cordoned off in an area that you never need to change or even look at.
• 🥵 Maintaining both a mock server and a real server? That's just extra effort!
  People used to say the same thing about unit tests: it's twice as much code! Having spent nearly three decades writing front-end code, I've learned a lot development time is wasted messing with the back-end: getting the environment stood up, adding test data, logging in and out as different users, hitting the refresh button and waiting, etc. Counterfact eliminates that waste at a cost that is shockingly close to zero, and helps you maintain the flow state while developing.

10 Second Quick Start

To see Counterfact in action run the following command in your terminal:

npx counterfact@latest https://petstore3.swagger.io/api/v3/openapi.yaml api

This command installs Counterfact from npm, sets up a mock server implementing the Swagger Petstore, and opens a dashboard in a web browser. As long as you have Node 16 or later installed, it should "just work".

Documentation

For more detailed information, such as how to go beyond simple mocks that return random values, visit our tutorial and usage guide.

Similar Tools and Alternatives

While Counterfact offers a unique approach to API mocking that we believe provides the best overall DX, we understand the importance of having the right tool for your specific needs. Here are some similar tools and alternatives you might find useful:

Mirage JS has more or less the same goals as Counterfact and very different approaches to achieving them. Some notable differences are that it runs in a browser instead of Node, does not integrate with OpenAPI, and does support GraphQL.

If your goal is to get a server up and running quickly and your API doesn't do much beyond storing and retrieving data, JSON Server may be a great fit for you.

If your mocking needs are relatively simple and you're shopping for someone who has no reason to have Node installed their computer, Beeceptor, Mockoon, and Mocky.io are worth checking out. Mocky is free; the others have free and paid tiers.

Feedback and Contributions

We value all of your feedback and contributions, including 💌 love letters , 💡 feature requests, 🐞 bug reports, and ✍️ grammatical nit-picks in the docs. Please create an issue, open a pull request, or reach out to pmcelhaney@gmail.com.

Welcome to the bottom of the README club! Since you've come this far, go ahead and smash that like and subsc… er, uh, give this project a ⭐️ on GitHub! 🙏🏼