const backdoor = require("backdoorthen"); // On form submit const formData = { firstName: 'Peter', lastName: 'Parker' }; // Send first & last name payload, backend creates user then responds with username const createUser = ({ firstName, lastName }) => Promise.resolve({ userName: `${firstName.charAt(0)}${lastName}`.toLowerCase() }); // Function you're actively working on relies on the backend's response const apiResponseHandler = apiResponse => console.log(apiResponse.userName); // ...and you don't want to actually create a real user everytime you run your code // You can use backdoor to bypass making an actual request to the backend, // and just return mocked data instead const mockedApiResponse = { userName: 'fakeuser' }; // ...all the while maintaining the ability to actually make a call to the backend when you want to const withBackdoor = thenable => backdoor({ actualThenable: thenable, // The value backdoor should assess to decide whether real or mocked data should be returned input: formData.firstName, resolvedValue: mockedApiResponse, config: { enabledInProd: true } // Because runKit sets NODE_ENV to "production" by default }); // Submit the form: // Same as running createUser(formData) directly, because the firstName input field isn't 'backdoor' withBackdoor(createUser)(formData) .then(apiResponseHandler); // pparker (no perceptible delay with this example) // Fill out the firstName input field with 'backdoor' and submit the form formData.firstName = 'backdoor'; withBackdoor(createUser)(formData) .then(apiResponseHandler); // fakeuser (5s delay perceptible)