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Mocking network requests can be useful for a variety of use-cases, such as testing your frontend without running a backend, overwriting feature flag configurations, or testing error states.

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Mocking a route

In this example, we’ll capture requests to our /todos endpoint, and return fake data instead of hitting the production API server. First, we’ll create a new mock route step, and set the URL matcher to a regex that will capture requests to GET /todos.
Then, we define a response generator to create fake data. 
const url = new URL(mock.request.url)

const fakeTodo = {
    id: url.searchParams.get("id"),
    name: "My todo item",
    done: false,
    description: "An item that I need to do",
}

return new Response(
    JSON.stringify([fakeTodo]),
    {
        status: 200,
        headers: {
            "content-type": "application/json"
        }
    }
)

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Modifying the original response from the server

Sometimes, it’s easier to simply modify the original response from the server, for example, in order to set a specific feature flag. In order complete the initial request and pass the response to the mock generator, you must first enable the “fetch real response” option. This will allow us to access the response at mock.response inside our JavaScript code.
Now we can configure our mock to intercept feature flag requests.
And define a response to overwrite one of the values. 
const originalJson = await mock.response.json()

return new Response(
  JSON.stringify({
    ...originalJson,
    flags: {
        ...originalJson.flags,
        momentic_test: true,
    }
  }),
  {
    status: mock.response.status,
    headers: mock.response.headers
  }
)

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Removing a mock

There are two ways to reference a mock in later steps:
  1. By configuring a key in the mock route step.
  2. If no key is provided, a random one will be generated. You can still reference it later by saving the output of the step to an environment variable. The output of the mock route step will have a key field containing the random string.
You can use a remove route mock step to remove a mock by key, or remove all mocks if you omit the key.

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Updating a mock

You can update a mock by registering a different handler with the same key using another mock route step. Registering a mock with the same URL matcher but a different key is not sufficient to replace the existing mock.

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Debugging mocked routes

Because network requests are handled asynchronously in a separate process, it can be hard to investigate when something goes wrong. If a response generator throws an error while handling a request, Momentic will still respond to the request with a 500 server error status code, and the error message in the body of the response. This makes it easy to figure out what’s going on either in your app’s UI, or using the network logs.