Testing Isolation

Isolation is extremely important for making both deterministic and valid tests. Due to the approach to architecture where the client is a global module, we need to bring some of our attention to it.

What you'll learn

1. How to use client.clear() to reset the client's state between tests.
2. Why you need to clear the module cache for full isolation.
3. How to set up test isolation in a Jest environment.

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Why Isolation is Important

The Client in Hyper Fetch is often used as a global singleton. While this is great for application development, it can cause problems during testing, especially when running tests in parallel. Without proper isolation, state from one test (like cached data or queued requests) can leak into another, leading to unpredictable and flaky tests.

To prevent this, we need to ensure that each test runs in a clean environment. This involves two steps: clearing the client's internal state and resetting the module cache.

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Clearing Client State

The client.clear() method is a built-in utility that resets all of the client's subsystems to their initial state. This includes:

• Cache
• Queue
• Storage
• Dispatcher

You should call client.clear() before each test to ensure that no data from previous tests interferes with the current one.

import { client } from "api/client";

beforeEach(() => {
  // Clean the client environment to make sure it's isolated for each test
  client.clear();
});

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Clearing Module Cache

While client.clear() resets the client's state, the client instance itself is still a singleton that is shared across test files. If you run tests in parallel (which most test runners do), you might still face issues with the global module.

To achieve full isolation, you also need to clear the Node.js require.cache or your test runner's module cache. This ensures that you get a fresh client instance for each test file.

Jest

If you are using Jest, you can use jest.resetModules() to clear the module cache before each test.

import { client } from "api/client";

beforeEach(() => {
  // We need to reset modules cache as client is a globally exported module.
  // This way it will not get mixed-up with other test cases running in parallel.
  jest.resetModules();

  // Then, we clean the client environment to make sure it's isolated.
  client.clear();
});

Other Environments

For other environments like Vitest or if you're not using a test runner with a built-in module cache reset, you might need to use a library like decache to clear the module cache manually.

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Congratulations!

You've learned how to properly isolate your tests in Hyper Fetch!

• You can use client.clear() to reset the client state.
• You understand the importance of clearing the module cache for full isolation.
• You can set up a fully isolated testing environment in Jest.