Can you use Mocha to test Zustand's computed values

Understanding Computed Values in Zustand

In Zustand, computed values are typically derived states calculated from the store's base state. These can be implemented either by:

- Using selectors in React components or hooks that compute derived data every time the state updates.
- Creating explicit computed state using middleware like the `zustand-computed` package, which wraps the store and adds computed fields to the state.

Selectors as computed values are efficient in React because they are recalculated on every render that accesses them. This contrasts with MobX's cached computed properties, which offer automatic memoization.

Example of computed state using `zustand-computed`:

js
import { createComputed } from "zustand-computed";

const computed = createComputed((state) => ({
  countSq: state.count  2,
}));

const useStore = create(computed((set, get) => ({
  count: 1,
  inc: () => set((state) => ({ count: state.count + 1 })),
  dec: () => set((state) => ({ count: state.count - 1 }))
})));

In this setup, `countSq` is a computed value derived from the base `count` state.

Setting Up Mocha to Test Zustand Stores

Mocha is a flexible JavaScript testing framework running in Node.js and browsers. It supports asynchronous testing and different styles like BDD and TDD. It does not directly support React but works well for testing plain JavaScript logic, including Zustand stores.

To test Zustand computed values with Mocha, the focus should be on testing:

- The base state and its mutations.
- The computed values and whether they update correctly in response to state changes.
- The behavior of any selectors or derived state logic.

Basic Mocha Test Structure for Zustand

A typical Mocha test suite for a Zustand store involves:

- Importing the store creation function.
- Using the store API (e.g., `getState()`, actions) to manipulate state.
- Using assertions to verify that computed values reflect expected results after state changes.

Example:

js
const assert = require("assert");
const { useStore } = require("./path/to/store");

describe("Zustand store computed values", function () {
  it("should start with initial computed value", function () {
    const state = useStore.getState();
    assert.strictEqual(state.count, 1);
    assert.strictEqual(state.countSq, 1);
  });

  it("should update computed value when base state changes", function () {
    const { inc } = useStore.getState();
    inc();
    const state = useStore.getState();
    assert.strictEqual(state.count, 2);
    assert.strictEqual(state.countSq, 4);
  });
});

This pattern is straightforward because Zustand stores expose synchronous APIs for getting and setting state, which can be directly tested without rendering components.

Testing Best Practices with Zustand and Mocha

1. Avoid Testing React Components for Computed Values Here: Zustand testing for computed values is best done at the store logic level. For React components, React Testing Library is recommended, but for pure state logic Mocha is suitable.

2. Reset Store Between Tests: Zustand stores are singleton-like unless recreated per test. To avoid state pollution between tests, reset the store state after each test. This can be done using store reset functions or by creating a fresh store instance in each test.

3. Use Selectors or Computed Middleware: Computed values should be clearly defined so that tests can assert expected outputs for given base states. Using `zustand-computed` middleware provides a clean separation of computed logic.

4. Test Both Base and Derived Values: Assertions should cover that computed values update correctly and remain consistent with base state updates.

5. Write Clear, Small Test Cases: Each test case should focus on a single mutation or computed value to isolate failures.

Resetting Zustand Store in Tests

Because Zustand's stores persist in memory, to ensure isolated tests you can reset the store state after each test. One approach is mocking Zustand or enhancing the store with a reset method:

js
afterEach(() => {
  useStore.setState(useStore.getState().initialState, true);
});

Alternatively, creating a factory function producing a new store instance per test is ideal to avoid shared state.

Handling Asynchronous State Updates

In advanced scenarios where state updates are asynchronous (e.g., fetching data), Mocha's asynchronous test support can be used with `done` callbacks or returning promises.

js
it("should handle async state updates", async function () {
  const { fetchData } = useStore.getState();
  await fetchData();
  const state = useStore.getState();
  assert.strictEqual(state.dataLoaded, true);
});

Using Assertion Libraries

Mocha itself does not include assertions but works with libraries like `assert` (Node built-in), Chai, or should.js. Chai provides expressive BDD style assertions making tests readable.

Example with Chai:

js
const chai = require("chai");
const expect = chai.expect;

expect(state.count).to.equal(1);
expect(state.countSq).to.equal(1);

Summary of Testing Workflow

- Create and export Zustand store with computed values (selectors or middleware).
- In test files, import the store instance.
- Use Mocha's `describe` and `it` blocks to organize tests by feature.
- Use synchronous calls like `getState` to get current state.
- Trigger updates via action calls.
- Assert computed values reflect expected transformations.
- Reset store state after each test to isolate.

Additional Considerations

- For large stores, modularize tests by store slice or computed logic.
- Consider memoization for computed values if costly calculations exist, though Zustand selectors recalculate by default.
- Utilize TypeScript typings to ensure compute functions have correct inputs/outputs.
- Combine Mocha with other tools like Sinon for spies/mocks if needed to observe internal store behavior.
- Use dynamic test generation in Mocha to run multiple test scenarios for computed values with different base states.

Example Complete Code Snippet Testing Zustand Computed Values with Mocha

js
const assert = require("assert");
const { createComputed } = require("zustand-computed");
const create = require("zustand").default;

// Create store with computed values
const computed = createComputed((state) => ({
  countSq: state.count  2,
}));

const useStore = create(
  computed((set) => ({
    count: 1,
    inc: () => set((state) => ({ count: state.count + 1 })),
    dec: () => set((state) => ({ count: state.count - 1 })),
  }))
);

describe("Zustand computed value tests", function () {
  afterEach(() => {
    useStore.setState({ count: 1 }, true);
  });

  it("initial count and computed squared value", function () {
    const state = useStore.getState();
    assert.strictEqual(state.count, 1);
    assert.strictEqual(state.countSq, 1);
  });

  it("increment updates count and computed square", function () {
    const { inc } = useStore.getState();
    inc();
    const state = useStore.getState();
    assert.strictEqual(state.count, 2);
    assert.strictEqual(state.countSq, 4);
  });

  it("decrement updates count and computed square", function () {
    const { dec } = useStore.getState();
    dec();
    const state = useStore.getState();
    assert.strictEqual(state.count, 0);
    assert.strictEqual(state.countSq, 0);
  });
});

This approach demonstrates a minimal, clear way to test Zustand computed values using Mocha as the test runner and `assert` for assertions. It shows how computed values created by `zustand-computed` middleware correctly reflect the underlying state changes and can be tested deterministically.

***

In conclusion, Mocha is well suited for testing Zustand's computed values, especially the pure state logic including selectors and computed middleware. By structuring tests to manipulate and inspect store state synchronously, resetting state between tests, and leveraging assertion libraries, developers can reliably verify the correctness of Zustand computed values in any JavaScript or Node.js environment. This testing strategy helps ensure more robust state management logic in React and general JavaScript applications.