How do you test state logic?

Question

Accepted Answer

Testing state logic builds confidence that your state transitions and updates are correct. The strategy depends on *how* the logic is structured — pure functions (reducers) are easiest to test, so well-architected state is also well-testable state.

## Test pure reducers directly (the easiest, highest-value case)

```js
// reducers are pure: (state, action) => newState → trivial to test, no mocks
describe("todosReducer", () => {
  it("adds a todo", () => {
    const initial = [];
    const next = todosReducer(initial, { type: "todos/added", payload: { id: 1, text: "A" } });
    expect(next).toEqual([{ id: 1, text: "A" }]);
    expect(initial).toEqual([]); // ✅ verify the original wasn't mutated (immutability)
  });
});
```

Because reducers are pure functions, you just call them with an input and assert the output — no rendering, no mocking, fast and deterministic. This is *why* the pure-reducer pattern is so valued.

## Test selectors

```js
it("selects the cart total", () => {
  const state = { cart: { items: [{ price: 10 }, { price: 5 }] } };
  expect(selectCartTotal(state)).toBe(15);
});
```

## Test custom hooks / store logic

```jsx
import { renderHook, act } from "@testing-library/react";

it("increments", () => {
  const { result } = renderHook(() => useCounter());
  act(() => result.current.increment()); // act wraps state updates
  expect(result.current.count).toBe(1);
});
```

`renderHook` + `act` let you test hook/store state logic in isolation.

## Test components by behavior, not internals

```jsx
import { render, screen, fireEvent } from "@testing-library/react";

it("shows the count after clicking", () => {
  render(<Counter />);
  fireEvent.click(screen.getByRole("button"));
  expect(screen.getByText("1")).toBeInTheDocument(); // assert the rendered RESULT
});
```

Test what the user sees/does (behavior), not implementation details — so refactors don't break tests.

## Test async state (mock the network)

```js
// mock fetch/API (e.g. MSW), then assert loading → success/error transitions
it("handles fetch error → error state", async () => { ... });
```

Use MSW or mocks to test loading/success/error transitions deterministically.

## What to prioritize

```text
Reducers/selectors/pure logic → unit test thoroughly (cheap, high value)
Component behavior           → integration test key user flows
Async flows                  → test transitions with mocked APIs
Don't test framework internals or trivial setters
```

## Why it matters

State bugs are among the most impactful, so testing state logic is high-value.

Crucially, **well-structured state is inherently testable**: pure reducers and selectors are trivial to unit test (no mocks, deterministic), which is a strong argument for that architecture.

Knowing to test reducers/selectors directly, hooks with `renderHook`, components by user-visible behavior, and async flows with mocked networks — while avoiding brittle implementation-detail tests — gives reliable coverage of the logic most likely to cause subtle bugs.