How to write tests for XState – CodeWithSwiz 12

Once you know how to refactor a useReducer to XState, you gotta prove you did it right.

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useAuth had existing tests for its core reducer and that makes refactoring easier.

In theory tests are great for refactoring. You can change the implementation, leave tests alone, get them passing. Verify you refactored without destroying the external API.

My tests were not that good.

// src/__tests__/authReducer.spec.ts
describe("login", () => {
    const currentTime = new Date().getTime();
    const state: AuthState = { // ... };
    const action: AuthAction = { // ... };

    it("adds user to local storage", () => {
        localStorage.removeItem("useAuth:expires_at");
        localStorage.removeItem("useAuth:user");

        authReducer(state, action);

        expect(
            JSON.parse(localStorage.getItem("useAuth:expires_at")!)
        ).toBeGreaterThanOrEqual(currentTime + 500 * 1000);

        expect(localStorage.getItem("useAuth:user")).toEqual(
            JSON.stringify(action.user)
        );
    });

    it("sets user", () => {
        expect(authReducer(state, action).user).toEqual(action.user);
    });
    it("sets expiresAt", () => {
        expect(authReducer(state, action).expiresAt).toBeGreaterThanOrEqual(
            currentTime + 500 * 1000
        );
    });
    // ...
});

Beautiful reducer tests. Take reducer as a function, pass-in a fake current state and an action, check for expected changes.

Every part of the reducer's "login" action gets tested.

// src/authReducer.ts
switch (action.type) {
    case "login":
        const { authResult, user } = action;
        const expiresAt =
            authResult.expiresIn! * 1000 + new Date().getTime();

        if (typeof localStorage !== "undefined") {
            localStorage.setItem(
                "useAuth:expires_at",
                JSON.stringify(expiresAt)
            );
            localStorage.setItem("useAuth:user", JSON.stringify(user));
        }

        return {
            ...state,
            user,
            expiresAt,
            authResult
        };

Can you spot the problem?

Testing too close to the implementation

My tests are looking at implementation details. They know how the reducer works and test that way.

That won't work for XState.

XState doesn't let you fake anything. You can't force it into a particular state, you can't trick it to do something it's not supposed to.

That's the whole point. "Make impossible states impossible" as the tagline goes.

How to test XState

As David says in his tweet, the trick is to go one level higher and write better tests. At the integration and API level.

Start by initializing your state machine.

// src/__tests__/authReducer.spec.ts

function initStateMachine() {
  const state = interpret(authMachine.withContext(initialContext)).start()

  return state
}

You'll use this in multiple tests so having a function helps. It takes your state machine – authMachine – seeds it with initial context and starts an interpreter.

Jest runs tests within a describe block in sequence[1]. That means you can share setup between tests.

// init with values to easily tell TypeScript the type
let authState = initStateMachine(),
  savedContext = initialContext

beforeEach(() => {
  authState = initStateMachine()

  authState.subscribe((state) => {
    savedContext = state.context
  })
})

Create a shared authState and savedContext, giving them values makes TypeScript's type inference kick in. Means you don't have to specify your own types.

In a beforeEach block you then instantiate a fresh state machine for each test and subscribe to changes. This lets you keep track of the latest application state.

Each test then triggers the right sequence of actions and checks the result.

// src/__tests__/authReducer.spec.ts
describe("LOGIN", // ...
	it("sets user", () => {
	    authState.send("LOGIN");
	    authState.send("AUTHENTICATED", loginPayload);

	    expect(savedContext.user).toEqual(loginPayload.user);
	});
	it("sets expiresAt", () => {
	    authState.send("LOGIN");
	    authState.send("AUTHENTICATED", loginPayload);

	    expect(savedContext.expiresAt).toBeGreaterThanOrEqual(
	        currentTime + 500 * 1000
	    );
	});
	it("sets authResult", () => {
	    authState.send("LOGIN");
	    authState.send("AUTHENTICATED", loginPayload);
	    expect(savedContext.authResult).toBe(loginPayload.authResult);
	});

We need 2 actions for each test:

The state subscription runs every time and updates savedContext. You can then verify the expected result.

You could move action firing into beforeEach to DRY this up further. I prefer to make it obvious what each test is testing.

Bonus: Clean side effects

Fun bonus from using XState 👉 clean, readable, predictable side-effects.

Look at this login action on the reducer:

case "login":
    const { authResult, user } = action;
    const expiresAt =
        authResult.expiresIn! * 1000 + new Date().getTime();

    if (typeof localStorage !== "undefined") {
        localStorage.setItem(
            "useAuth:expires_at",
            JSON.stringify(expiresAt)
        );
        localStorage.setItem("useAuth:user", JSON.stringify(user));
    }

    return {
        ...state,
        user,
        expiresAt,
        authResult
    };

Bleh. Deals with local storage, handles time, hard to read. You can't look at this and immediately know what's going on and why.

Compare to the XState version:

authenticated: {
    on: {
        LOGOUT: "unauthenticated"
    },
    entry: ["saveUserToContext", "saveToLocalStorage"],
    exit: ["clearUserFromContext", "clearLocalStorage"]
},

Ah! When the user enters the authenticatd state, we saveUserToContext and saveToLocalStorage. When they exit this state, we clear.

Methods for that are defined later. Each does 1 thing.

actions: {
    saveUserToContext: assign((context, event) => {
        const { authResult, user } = event;
        const expiresAt =
            authResult.expiresIn! * 1000 + new Date().getTime();

        return {
            user,
            authResult,
            expiresAt
        };
    }),
    saveToLocalStorage: (context, event) => {
        const { expiresAt, user } = context;

        if (typeof localStorage !== "undefined") {
            localStorage.setItem(
                "useAuth:expires_at",
                JSON.stringify(expiresAt)
            );
            localStorage.setItem("useAuth:user", JSON.stringify(user));
        }
    },

Same code as before but readable and organized. Beautiful 😍

Cheers,
~Swizec

PS: continue reading with part 3 👉 swap useReducer with XState