SKILL.md

name: writing-tests description: Guides writing Vitest tests for Chrome extension code. Use when creating tests, improving coverage, or testing Vue components, content scripts, or Chrome APIs.

Writing Tests

Testing patterns for the AnimeList Chrome extension using Vitest.

Commands

npm run test:unit              # Run all tests
npm run test:unit:watch        # Watch mode for TDD
npm run test:unit:coverage     # Coverage report
npm run test:unit:coverage -- test/path/file.test.ts  # Single file

Test File Structure

Mirror source structure: src/commons/services/animeService.ts → test/commons/services/animeService.test.ts

Basic Template

import { describe, it, expect, beforeEach, vi } from "vitest";

describe("ComponentName", () => {
    beforeEach(() => {
        vi.clearAllMocks();
        (chrome.storage.local.get as any).mockResolvedValue({});
    });

    describe("methodName", () => {
        it("should handle normal case", async () => {
            const result = await method();
            expect(result).toEqual(expected);
        });

        it("should handle error case", async () => {
            (chrome.storage.local.get as any).mockRejectedValue(new Error("Storage error"));
            await expect(method()).rejects.toThrow("Storage error");
        });
    });
});

Chrome API Mocking

Mocks are set up in test/setup.ts. Available APIs:

chrome.storage.local.get
chrome.storage.local.set
chrome.storage.local.remove
chrome.storage.local.clear

Usage Patterns

// Mock successful retrieval
(chrome.storage.local.get as any).mockResolvedValue({ key: { data: "value" } });

// Mock storage error
(chrome.storage.local.set as any).mockRejectedValue(new Error("Quota exceeded"));

// Verify calls
expect(chrome.storage.local.set).toHaveBeenCalledWith({ key: data });
expect(chrome.storage.local.set).toHaveBeenCalledTimes(1);

Content Script Testing (JSDOM)

// @vitest-environment jsdom
import { JSDOM } from "jsdom";
import { beforeEach, afterEach, describe, it, vi } from "vitest";

describe("Content Script", () => {
    let dom: JSDOM;

    beforeEach(() => {
        dom = new JSDOM(`
            <!DOCTYPE html>
            <html><body>
                <div class="flw-item" data-testid="anime-item">
                    <a href="/anime-123" class="dynamic-name">Test Anime</a>
                </div>
            </body></html>
        `);
        global.document = dom.window.document;
        global.window = dom.window as any;

        global.MutationObserver = vi.fn(() => ({
            observe: vi.fn(),
            disconnect: vi.fn(),
        })) as any;

        vi.spyOn(console, "log").mockImplementation(() => {});
        vi.clearAllMocks();
    });

    afterEach(() => {
        dom.window.close();
    });
});

Vue Component Testing

import { mount } from "@vue/test-utils";
import Component from "@/popup/Component.vue";

it("should render correctly", () => {
    const wrapper = mount(Component);
    expect(wrapper.find('[data-testid="element"]').exists()).toBe(true);
});

it("should handle interactions", async () => {
    const wrapper = mount(Component);
    await wrapper.find('[data-testid="button"]').trigger("click");
    expect(mockFunction).toHaveBeenCalled();
});

Always use data-testid attributes for element selection, not CSS classes.

Branch Coverage Patterns

Every conditional creates branches that need testing:

// For: const value = getValue() || "default";
it("should use returned value when truthy", () => {
    vi.mocked(getValue).mockReturnValue("actual");
    // Verify "actual" is used
});

it("should use default when falsy", () => {
    vi.mocked(getValue).mockReturnValue(null);
    // Verify "default" is used
});

// For: const result = condition ? "yes" : "no";
it("should return yes when condition is true", () => { /* ... */ });
it("should return no when condition is false", () => { /* ... */ });

Error Handling Coverage

Test every try-catch block:

it("should handle async operation errors", async () => {
    vi.mocked(asyncOperation).mockRejectedValue(new Error("Test error"));
    const consoleSpy = vi.spyOn(console, "error").mockImplementation(() => {});

    await functionWithTryCatch();

    expect(consoleSpy).toHaveBeenCalledWith("Error message:", expect.any(Error));
});

Async Testing

// Wait for promises to resolve
await new Promise((resolve) => setTimeout(resolve, 0));

// Wait for DOM updates
await wrapper.vm.$nextTick();

// Wait for specific timeout
vi.useFakeTimers();
vi.advanceTimersByTime(2000);
vi.useRealTimers();

Coverage Requirements

Statements: 85%+
Branches: 80%+
Functions: 85%+
Lines: 85%+

Reading Coverage Reports

File                     | % Stmts | % Branch | Uncovered Line #s
content/index.ts         |   92.43 |    84.48 | 202-204,227

Lines 202-204 are uncovered - examine the source to understand what conditions trigger those lines, then write targeted tests.

Test Organization

describe("Service", () => {
    describe("Normal Operations", () => {
        it("should handle typical use case");
        it("should handle edge cases");
    });

    describe("Error Scenarios", () => {
        it("should handle dependency errors");
        it("should handle invalid input");
    });

    describe("Branch Coverage", () => {
        it("should handle condition=true path");
        it("should handle condition=false path");
    });
});

Naming Convention

Format: should [expected behavior] when [condition]

should return empty array when no episodes exist
should throw error when storage is unavailable
should update existing episode progress

Common Pitfalls

Not resetting mocks: Always use vi.clearAllMocks() in beforeEach
Testing implementation: Test behavior/outcomes, not internal methods
Missing error tests: Test both success and failure paths
Ignoring async timing: Use proper async patterns with await

Checklist

Happy path tests
Error handling tests
Edge case tests
Branch coverage (all conditionals)
Mocks properly reset
Descriptive test names
data-testid for DOM selection

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