JavaScript/TypeScript Testing Skill

When to Activate

Activate this skill when:

• Writing JavaScript or TypeScript tests
• Testing Svelte, React, or Vue components
• Setting up Vitest or Jest configuration
• Working with mocks, spies, or test utilities
• Running tests or checking coverage

Framework Selection

Quick Commands

Vitest

npx vitest              # Watch mode
npx vitest run          # Single run (CI)
npx vitest run --coverage
npx vitest --ui         # Visual UI

Jest

pnpm test
pnpm test --watch
pnpm test --coverage

Test Structure: AAA Pattern

import { describe, it, expect, beforeEach } from 'vitest';

describe('UserService', () => {
    let userService: UserService;

    beforeEach(() => {
        userService = new UserService();
    });

    it('should create a new user with valid data', () => {
        // Arrange
        const email = 'test@example.com';
        const password = 'secure_pass123';

        // Act
        const result = userService.register(email, password);

        // Assert
        expect(result.success).toBe(true);
        expect(result.user.email).toBe(email);
    });
});

Vitest Setup (SvelteKit)

// vite.config.ts
import { defineConfig } from 'vitest/config';
import { sveltekit } from '@sveltejs/kit/vite';

export default defineConfig({
    plugins: [sveltekit()],
    test: {
        include: ['src/**/*.{test,spec}.{js,ts}'],
        globals: true,
        environment: 'jsdom',
        setupFiles: ['./src/tests/setup.ts'],
    }
});

Mocking

Vitest

import { vi } from 'vitest';

vi.mock('./api', () => ({
    fetchUser: vi.fn()
}));

vi.mocked(fetchUser).mockResolvedValue({ id: 1, name: 'John' });

Jest

jest.mock('./api', () => ({
    fetchUser: jest.fn()
}));

Component Testing (Svelte)

import { render, screen, fireEvent } from '@testing-library/svelte';
import Counter from './Counter.svelte';

it('should increment count on click', async () => {
    render(Counter, { props: { initialCount: 0 } });

    const button = screen.getByRole('button', { name: /increment/i });
    await fireEvent.click(button);

    expect(screen.getByText('Count: 1')).toBeInTheDocument();
});

Common Assertions

// Equality
expect(value).toBe(expected);           // Strict ===
expect(value).toEqual(expected);        // Deep equality

// Truthiness
expect(value).toBeTruthy();
expect(value).toBeNull();

// Arrays/Objects
expect(array).toContain(item);
expect(obj).toHaveProperty('key');

// Exceptions
expect(() => fn()).toThrow('error');

// Async
await expect(promise).resolves.toBe(value);
await expect(promise).rejects.toThrow();

Query Priority (Testing Library)

1. getByRole - Accessible queries (best)
2. getByLabelText - Form fields
3. getByPlaceholderText - Inputs
4. getByText - Non-interactive elements
5. getByTestId - Last resort

Directory Structure

src/
├── lib/
│   ├── components/
│   │   ├── Button.svelte
│   │   └── Button.test.ts
│   └── utils/
│       ├── format.ts
│       └── format.test.ts
└── tests/
    ├── setup.ts
    └── integration/

SvelteKit Testing

Load Functions

import { load } from './+page.server';

it('should fetch posts', async () => {
    const mockFetch = vi.fn().mockResolvedValue({
        json: () => Promise.resolve([{ id: 1 }])
    });

    const result = await load({ fetch: mockFetch } as any);
    expect(result.posts).toHaveLength(1);
});

Form Actions

import { actions } from './+page.server';

it('should validate login', async () => {
    const formData = new FormData();
    formData.set('email', 'test@example.com');

    const request = new Request('http://localhost', {
        method: 'POST',
        body: formData
    });

    const result = await actions.default({ request } as any);
    expect(result.success).toBe(true);
});

Related Resources

See AgentUsage/testing_javascript.md for complete documentation including:

• Jest configuration
• Async testing patterns
• SvelteKit-specific patterns
• CI/CD integration