# Frontend testing best practices

> Use when writing frontend tests that favour end-to-end coverage over mocks.

Testing best practices for the frontend. Emphasizes E2E tests over unit tests, minimal mocking, and testing behavior over implementation details. Use when writing tests or reviewing test code.

## Facts

- Type: Agent skill
- Discipline: Engineering
- Author: Sergio Xalambri (https://github.com/sergiodxa)
- Upstream: https://github.com/sergiodxa/agent-skills/blob/main/skills/frontend-testing-best-practices/SKILL.md
- Source file: https://raw.githubusercontent.com/sergiodxa/agent-skills/main/skills/frontend-testing-best-practices/SKILL.md (4720 bytes, sha256:22a9d05432d447e3cef47c4271a86603f254bf0f3612bf87beece08da35c02b6)
- Licence: MIT
- Upstream copy last changed: 2026-09-17
- Status: listed, not yet ruled on by the curator

## Use it

I want to use the Frontend testing best practices skill by Sergio Xalambri.
This is what it does, in the author's own words: Testing best practices for the frontend. Emphasizes E2E tests over unit tests, minimal mocking, and testing behavior over implementation details. Use when writing tests or reviewing test code.
Download the whole folder at https://github.com/sergiodxa/agent-skills/tree/main/skills/frontend-testing-best-practices, SKILL.md and every file beside it, into your skills directory: ~/.claude/skills/frontend-testing-best-practices/ for Claude Code, ~/.agents/skills/frontend-testing-best-practices/ for Codex, ~/.cursor/skills/frontend-testing-best-practices/ for Cursor. Get it from the author's repo, not from startupskills.dev.
Then tell me the path you installed it to and how to invoke it.

Nothing here is vendored: install from the author's repo, not from this site. A verbatim copy of the author's file follows for reading, under the licence reproduced after it; the current file is in the author's repo.

## Links

- Page: https://startupskills.dev/sergiodxa/frontend-testing-best-practices
- Author: https://startupskills.dev/sergiodxa
- Index: https://startupskills.dev/llms.txt

## The file itself

---
name: frontend-testing-best-practices
description: Testing best practices for the frontend. Emphasizes E2E tests over unit tests, minimal mocking, and testing behavior over implementation details. Use when writing tests or reviewing test code.
---

# Testing Best Practices

Guidelines for writing effective, maintainable tests that provide real confidence. Contains 6 rules focused on preferring E2E tests, minimizing mocking, and testing behavior over implementation.

## Core Philosophy

1. **Prefer E2E tests over unit tests** - Test the whole system, not isolated pieces
2. **Minimize mocking** - If you need complex mocks, write an E2E test instead
3. **Test behavior, not implementation** - Test what users see and do
4. **Avoid testing React components directly** - Test them through E2E

## When to Apply

Reference these guidelines when:

- Deciding what type of test to write
- Writing new E2E or unit tests
- Reviewing test code
- Refactoring tests

## Rules Summary

### Testing Strategy (CRITICAL)

#### prefer-e2e-tests - @rules/prefer-e2e-tests.md

Default to E2E tests. Only write unit tests for pure functions.

```typescript
// E2E test (PREFERRED) - tests real user flow
test("user can place an order", async ({ page }) => {
  await createTestingAccount(page, { account_status: "active" });
  await page.goto("/catalog");
  await page.getByRole("heading", { name: "Example Item" }).click();
  await page.getByRole("link", { name: "Buy" }).click();
  // ... complete flow
  await expect(page.getByAltText("Thank you")).toBeVisible();
});

// Unit test - ONLY for pure functions
test("formatCurrency formats with two decimals", () => {
  expect(formatCurrency(1234.5)).toBe("$1,234.50");
});
```

#### avoid-component-tests - @rules/avoid-component-tests.md

Don't unit test React components. Test them through E2E or not at all.

```typescript
// BAD: Component unit test
describe("OrderCard", () => {
  test("renders amount", () => {
    render(<OrderCard amount={100} />);
    expect(screen.getByText("$100")).toBeInTheDocument();
  });
});

// GOOD: E2E test covers the component naturally
test("order history shows orders", async ({ page }) => {
  await page.goto("/orders");
  await expect(page.getByText("$100")).toBeVisible();
});
```

#### minimize-mocking - @rules/minimize-mocking.md

Keep mocks simple. If you need 3+ mocks, write an E2E test instead.

```typescript
// BAD: Too many mocks = write E2E test
vi.mock("~/lib/auth");
vi.mock("~/lib/transactions");
vi.mock("~/hooks/useAccount");

// GOOD: Simple MSW mock for loader test
mockServer.use(
  http.get("/api/user", () => HttpResponse.json({ name: "John" })),
);
```

### E2E Tests (HIGH)

#### e2e-test-structure - @rules/e2e-test-structure.md

E2E tests go in `e2e/tests/`, not `frontend/`.

```typescript
// e2e/tests/order.spec.ts
import { test, expect } from "@playwright/test";
import { addAccountBalance, createTestingAccount } from "./utils";

test.describe("Orders", () => {
  test.beforeEach(async ({ page, context }) => {
    await createTestingAccount(page, { account_status: "active" });
    let cookies = await context.cookies();
    let account_id = cookies.find((c) => c.name === "account_id").value;
    await addAccountBalance({ account_id, amount: 10000, replaceBalance: true });
  });

  test("place order with default values", async ({ page }) => {
    await page.goto("/catalog");
    // ... user flow
  });
});
```

#### e2e-selectors - @rules/e2e-selectors.md

Use accessible selectors: role > label > text > testid.

```typescript
// GOOD: Role-based (preferred)
await page.getByRole("button", { name: "Submit" }).click();
await page.getByRole("heading", { name: "Dashboard" });

// GOOD: Label-based
await page.getByLabel("Email").fill("test@example.com");

// OK: Test ID when no accessible selector exists
await expect(page.getByTestId("balance")).toHaveText("$1,234");

// BAD: CSS selectors
await page.locator(".btn-primary").click();
```

### Unit Tests (MEDIUM)

#### unit-test-structure - @rules/unit-test-structure.md

Unit tests for pure functions only. Co-locate with source files.

```typescript
// app/utils/format.test.ts
import { describe, test, expect } from "vitest";
import { formatCurrency } from "./format";

describe("formatCurrency", () => {
  test("formats positive amounts", () => {
    expect(formatCurrency(1234.5)).toBe("$1,234.50");
  });

  test("handles zero", () => {
    expect(formatCurrency(0)).toBe("$0.00");
  });
});
```

## Key Files

- `e2e/tests/` - E2E tests (Playwright)
- `e2e/tests/utils.ts` - E2E test utilities
- `vitest.config.ts` - Unit test configuration
- `vitest.setup.ts` - Global test setup with MSW
- `app/utils/test-utils.ts` - Unit test utilities

## Licence of the file above

The file above is https://raw.githubusercontent.com/sergiodxa/agent-skills/main/skills/frontend-testing-best-practices/SKILL.md, by Sergio Xalambri, reproduced without changes.
- Source: https://github.com/sergiodxa/agent-skills/blob/main/skills/frontend-testing-best-practices/SKILL.md
- Copyright: Copyright (c) 2026 Sergio Xalambrí
- Licence: MIT, https://raw.githubusercontent.com/sergiodxa/agent-skills/main/LICENSE

The repo's licence file, verbatim from https://raw.githubusercontent.com/sergiodxa/agent-skills/main/LICENSE:

MIT License

Copyright (c) 2026 Sergio Xalambrí

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.