---

name: unit-testing description: This skill should be used when the user asks to "add unit tests", "write tests", "set up Vitest", "create a utility function", "add test coverage", or mentions TDD, test-driven development, or testing utilities. Provides guidance for unit testing with Vitest in TypeScript/Next.js projects using TDD approach.

Unit Testing with Vitest

Scope: Unit tests only. See separate skills for integration/E2E.

TDD Workflow

CopyRED → GREEN → REFACTOR

1. RED: Write failing test first (function doesn't exist yet)
2. GREEN: Write minimal code to pass
3. REFACTOR: Clean up, run tests after each change

Setup

Copypnpm add -D vitest

Create vitest.config.mts:

Copyimport { defineConfig } from "vitest/config";

export default defineConfig({
  test: {
    globals: true,
    environment: "node",
    include: ["**/*.test.ts"],
    exclude: ["node_modules", ".next", ".content-collections", "e2e"],
  },
});

Add to tsconfig.json:

Copy{ "compilerOptions": { "types": ["vitest/globals"] } }

Add scripts:

Copy{ "test": "vitest", "test:run": "vitest run" }

For complete setup options, see references/vitest-setup.md.

Test Structure

Place tests alongside code: myFunction.ts → myFunction.test.ts

Copyimport { describe, expect, it } from "vitest";
import { myFunction } from "./myFunction";

describe("myFunction", () => {
  describe("valid inputs", () => {
    it("handles primary case", () => {
      expect(myFunction("input")).toBe("expected");
    });
  });

  describe("invalid inputs", () => {
    it("throws for bad input", () => {
      expect(() => myFunction("bad")).toThrow();
    });
  });
});

Key Patterns

Zod validation - use safeParse:

Copyit("rejects invalid", () => {
  const result = Schema.safeParse(badData);
  expect(result.success).toBe(false);
});

it("checks error message", () => {
  const result = Schema.safeParse(badData);
  if (!result.success) {
    expect(result.error.errors[0].message).toContain("expected");
  }
});

For mocking, async, parameterized tests, see references/test-patterns.md.

Run Tests

Copypnpm test:run                              # all tests
pnpm test:run lib/utils/myFunction.test.ts # specific file
pnpm test:run -t "pattern"                 # matching pattern

Checklist

• Tests cover primary use case
• Tests cover edge cases (empty, boundary)
• Tests cover error cases
• Descriptive test names
• Independent tests (no shared state)
• Mocks cleared (vi.clearAllMocks() in beforeEach)
• All tests pass