Comprehensive unit testing expertise covering Vitest, test-driven development (TDD), mocking strategies, and production-grade best practices. Activates for unit testing, Vitest, TDD, Red-Green-Refactor, mocking, stubbing, spying, test coverage, and test architecture in TypeScript/Node projects.

Vitest Standard

This skill provides opinionated, production-tested guidance for high-integrity unit testing with Vitest. It emphasizes behavior-driven design, strictly isolated tests, and the TDD lifecycle.

Golden Rules (Mandatory)

Test Behavior, Not Implementation — Assert what the code does, not how it looks internally.
Strict Isolation — NO shared state between tests. Create new instances in beforeEach.
Clean Mocks — Use vi.clearAllMocks() in beforeEach to prevent call history leaks.
No Magic Numbers — Use descriptive variables for expected values.
AAA Pattern — Every test must follow Arrange-Act-Assert.
Async/Await — Always await promises; use rejects.toThrow() for error paths.
Deterministic Tests — No dependence on system time, environment variables, or randomness (mock them).
Meaningful Names — Use it('should [action] when [condition]') or Given-When-Then.
Mock at Boundaries — Mock external APIs, databases, and third-party SDKs; test your own logic.
Red-Green-Refactor — Prefer writing tests before code to drive API design.

Core Patterns

AAA (Arrange-Act-Assert)

it('should calculate total price', () => {
  // Arrange: Setup data and environment
  const cart = new ShoppingCart()
  cart.addItem({ price: 10, quantity: 2 })

  // Act: Execute the method being tested
  const total = cart.getTotal()

  // Assert: Verify the outcome
  expect(total).toBe(20)
})

Mocking Example

import { vi, it, expect } from 'vitest'

it('mocks dependencies', () => {
  const mockFn = vi.fn().mockReturnValue(42)
  expect(mockFn()).toBe(42)
  expect(mockFn).toHaveBeenCalledOnce()
})

Quick Reference: Assertions

Assertion	Purpose
`toBe(val)`	Strict equality (`===`)
`toEqual(val)`	Deep equality (objects/arrays)
`toMatchObject(obj)`	Partial match on an object
`toThrow(error?)`	Validates a thrown error
`toHaveBeenCalledWith()`	Verifies mock call arguments
`resolves.toEqual()`	Validates a fulfilled promise
`rejects.toThrow()`	Validates a rejected promise

Specialized References (Load On-Demand)

Scenario	Reference File
Mocking & Doubles	mocking.md
Async & Error Handling	async-and-errors.md
Coverage & Configuration	coverage-and-config.md
TDD Cycle	tdd-patterns.md

Example: TDD Calculator

See tdd-calculator.ts for the Red-Green-Refactor workflow.

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