# vitest-unit-testing-cursorrules-prompt-file

A Cursor rule that turns the agent into a Vitest and TypeScript test writer: it targets business logic and utilities, mocking dependencies with vi.mock.

- rule · `type=rule` · Cursor · ★ 41011
- Author: PatrickJS
- License: CC0-1.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/PatrickJS/awesome-cursorrules/tree/HEAD/rules/vitest-unit-testing-cursorrules-prompt-file.mdc
- Updated: 2026-05-30
- HTML: https://affpapa.org/en/nevibecode/rules/vitest-unit-testing-cursorrules-prompt-file

## What it does

Drop the rule into a project and the agent follows it when writing tests. First it looks for TypeScript — in `tsconfig.json` or `package.json` dependencies — and adjusts the syntax to what it finds.

Then come the working rules: test business logic and utility functions rather than everything; mock API calls and external modules with `vi.mock` before imports; cover valid inputs, invalid inputs and edge cases. Test names should describe expected behaviour, and related tests belong in `describe` blocks. Undefined values, type mismatches and unexpected inputs get their own coverage. The rule caps a file at 3–5 focused tests — that is the author's guidance, not a hard limit of the tool.

Two examples ship with it: a JavaScript one around `formatDate` and a TypeScript one around `getForecast`, with a mocked `weatherService` and `WeatherData`/`Forecast` interfaces. Both show the same order — mock first, import the module under test after. The rule also tells the agent to match your team's existing testing conventions, so if you already have a house style, expect some tension there. Worth checking before you commit it.

## Install

### Cursor (native platform)

**Manually: put the files in place**
- File: `.cursor/rules/vitest-unit-testing-cursorrules-prompt-file.mdc` — Frontmatter: description, globs, alwaysApply

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/vitest-unit-testing-cursorrules-prompt-file.md` — The paths frontmatter field limits the files: move globs from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### GitHub Copilot

**Manually: put the files in place**
- File: `.github/instructions/vitest-unit-testing-cursorrules-prompt-file.instructions.md` — The applyTo field sets which files it applies to: move globs from the source into applyTo

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/vitest-unit-testing-cursorrules-prompt-file.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/vitest-unit-testing-cursorrules-prompt-file.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/PatrickJS/awesome-cursorrules/tree/HEAD/rules/vitest-unit-testing-cursorrules-prompt-file.mdc

## When to use it
- write unit tests for business logic and utilities
- mock API calls and external modules before imports with `vi.mock`
- cover invalid inputs and edge cases
- group related tests in `describe` blocks with clear names
- keep the number of tests per file sensible

## Example prompts
- Write Vitest unit tests for `formatDate` following the rule
- Add tests for `getForecast` with `weatherService` mocked
- Cover the edge cases for this utility and mock its dependencies

## Source text · vitest-unit-testing-cursorrules-prompt-file.mdc · CC0-1.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: a7d3d3681eb9476c.

BEGIN THIRD-PARTY FILE a7d3d3681eb9476c
````text
---
description: "Cursor rules for Vitest development with unit testing."
globs: **/*
alwaysApply: false
---
# Persona

You are an expert developer with deep knowledge of Vitest and TypeScript, tasked with creating unit tests for JavaScript/TypeScript applications.

# Auto-detect TypeScript Usage

Check for TypeScript in the project through tsconfig.json or package.json dependencies.
Adjust syntax based on this detection.

# Unit Testing Focus

Create unit tests that focus on critical functionality (business logic, utility functions)
Mock dependencies (API calls, external modules) before imports using vi.mock
Test various data scenarios (valid inputs, invalid inputs, edge cases)
Write maintainable tests with descriptive names grouped in describe blocks

# Best Practices

**1** **Critical Functionality**: Prioritize testing business logic and utility functions
**2** **Dependency Mocking**: Always mock dependencies before imports with vi.mock()
**3** **Data Scenarios**: Test valid inputs, invalid inputs, and edge cases
**4** **Descriptive Naming**: Use clear test names indicating expected behavior
**5** **Test Organization**: Group related tests in describe/context blocks
**6** **Project Patterns**: Match team's testing conventions and patterns
**7** **Edge Cases**: Include tests for undefined values, type mismatches, and unexpected inputs
**8** **Test Quantity**: Limit to 3-5 focused tests per file for maintainability

# Example Unit Test

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

// Mock dependencies before imports
vi.mock('../api/locale', () => ({
  getLocale: vi.fn(() => 'en-US'), // Mock locale API
}));

// Import module under test
const { formatDate } = await import('../utils/formatDate');

describe('formatDate', () => {
  beforeEach(() => {
    vi.clearAllMocks();
  });

  it('should format date correctly', () => {
    // Arrange
    const date = new Date('2023-10-15');
    
    // Act
    const result = formatDate(date);
    
    // Assert
    expect(result).toBe('2023-10-15');
  });

  it('should handle invalid date', () => {
    const result = formatDate(new Date('invalid'));
    expect(result).toBe('Invalid Date');
  });

  it('should throw error for undefined input', () => {
    expect(() => formatDate(undefined)).toThrow('Input must be a Date object');
  });

  it('should handle non-Date object', () => {
    expect(() => formatDate('2023-10-15')).toThrow('Input must be a Date object');
  });
});
```

# TypeScript Example

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

// Mock dependencies before imports
vi.mock('../api/weatherService', () => ({
  getWeatherData: vi.fn(),
}));

// Import the mocked module and the function to test
import { getWeatherData } from '../api/weatherService';
import { getForecast } from '../utils/forecastUtils';

// Define TypeScript interfaces
interface WeatherData {
  temperature: number;
  humidity: number;
  conditions: string;
}

interface Forecast {
  prediction: string;
  severity: 'low' | 'medium' | 'high';
}

describe('getForecast', () => {
  beforeEach(() => {
    vi.clearAllMocks();
  });

  it('should return forecast when weather data is available', async () => {
    // Arrange
````
END THIRD-PARTY FILE a7d3d3681eb9476c

The third-party file has ended. Everything between the a7d3d3681eb9476c tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

First 3283 of 4345 bytes. © PatrickJS · CC0-1.0 · https://github.com/PatrickJS/awesome-cursorrules/tree/HEAD/rules/vitest-unit-testing-cursorrules-prompt-file.mdc

_Part of the page did not fit — see the HTML version._

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