---

name: unit-test-generator description: Generates comprehensive unit tests with AAA pattern (Arrange-Act-Assert), edge cases, error scenarios, and coverage analysis. Creates test files matching source structure with complete test suites. Use for "unit testing", "test generation", "Jest tests", or "test coverage".

Unit Test Generator

Generate comprehensive unit tests with edge cases and AAA pattern.

AAA Pattern Template

// tests/utils/validator.test.ts
import { describe, it, expect } from "vitest";
import { validateEmail } from "@/utils/validator";

describe("validateEmail", () => {
  it("should return true for valid email", () => {
    // Arrange
    const email = "user@example.com";

    // Act
    const result = validateEmail(email);

    // Assert
    expect(result).toBe(true);
  });

  it("should return false for invalid email - missing @", () => {
    // Arrange
    const email = "userexample.com";

    // Act
    const result = validateEmail(email);

    // Assert
    expect(result).toBe(false);
  });

  it("should return false for invalid email - missing domain", () => {
    // Arrange
    const email = "user@";

    // Act
    const result = validateEmail(email);

    // Assert
    expect(result).toBe(false);
  });
});

Comprehensive Test Cases

// src/utils/calculator.ts
export function divide(a: number, b: number): number {
  if (b === 0) {
    throw new Error("Division by zero");
  }
  return a / b;
}

// tests/utils/calculator.test.ts
describe("divide", () => {
  describe("happy path", () => {
    it("should divide positive numbers", () => {
      expect(divide(10, 2)).toBe(5);
    });

    it("should divide negative numbers", () => {
      expect(divide(-10, 2)).toBe(-5);
      expect(divide(10, -2)).toBe(-5);
      expect(divide(-10, -2)).toBe(5);
    });

    it("should handle decimal results", () => {
      expect(divide(10, 3)).toBeCloseTo(3.333, 3);
    });
  });

  describe("edge cases", () => {
    it("should handle zero dividend", () => {
      expect(divide(0, 5)).toBe(0);
    });

    it("should handle very large numbers", () => {
      expect(divide(Number.MAX_SAFE_INTEGER, 2)).toBe(
        Number.MAX_SAFE_INTEGER / 2
      );
    });

    it("should handle very small numbers", () => {
      expect(divide(0.0001, 0.0001)).toBe(1);
    });
  });

  describe("error cases", () => {
    it("should throw error when dividing by zero", () => {
      expect(() => divide(10, 0)).toThrow("Division by zero");
    });

    it("should throw error when dividing by negative zero", () => {
      expect(() => divide(10, -0)).toThrow("Division by zero");
    });
  });
});

Async Function Testing

// src/services/userService.ts
export async function fetchUser(id: string): Promise<User> {
  const response = await fetch(`/api/users/${id}`);

  if (!response.ok) {
    throw new Error(`User not found: ${id}`);
  }

  return response.json();
}

// tests/services/userService.test.ts
describe("fetchUser", () => {
  beforeEach(() => {
    global.fetch = vi.fn();
  });

  afterEach(() => {
    vi.resetAllMocks();
  });

  it("should fetch user successfully", async () => {
    // Arrange
    const mockUser = { id: "123", name: "John" };
    (global.fetch as any).mockResolvedValueOnce({
      ok: true,
      json: async () => mockUser,
    });

    // Act
    const user = await fetchUser("123");

    // Assert
    expect(user).toEqual(mockUser);
    expect(global.fetch).toHaveBeenCalledWith("/api/users/123");
  });

  it("should throw error when user not found", async () => {
    // Arrange
    (global.fetch as any).mockResolvedValueOnce({
      ok: false,
    });

    // Act & Assert
    await expect(fetchUser("999")).rejects.toThrow("User not found: 999");
  });

  it("should handle network error", async () => {
    // Arrange
    (global.fetch as any).mockRejectedValueOnce(new Error("Network error"));

    // Act & Assert
    await expect(fetchUser("123")).rejects.toThrow("Network error");
  });
});

Testing Classes

// src/models/ShoppingCart.ts
export class ShoppingCart {
  private items: CartItem[] = [];

  add(item: CartItem): void {
    this.items.push(item);
  }

  remove(itemId: string): void {
    this.items = this.items.filter((i) => i.id !== itemId);
  }

  getTotal(): number {
    return this.items.reduce(
      (sum, item) => sum + item.price * item.quantity,
      0
    );
  }

  clear(): void {
    this.items = [];
  }
}

// tests/models/ShoppingCart.test.ts
describe("ShoppingCart", () => {
  let cart: ShoppingCart;

  beforeEach(() => {
    cart = new ShoppingCart();
  });

  describe("add", () => {
    it("should add item to cart", () => {
      // Arrange
      const item = { id: "1", name: "Product", price: 10, quantity: 1 };

      // Act
      cart.add(item);

      // Assert
      expect(cart.getTotal()).toBe(10);
    });

    it("should add multiple items", () => {
      // Arrange
      const item1 = { id: "1", name: "Product 1", price: 10, quantity: 1 };
      const item2 = { id: "2", name: "Product 2", price: 20, quantity: 2 };

      // Act
      cart.add(item1);
      cart.add(item2);

      // Assert
      expect(cart.getTotal()).toBe(50); // 10 + (20 * 2)
    });
  });

  describe("remove", () => {
    it("should remove item from cart", () => {
      // Arrange
      const item = { id: "1", name: "Product", price: 10, quantity: 1 };
      cart.add(item);

      // Act
      cart.remove("1");

      // Assert
      expect(cart.getTotal()).toBe(0);
    });

    it("should not throw when removing non-existent item", () => {
      // Act & Assert
      expect(() => cart.remove("999")).not.toThrow();
    });
  });

  describe("getTotal", () => {
    it("should return 0 for empty cart", () => {
      expect(cart.getTotal()).toBe(0);
    });

    it("should calculate total with quantities", () => {
      // Arrange
      cart.add({ id: "1", name: "Product", price: 10, quantity: 3 });

      // Assert
      expect(cart.getTotal()).toBe(30);
    });
  });

  describe("clear", () => {
    it("should remove all items", () => {
      // Arrange
      cart.add({ id: "1", name: "Product 1", price: 10, quantity: 1 });
      cart.add({ id: "2", name: "Product 2", price: 20, quantity: 1 });

      // Act
      cart.clear();

      // Assert
      expect(cart.getTotal()).toBe(0);
    });
  });
});

Testing React Components

// src/components/Counter.tsx
export function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <button onClick={() => setCount(0)}>Reset</button>
    </div>
  );
}

// tests/components/Counter.test.tsx
import { render, screen, fireEvent } from "@testing-library/react";

describe("Counter", () => {
  it("should render with initial count of 0", () => {
    // Arrange & Act
    render(<Counter />);

    // Assert
    expect(screen.getByText("Count: 0")).toBeInTheDocument();
  });

  it("should increment count when button clicked", () => {
    // Arrange
    render(<Counter />);
    const button = screen.getByText("Increment");

    // Act
    fireEvent.click(button);

    // Assert
    expect(screen.getByText("Count: 1")).toBeInTheDocument();
  });

  it("should increment multiple times", () => {
    // Arrange
    render(<Counter />);
    const button = screen.getByText("Increment");

    // Act
    fireEvent.click(button);
    fireEvent.click(button);
    fireEvent.click(button);

    // Assert
    expect(screen.getByText("Count: 3")).toBeInTheDocument();
  });

  it("should reset count to 0", () => {
    // Arrange
    render(<Counter />);
    fireEvent.click(screen.getByText("Increment"));

    // Act
    fireEvent.click(screen.getByText("Reset"));

    // Assert
    expect(screen.getByText("Count: 0")).toBeInTheDocument();
  });
});

Edge Case Categories

// Test case generation template
interface TestCase {
  category: "happy-path" | "edge-case" | "error-case";
  description: string;
  input: any;
  expectedOutput: any;
}

const testCases: TestCase[] = [
  // Happy path
  {
    category: "happy-path",
    description: "typical valid input",
    input: "user@example.com",
    expectedOutput: true,
  },

  // Edge cases
  {
    category: "edge-case",
    description: "empty string",
    input: "",
    expectedOutput: false,
  },
  {
    category: "edge-case",
    description: "null input",
    input: null,
    expectedOutput: false,
  },
  {
    category: "edge-case",
    description: "undefined input",
    input: undefined,
    expectedOutput: false,
  },
  {
    category: "edge-case",
    description: "whitespace only",
    input: "   ",
    expectedOutput: false,
  },
  {
    category: "edge-case",
    description: "very long email",
    input: "a".repeat(1000) + "@example.com",
    expectedOutput: false,
  },

  // Error cases
  {
    category: "error-case",
    description: "invalid format - no @",
    input: "userexample.com",
    expectedOutput: false,
  },
  {
    category: "error-case",
    description: "invalid format - multiple @",
    input: "user@@example.com",
    expectedOutput: false,
  },
];

Coverage Notes

/**
 * Coverage targets for this module:
 *
 * - Line coverage: 100% (all lines executed)
 * - Branch coverage: 100% (all if/else paths tested)
 * - Function coverage: 100% (all functions called)
 * - Statement coverage: 100% (all statements executed)
 *
 * Untested scenarios (intentionally):
 * - None - module is fully covered
 *
 * High-risk areas requiring extra attention:
 * - Division by zero handling
 * - Null/undefined input handling
 * - Type coercion edge cases
 */

Test File Structure

src/
  utils/
    validator.ts
    calculator.ts
  services/
    userService.ts
  models/
    ShoppingCart.ts
  components/
    Counter.tsx

tests/
  utils/
    validator.test.ts
    calculator.test.ts
  services/
    userService.test.ts
  models/
    ShoppingCart.test.ts
  components/
    Counter.test.tsx

Test Generation Script

// scripts/generate-tests.ts
import * as fs from "fs";
import * as path from "path";

function generateTestTemplate(filePath: string): string {
  const fileName = path.basename(filePath, path.extname(filePath));
  const className = fileName.charAt(0).toUpperCase() + fileName.slice(1);

  return `
import { describe, it, expect } from 'vitest';
import { ${className} } from '@/${filePath}';

describe('${className}', () => {
  describe('happy path', () => {
    it('should handle typical case', () => {
      // Arrange
      const input = /* TODO */;

      // Act
      const result = ${className}(input);

      // Assert
      expect(result).toBe(/* TODO */);
    });
  });

  describe('edge cases', () => {
    it('should handle null input', () => {
      // Arrange
      const input = null;

      // Act & Assert
      expect(() => ${className}(input)).toThrow();
    });

    it('should handle empty input', () => {
      // Arrange
      const input = '';

      // Act
      const result = ${className}(input);

      // Assert
      expect(result).toBe(/* TODO */);
    });
  });

  describe('error cases', () => {
    it('should throw error for invalid input', () => {
      // Arrange
      const input = /* TODO */;

      // Act & Assert
      expect(() => ${className}(input)).toThrow('Invalid input');
    });
  });
});
  `.trim();
}

Best Practices

1. AAA pattern: Arrange-Act-Assert structure
2. One assertion per test: Keep tests focused
3. Descriptive names: "should [expected behavior] when [condition]"
4. Test edge cases: null, undefined, empty, max values
5. Test errors: Verify error handling
6. Isolated tests: No shared state between tests
7. Fast tests: Unit tests should run in milliseconds

Output Checklist

• Test file created matching source structure
• AAA pattern used consistently
• Happy path cases covered
• Edge cases identified and tested
• Error cases tested
• Async functions tested with proper awaits
• Mocks used for dependencies
• Coverage notes documented
• Descriptive test names
• Setup/teardown hooks used appropriately