Jest vs Vitest: The King vs New Kid On The Block

When building modern JavaScript/TypeScript applications, choosing the right testing framework can significantly impact your development experience. After working with both Jest and Vitest, here’s what I’ve learned about their strengths and the powerful world of runtime mocking.

Jest: The Veteran Champion

Jest has been the go-to testing framework for years, particularly excelling in:

• CommonJS and React ecosystems - Rock-solid compatibility
• Mature ecosystem - Extensive supporting packages and community resources
• Battle-tested stability - Years of production use

However, Jest does come with a notable drawback: speed. It can be slower, especially with larger codebases. Not to mention, TypeScript code needs to transpile to JavaScript.

Vitest: The Speed Demon

Vitest emerges as the modern alternative, particularly shining with:

• ESM and TypeScript - Native support without configuration headaches
• Blazing fast execution - Significantly faster test runs
• Modern tooling integration - Built for the Vite ecosystem

The Node.js Wild Card

Since Node 20, there’s also node:test - Node’s built-in testing solution. While still maturing, it’s worth considering for simple projects wanting to minimize dependencies.

New and Shiny but.. Not All That Glitters Is Gold

Instead of complex dependency injection patterns, Jest/Vitest embraces runtime mocking. Here’s a balanced analysis:

Runtime Mocking Pros:

• Co-location benefit - Mocking logic lives with the test, making it easier to understand test intent
• Self-contained tests - Easier to read and maintain
• Performance - Tests run faster (no real dependencies)
• Deterministic - Consistent, controllable test conditions
• Isolation - Pure unit testing, only tests your code

Runtime Mocking Cons:

• False confidence - Unit tests pass easily but integration/e2e tests will be more difficult
• Mock drift - Mocks can become stale contracts that don’t reflect reality
• Refactoring blindness - Changes to interfaces might not break mocked tests
• Testing strategy issue - Harder to catch runtime errors when everything is mocked

DI Advantages We Shouldn’t Ignore:

• Contract enforcement - Interfaces ensure real implementations match expectations (I feel this is a strong point why I still prefer Golang)
• Refactoring safety - Interface changes break tests immediately
• Hybrid testing - Same test can run with mocks or real implementations

Mastering Jest’s(or Vitest’s) spyOn: Your Mocking Superpower

Understanding spyOn is crucial for effective testing, but before that you need to know what are mock functions.

Here’s the crash course:

Mock Functions: The Shape-Shifters

Think of mock functions as the “Ditto” (Pokemon creature) of the programming world - they can morph into any function signature:

const mockFoo = jest.fn<typeof Foo>();
// Now mockFoo can be used anywhere Foo is expected

mockFoo.mockImplementation(() => console.log("bar"));
// Instead of complex original logic, do whatever you want

spyOn: The Method Interceptor

spyOn takes this further by automatically creating mocks for existing methods:

const foo = jest.spyOn(MyClass, "methodName");
// foo is now a mock function that intercepts MyClass.methodName()

// Track all calls and results
foo.mock.calls(); // Array of call arguments
foo.mock.results(); // Array of return values

jest.mock(): The Module Replacer

While spyOn targets specific methods, jest.mock() takes a more aggressive approach:

jest.mock("path/to/module", mock_implementation_function);

Key differences:

• spyOn: Only changes the specific method you spied on, other methods remain unchanged
• jest.mock: Replaces ALL exported functions of a module with jest.fn()

Real-World Applications: Testing Layered Architecture

Scenario 1: Testing Service Layer with spyOn

Consider a typical router → service → repository architecture. To unit test router functions, you need to mock service and repository calls.

Traditional approach: Create interfaces and mock instances
Modern approach: Use spyOn to intercept specific methods

// Instead of complex interface mocking
const serviceSpy = jest.spyOn(catalogService, "createProduct");
serviceSpy.mockResolvedValue(mockProduct);

// Clean, focused, and maintainable

Scenario 2: Testing Repository Layer with jest.mock()

For database repositories, we can use jest.mock() to replace the entire PrismaClient module:

const mockPrismaProduct = {
  create: jest.fn(),
  update: jest.fn(),
  delete: jest.fn(),
  findMany: jest.fn(),
  findUnique: jest.fn(),
};

// Mock the entire PrismaClient module
jest.mock("../../generated/prisma", () => ({
  PrismaClient: jest.fn().mockImplementation(() => ({
    product: mockPrismaProduct,
  })),
}));

// Our test assertions focus on calls and arguments
expect(mockPrismaProduct.create).toHaveBeenCalledWith({
  data: productData,
});

This saves us from spinning up a database just to test PrismaClient. (Perhaps that’s more of kicking the can down the road, since we have to do that eventually in integration tests)

Conclusion: The Sweet Spot

The optimal approach isn’t choosing one over the other—it’s using both strategically:

• Runtime mocking for fast unit tests or for prototyping/proof-of-concept projects.
• DI for slower integration tests
• Both approaches serving different purposes in your testing pyramid

For testing strategy, avoid the false dichotomy between runtime mocking and dependency injection. The real power comes from understanding when to use each approach. Runtime mocking excels at fast, isolated unit tests, while DI shines in integration scenarios where contract enforcement matters.

The key is building a comprehensive testing strategy that leverages the strengths of both approaches.

Happy testing! 🧪

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