Mocking Stubbing

Mocking and Stubbing

Table of Contents

• Overview
• When to Use
• Quick Start
• Reference Guides
• Best Practices

Overview

Mocking and stubbing are essential techniques for isolating units of code during testing by replacing dependencies with controlled test doubles. This enables fast, reliable, and focused unit tests that don't depend on external systems like databases, APIs, or file systems.

When to Use

• Isolating unit tests from external dependencies
• Testing code that depends on slow operations (DB, network)
• Simulating error conditions and edge cases
• Verifying interactions between objects
• Testing code with non-deterministic behavior (time, randomness)
• Avoiding expensive operations in tests
• Testing error handling without triggering real failures

Quick Start

Minimal working example:

// services/UserService.ts
import { UserRepository } from "./UserRepository";
import { EmailService } from "./EmailService";

export class UserService {
  constructor(
    private userRepository: UserRepository,
    private emailService: EmailService,
  ) {}