chore: restore files unintentionally deleted in 6c61a4e
This commit is contained in:
yeuimu
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---
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name: javascript-testing-patterns
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description: Implement comprehensive testing strategies using Jest, Vitest, and Testing Library for unit tests, integration tests, and end-to-end testing with mocking, fixtures, and test-driven development. Use when writing JavaScript/TypeScript tests, setting up test infrastructure, or implementing TDD/BDD workflows.
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---
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# JavaScript Testing Patterns
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Comprehensive guide for implementing robust testing strategies in JavaScript/TypeScript applications using modern testing frameworks and best practices.
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## When to Use This Skill
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- Setting up test infrastructure for new projects
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- Writing unit tests for functions and classes
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- Creating integration tests for APIs and services
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- Implementing end-to-end tests for user flows
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- Mocking external dependencies and APIs
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- Testing React, Vue, or other frontend components
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- Implementing test-driven development (TDD)
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- Setting up continuous testing in CI/CD pipelines
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## Testing Frameworks
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### Jest - Full-Featured Testing Framework
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**Setup:**
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```typescript
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// jest.config.ts
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import type { Config } from "jest";
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const config: Config = {
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preset: "ts-jest",
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testEnvironment: "node",
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roots: ["<rootDir>/src"],
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testMatch: ["**/__tests__/**/*.ts", "**/?(*.)+(spec|test).ts"],
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collectCoverageFrom: [
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"src/**/*.ts",
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"!src/**/*.d.ts",
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"!src/**/*.interface.ts",
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],
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coverageThreshold: {
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global: {
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branches: 80,
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functions: 80,
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lines: 80,
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statements: 80,
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},
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},
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setupFilesAfterEnv: ["<rootDir>/src/test/setup.ts"],
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};
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export default config;
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```
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### Vitest - Fast, Vite-Native Testing
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**Setup:**
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```typescript
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// vitest.config.ts
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import { defineConfig } from "vitest/config";
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export default defineConfig({
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test: {
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globals: true,
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environment: "node",
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coverage: {
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provider: "v8",
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reporter: ["text", "json", "html"],
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exclude: ["**/*.d.ts", "**/*.config.ts", "**/dist/**"],
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},
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setupFiles: ["./src/test/setup.ts"],
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},
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});
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```
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## Unit Testing Patterns
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### Pattern 1: Testing Pure Functions
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```typescript
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// utils/calculator.ts
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export function add(a: number, b: number): number {
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return a + b;
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}
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export function divide(a: number, b: number): number {
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if (b === 0) {
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throw new Error("Division by zero");
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}
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return a / b;
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}
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// utils/calculator.test.ts
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import { describe, it, expect } from "vitest";
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import { add, divide } from "./calculator";
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describe("Calculator", () => {
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describe("add", () => {
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it("should add two positive numbers", () => {
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expect(add(2, 3)).toBe(5);
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});
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it("should add negative numbers", () => {
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expect(add(-2, -3)).toBe(-5);
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});
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it("should handle zero", () => {
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expect(add(0, 5)).toBe(5);
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expect(add(5, 0)).toBe(5);
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});
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});
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describe("divide", () => {
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it("should divide two numbers", () => {
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expect(divide(10, 2)).toBe(5);
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});
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it("should handle decimal results", () => {
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expect(divide(5, 2)).toBe(2.5);
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});
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it("should throw error when dividing by zero", () => {
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expect(() => divide(10, 0)).toThrow("Division by zero");
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});
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});
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});
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```
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### Pattern 2: Testing Classes
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```typescript
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// services/user.service.ts
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export class UserService {
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private users: Map<string, User> = new Map();
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create(user: User): User {
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if (this.users.has(user.id)) {
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throw new Error("User already exists");
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}
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this.users.set(user.id, user);
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return user;
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}
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findById(id: string): User | undefined {
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return this.users.get(id);
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}
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update(id: string, updates: Partial<User>): User {
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const user = this.users.get(id);
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if (!user) {
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throw new Error("User not found");
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}
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const updated = { ...user, ...updates };
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this.users.set(id, updated);
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return updated;
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}
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delete(id: string): boolean {
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return this.users.delete(id);
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}
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}
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// services/user.service.test.ts
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import { describe, it, expect, beforeEach } from "vitest";
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import { UserService } from "./user.service";
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describe("UserService", () => {
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let service: UserService;
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beforeEach(() => {
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service = new UserService();
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});
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describe("create", () => {
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it("should create a new user", () => {
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const user = { id: "1", name: "John", email: "john@example.com" };
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const created = service.create(user);
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expect(created).toEqual(user);
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expect(service.findById("1")).toEqual(user);
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});
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it("should throw error if user already exists", () => {
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const user = { id: "1", name: "John", email: "john@example.com" };
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service.create(user);
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expect(() => service.create(user)).toThrow("User already exists");
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});
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});
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describe("update", () => {
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it("should update existing user", () => {
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const user = { id: "1", name: "John", email: "john@example.com" };
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service.create(user);
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const updated = service.update("1", { name: "Jane" });
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expect(updated.name).toBe("Jane");
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expect(updated.email).toBe("john@example.com");
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});
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it("should throw error if user not found", () => {
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expect(() => service.update("999", { name: "Jane" })).toThrow(
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"User not found",
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);
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});
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});
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});
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```
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### Pattern 3: Testing Async Functions
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```typescript
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// services/api.service.ts
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export class ApiService {
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async fetchUser(id: string): Promise<User> {
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const response = await fetch(`https://api.example.com/users/${id}`);
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if (!response.ok) {
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throw new Error("User not found");
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}
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return response.json();
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}
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async createUser(user: CreateUserDTO): Promise<User> {
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const response = await fetch("https://api.example.com/users", {
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method: "POST",
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headers: { "Content-Type": "application/json" },
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body: JSON.stringify(user),
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});
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return response.json();
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}
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}
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// services/api.service.test.ts
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import { describe, it, expect, vi, beforeEach } from "vitest";
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import { ApiService } from "./api.service";
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// Mock fetch globally
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global.fetch = vi.fn();
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describe("ApiService", () => {
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let service: ApiService;
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beforeEach(() => {
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service = new ApiService();
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vi.clearAllMocks();
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});
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describe("fetchUser", () => {
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it("should fetch user successfully", async () => {
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const mockUser = { id: "1", name: "John", email: "john@example.com" };
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(fetch as any).mockResolvedValueOnce({
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ok: true,
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json: async () => mockUser,
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});
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const user = await service.fetchUser("1");
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expect(user).toEqual(mockUser);
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expect(fetch).toHaveBeenCalledWith("https://api.example.com/users/1");
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});
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it("should throw error if user not found", async () => {
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(fetch as any).mockResolvedValueOnce({
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ok: false,
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});
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await expect(service.fetchUser("999")).rejects.toThrow("User not found");
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});
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});
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describe("createUser", () => {
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it("should create user successfully", async () => {
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const newUser = { name: "John", email: "john@example.com" };
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const createdUser = { id: "1", ...newUser };
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(fetch as any).mockResolvedValueOnce({
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ok: true,
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json: async () => createdUser,
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});
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const user = await service.createUser(newUser);
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expect(user).toEqual(createdUser);
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expect(fetch).toHaveBeenCalledWith(
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"https://api.example.com/users",
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expect.objectContaining({
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method: "POST",
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body: JSON.stringify(newUser),
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}),
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);
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});
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});
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});
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```
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## Mocking Patterns
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### Pattern 1: Mocking Modules
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```typescript
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// services/email.service.ts
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import nodemailer from "nodemailer";
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export class EmailService {
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private transporter = nodemailer.createTransport({
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host: process.env.SMTP_HOST,
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port: 587,
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auth: {
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user: process.env.SMTP_USER,
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pass: process.env.SMTP_PASS,
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},
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});
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async sendEmail(to: string, subject: string, html: string) {
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await this.transporter.sendMail({
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from: process.env.EMAIL_FROM,
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to,
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subject,
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html,
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});
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}
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}
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// services/email.service.test.ts
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import { describe, it, expect, vi, beforeEach } from "vitest";
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import { EmailService } from "./email.service";
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vi.mock("nodemailer", () => ({
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default: {
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createTransport: vi.fn(() => ({
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sendMail: vi.fn().mockResolvedValue({ messageId: "123" }),
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})),
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},
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}));
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describe("EmailService", () => {
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let service: EmailService;
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beforeEach(() => {
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service = new EmailService();
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});
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it("should send email successfully", async () => {
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await service.sendEmail(
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"test@example.com",
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"Test Subject",
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"<p>Test Body</p>",
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);
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expect(service["transporter"].sendMail).toHaveBeenCalledWith(
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expect.objectContaining({
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to: "test@example.com",
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subject: "Test Subject",
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}),
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);
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});
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});
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```
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### Pattern 2: Dependency Injection for Testing
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```typescript
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// services/user.service.ts
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export interface IUserRepository {
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findById(id: string): Promise<User | null>;
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create(user: User): Promise<User>;
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}
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export class UserService {
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constructor(private userRepository: IUserRepository) {}
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async getUser(id: string): Promise<User> {
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const user = await this.userRepository.findById(id);
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if (!user) {
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throw new Error("User not found");
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}
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return user;
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}
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async createUser(userData: CreateUserDTO): Promise<User> {
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// Business logic here
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const user = { id: generateId(), ...userData };
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return this.userRepository.create(user);
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}
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}
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// services/user.service.test.ts
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import { describe, it, expect, vi, beforeEach } from "vitest";
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import { UserService, IUserRepository } from "./user.service";
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describe("UserService", () => {
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let service: UserService;
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let mockRepository: IUserRepository;
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beforeEach(() => {
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mockRepository = {
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findById: vi.fn(),
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create: vi.fn(),
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};
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service = new UserService(mockRepository);
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});
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describe("getUser", () => {
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it("should return user if found", async () => {
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const mockUser = { id: "1", name: "John", email: "john@example.com" };
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vi.mocked(mockRepository.findById).mockResolvedValue(mockUser);
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const user = await service.getUser("1");
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expect(user).toEqual(mockUser);
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expect(mockRepository.findById).toHaveBeenCalledWith("1");
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});
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it("should throw error if user not found", async () => {
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vi.mocked(mockRepository.findById).mockResolvedValue(null);
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await expect(service.getUser("999")).rejects.toThrow("User not found");
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});
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});
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describe("createUser", () => {
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it("should create user successfully", async () => {
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const userData = { name: "John", email: "john@example.com" };
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const createdUser = { id: "1", ...userData };
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vi.mocked(mockRepository.create).mockResolvedValue(createdUser);
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const user = await service.createUser(userData);
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expect(user).toEqual(createdUser);
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expect(mockRepository.create).toHaveBeenCalled();
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||||
});
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});
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});
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```
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### Pattern 3: Spying on Functions
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|
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```typescript
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// utils/logger.ts
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export const logger = {
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info: (message: string) => console.log(`INFO: ${message}`),
|
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error: (message: string) => console.error(`ERROR: ${message}`),
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};
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// services/order.service.ts
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import { logger } from "../utils/logger";
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export class OrderService {
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async processOrder(orderId: string): Promise<void> {
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logger.info(`Processing order ${orderId}`);
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// Process order logic
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logger.info(`Order ${orderId} processed successfully`);
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||||
}
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||||
}
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||||
|
||||
// services/order.service.test.ts
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import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
|
||||
import { OrderService } from "./order.service";
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||||
import { logger } from "../utils/logger";
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||||
|
||||
describe("OrderService", () => {
|
||||
let service: OrderService;
|
||||
let loggerSpy: any;
|
||||
|
||||
beforeEach(() => {
|
||||
service = new OrderService();
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||||
loggerSpy = vi.spyOn(logger, "info");
|
||||
});
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||||
|
||||
afterEach(() => {
|
||||
loggerSpy.mockRestore();
|
||||
});
|
||||
|
||||
it("should log order processing", async () => {
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await service.processOrder("123");
|
||||
|
||||
expect(loggerSpy).toHaveBeenCalledWith("Processing order 123");
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||||
expect(loggerSpy).toHaveBeenCalledWith("Order 123 processed successfully");
|
||||
expect(loggerSpy).toHaveBeenCalledTimes(2);
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||||
});
|
||||
});
|
||||
```
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||||
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||||
## Integration Testing
|
||||
|
||||
Integration tests verify real database operations and HTTP endpoints using `supertest` and a test database instance. Always truncate tables in `beforeEach` and tear down in `afterAll`.
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||||
|
||||
For full API integration test examples (supertest + PostgreSQL) and database repository integration tests, see [references/advanced-testing-patterns.md](references/advanced-testing-patterns.md).
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||||
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||||
## Frontend Testing with Testing Library
|
||||
|
||||
Test React components by rendering them and querying by role, placeholder, or test ID. Test hooks with `renderHook` + `act`. Prefer semantic queries (`getByRole`, `getByPlaceholderText`) over `data-testid`.
|
||||
|
||||
For complete React component test examples (UserForm, hooks with `renderHook`/`act`), see [references/advanced-testing-patterns.md](references/advanced-testing-patterns.md).
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||||
|
||||
## Test Fixtures and Factories
|
||||
|
||||
Use `@faker-js/faker` to generate realistic test data factories. Factories accept optional `overrides` so tests can set only the fields they care about:
|
||||
|
||||
```typescript
|
||||
// tests/fixtures/user.fixture.ts
|
||||
import { faker } from "@faker-js/faker";
|
||||
|
||||
export function createUserFixture(overrides?: Partial<User>): User {
|
||||
return {
|
||||
id: faker.string.uuid(),
|
||||
name: faker.person.fullName(),
|
||||
email: faker.internet.email(),
|
||||
createdAt: faker.date.past(),
|
||||
...overrides,
|
||||
};
|
||||
}
|
||||
```
|
||||
|
||||
For snapshot testing, coverage configuration, test organization patterns, promise testing, and timer mocking, see [references/advanced-testing-patterns.md](references/advanced-testing-patterns.md).
|
||||
|
||||
## Best Practices
|
||||
|
||||
1. **Follow AAA Pattern**: Arrange, Act, Assert
|
||||
2. **One assertion per test**: Or logically related assertions
|
||||
3. **Descriptive test names**: Should describe what is being tested
|
||||
4. **Use beforeEach/afterEach**: For setup and teardown
|
||||
5. **Mock external dependencies**: Keep tests isolated
|
||||
6. **Test edge cases**: Not just happy paths
|
||||
7. **Avoid implementation details**: Test behavior, not implementation
|
||||
8. **Use test factories**: For consistent test data
|
||||
9. **Keep tests fast**: Mock slow operations
|
||||
10. **Write tests first (TDD)**: When possible
|
||||
11. **Maintain test coverage**: Aim for 80%+ coverage
|
||||
12. **Use TypeScript**: For type-safe tests
|
||||
13. **Test error handling**: Not just success cases
|
||||
14. **Use data-testid sparingly**: Prefer semantic queries
|
||||
15. **Clean up after tests**: Prevent test pollution
|
||||
Reference in New Issue
Block a user