Build: Unit Test a Service with Mocked Dependencies

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◆ Before you start

Prerequisites

VitestRequired
npm install -D vitest
Get it ↗
TypeScriptRequired
npm install -D typescript

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Step 1 of 1

Unit Test a Service with Mocked Dependencies

Write unit tests for a UserService using vi.fn() mocks for the repository.

Tasks

Create UserService with injected UserRepo

Write tests using vi.fn()

Test getUser success, not-found, error cases

Verify mock was called with correct args

🎯 What You'll Learn

▸Mock dependencies with vi.fn()
▸Test happy paths and error paths
▸Verify interactions with mocked dependencies

Code

TypeScriptsrc/user-service.ts
1 export interface User { id: string; name: string; email: string; }
2  
3 export interface UserRepo {
4   findById(id: string): Promise<User | null>;
5   save(user: Omit<User, 'id'>): Promise<User>;
6 }
7  
8 export class UserService {
9   constructor(private repo: UserRepo) {}
10  
11   async getUser(id: string): Promise<User> {
12     const user = await this.repo.findById(id);
13     if (!user) throw new Error(`User ${id} not found`);
14     return user;
15   }
16  
17   async createUser(data: { name: string; email: string }): Promise<User> {
18     if (!data.email.includes('@')) throw new Error('Invalid email');
19     return this.repo.save(data);
20   }
21 }

💡 Pro Tips

▸beforeEach creates fresh mocks — no state leakage between tests
▸Test error paths — they contain 40% of bugs

Expected Output

>3 tests passing: success, not-found, creates-correctly

Discussion

1 export interface User { id: string; name: string; email: string; } 2 3 export interface UserRepo { 4 findById(id: string): Promise<User | null>; 5 save(user: Omit<User, 'id'>): Promise<User>; 6 } 7 8 export class UserService { 9 constructor(private repo: UserRepo) {} 10 11 async getUser(id: string): Promise<User> { 12 const user = await this.repo.findById(id); 13 if (!user) throw new Error(`User ${id} not found`); 14 return user; 15 } 16 17 async createUser(data: { name: string; email: string }): Promise<User> { 18 if (!data.email.includes('@')) throw new Error('Invalid email'); 19 return this.repo.save(data); 20 } 21 }

1	export interface User { id: string; name: string; email: string; }
2
3	export interface UserRepo {
4	findById(id: string): Promise<User \| null>;
5	save(user: Omit<User, 'id'>): Promise<User>;
6	}
7
8	export class UserService {
9	constructor(private repo: UserRepo) {}
10
11	async getUser(id: string): Promise<User> {
12	const user = await this.repo.findById(id);
13	if (!user) throw new Error(`User ${id} not found`);
14	return user;
15	}
16
17	async createUser(data: { name: string; email: string }): Promise<User> {
18	if (!data.email.includes('@')) throw new Error('Invalid email');
19	return this.repo.save(data);
20	}
21	}