API Mocking - 🌲 Sonamu

Learn how to write independent tests by mocking external dependencies like APIs, file systems, and timers.

What is API Mocking?

In testing, Mocking refers to providing fake behavior for external dependencies (APIs, file systems, time, etc.) without actually executing them. This enables:

Tests run regardless of external service outages
Fast tests without network calls
Stable tests with predictable results
Avoiding costly operations (payments, SMS, etc.)

Types of Mocking

Sonamu supports three types of mocking:

Vitest Mock

Vitest’s built-in mocking features

Naite Mock

Dynamic mocking with Naite

Manual Mock

Manually implemented mock objects

Using Vitest Mock

Module Mocking

Replace external module behavior with fakes:

import { test, vi } from "vitest";
import axios from "axios";

// Replace entire axios module with Mock
vi.mock("axios");

test("external API call", async () => {
  // Set mock function return value
  vi.mocked(axios.get).mockResolvedValue({
    data: { id: 1, name: "John" },
  });
  
  const result = await userService.fetchUser(1);
  
  expect(result.name).toBe("John");
  expect(axios.get).toHaveBeenCalledWith("/api/users/1");
});

Function Spy

Track actual function calls:

import { test, vi } from "vitest";
import { emailService } from "../services/email.service";

test("verify email sending", async () => {
  // Create Spy to track function calls
  const sendSpy = vi.spyOn(emailService, "send").mockResolvedValue(true);
  
  await userService.register({ email: "user@example.com" });
  
  // Verify email send function was called
  expect(sendSpy).toHaveBeenCalledWith({
    to: "user@example.com",
    subject: "Welcome!",
  });
  
  // Clean up Spy
  sendSpy.mockRestore();
});

Timer Mocking

Control time-related functions:

import { test, vi } from "vitest";

test("setTimeout test", async () => {
  // Use fake timers
  vi.useFakeTimers();
  
  let executed = false;
  setTimeout(() => {
    executed = true;
  }, 1000);
  
  // Before 1 second
  expect(executed).toBe(false);
  
  // Advance time by 1 second
  vi.advanceTimersByTime(1000);
  
  // After 1 second
  expect(executed).toBe(true);
  
  // Restore to real timers
  vi.useRealTimers();
});

bootstrap() and timers: Sonamu’s bootstrap() function automatically calls vi.useRealTimers() in afterEach to restore timers. No need to call it separately.

Date Mocking

Fix the current time:

import { test, vi } from "vitest";

test("check today's date", () => {
  // Fix to January 1, 2025
  vi.setSystemTime(new Date("2025-01-01T00:00:00Z"));
  
  const today = new Date();
  expect(today.getFullYear()).toBe(2025);
  expect(today.getMonth()).toBe(0);  // 0 = January
  
  // Restore system time
  vi.useRealTimers();
});

Using Naite Mock

Use Naite’s special key patterns to create mocks dynamically.

File System Mocking

Creating Virtual Files

import { test } from "sonamu/test";
import { Naite } from "sonamu";
import { access, constants } from "fs/promises";
import { expect } from "vitest";

test("virtual file system", async () => {
  const filePath = "/tmp/my-virtual-file.txt";
  
  // 1. File doesn't actually exist
  await expect(
    access(filePath, constants.F_OK)
  ).rejects.toThrow();
  
  // 2. Register virtual file with Naite
  Naite.t("mock:fs/promises:virtualFileSystem", filePath);
  
  // 3. Now behaves as if file "exists"
  await expect(
    access(filePath, constants.F_OK)
  ).resolves.toBeUndefined();
  
  // 4. Delete Mock
  Naite.del("mock:fs/promises:virtualFileSystem");
  
  // 5. No longer exists
  await expect(
    access(filePath, constants.F_OK)
  ).rejects.toThrow();
});

Practical Example: File Upload Test

import { test } from "sonamu/test";
import { Naite } from "sonamu";
import { fileService } from "../services/file.service";
import { expect } from "vitest";

test("check file existence", async () => {
  const uploadPath = "/uploads/user/123/profile.jpg";
  
  // Register virtual file
  Naite.t("mock:fs/promises:virtualFileSystem", uploadPath);
  
  // Test file service
  const exists = await fileService.checkExists(uploadPath);
  expect(exists).toBe(true);
  
  // Clean up Mock
  Naite.del("mock:fs/promises:virtualFileSystem");
});

test("check multiple files", async () => {
  const files = [
    "/uploads/file1.pdf",
    "/uploads/file2.pdf",
    "/uploads/file3.pdf",
  ];
  
  // Register multiple files
  for (const file of files) {
    Naite.t("mock:fs/promises:virtualFileSystem", file);
  }
  
  // Batch check
  const results = await fileService.checkMultiple(files);
  expect(results.every(r => r.exists)).toBe(true);
  
  // Clean up
  Naite.del("mock:fs/promises:virtualFileSystem");
});

How Naite Mock Works

Naite detects special key patterns (mock:*) to intercept corresponding behavior:

// Sonamu internal (pseudocode)
async function access(path: string) {
  // Check mock:fs/promises:virtualFileSystem in Naite
  const mocked = Naite.get("mock:fs/promises:virtualFileSystem")
    .result()
    .includes(path);
  
  if (mocked) {
    // If virtual file is registered, succeed
    return Promise.resolve();
  }
  
  // Check actual file system
  return realAccess(path);
}

Implementing Manual Mock

Implement Mock classes directly for complex external services.

Creating Mock Class

// __mocks__/payment.service.ts
export class MockPaymentService {
  private payments: Map<string, any> = new Map();
  
  async charge(amount: number, userId: number) {
    const paymentId = `mock_${Date.now()}`;
    
    this.payments.set(paymentId, {
      id: paymentId,
      amount,
      userId,
      status: "success",
      createdAt: new Date(),
    });
    
    return {
      success: true,
      paymentId,
    };
  }
  
  async getPayment(paymentId: string) {
    return this.payments.get(paymentId) ?? null;
  }
  
  async refund(paymentId: string) {
    const payment = this.payments.get(paymentId);
    if (!payment) {
      throw new Error("Payment not found");
    }
    
    payment.status = "refunded";
    return { success: true };
  }
  
  // Test helper
  clear() {
    this.payments.clear();
  }
}

Using Mock

import { test, beforeEach } from "vitest";
import { MockPaymentService } from "./__mocks__/payment.service";
import { orderService } from "../services/order.service";

let mockPayment: MockPaymentService;

beforeEach(() => {
  mockPayment = new MockPaymentService();
  // Replace real service with Mock
  orderService.paymentService = mockPayment as any;
});

test("order payment", async () => {
  const order = await orderService.create({
    userId: 1,
    items: [{ productId: 1, quantity: 2 }],
    totalAmount: 50000,
  });
  
  // Execute Mock payment
  const result = await orderService.processPayment(order.id);
  
  expect(result.success).toBe(true);
  expect(result.paymentId).toBeDefined();
  
  // Verify payment recorded in Mock
  const payment = await mockPayment.getPayment(result.paymentId);
  expect(payment?.amount).toBe(50000);
  expect(payment?.status).toBe("success");
});

test("payment failure scenario", async () => {
  // Configure Mock to throw error
  mockPayment.charge = async () => {
    throw new Error("Card declined");
  };
  
  const order = await orderService.create({
    userId: 1,
    items: [{ productId: 1, quantity: 1 }],
    totalAmount: 10000,
  });
  
  await expect(
    orderService.processPayment(order.id)
  ).rejects.toThrow("Card declined");
});

Practical Examples

External API Call Mocking

import { test, vi } from "vitest";
import axios from "axios";
import { weatherService } from "../services/weather.service";

vi.mock("axios");

test("fetch weather info", async () => {
  // Mock API response
  vi.mocked(axios.get).mockResolvedValue({
    data: {
      temperature: 25,
      condition: "Sunny",
      humidity: 60,
    },
  });
  
  const weather = await weatherService.getCurrentWeather("Seoul");
  
  expect(weather.temperature).toBe(25);
  expect(weather.condition).toBe("Sunny");
  expect(axios.get).toHaveBeenCalledWith(
    expect.stringContaining("Seoul")
  );
});

test("API error handling", async () => {
  // Simulate network error
  vi.mocked(axios.get).mockRejectedValue(
    new Error("Network Error")
  );
  
  await expect(
    weatherService.getCurrentWeather("Seoul")
  ).rejects.toThrow("Failed to fetch weather information");
});

SMS Sending Mocking

import { test, vi } from "vitest";
import { smsService } from "../services/sms.service";
import { userService } from "../services/user.service";

test("registration SMS sending", async () => {
  // Mock SMS send function
  const sendSpy = vi
    .spyOn(smsService, "send")
    .mockResolvedValue({ success: true, messageId: "mock_123" });
  
  await userService.register({
    phone: "010-1234-5678",
    name: "John Doe",
  });
  
  // Verify SMS was sent
  expect(sendSpy).toHaveBeenCalledWith({
    to: "010-1234-5678",
    message: expect.stringContaining("verification code"),
  });
  
  sendSpy.mockRestore();
});

File Upload Mocking

import { test } from "sonamu/test";
import { Naite } from "sonamu";
import { imageService } from "../services/image.service";
import { expect } from "vitest";

test("image upload and processing", async () => {
  const originalPath = "/tmp/upload/original.jpg";
  const thumbnailPath = "/tmp/upload/thumbnail.jpg";
  
  // Register virtual file
  Naite.t("mock:fs/promises:virtualFileSystem", originalPath);
  
  // Image processing (thumbnail creation)
  await imageService.createThumbnail(originalPath, thumbnailPath);
  
  // Assume thumbnail is also virtually created
  Naite.t("mock:fs/promises:virtualFileSystem", thumbnailPath);
  
  // Verify both files exist
  const exists = await imageService.checkBothExist(
    originalPath,
    thumbnailPath
  );
  expect(exists).toBe(true);
  
  // Clean up
  Naite.del("mock:fs/promises:virtualFileSystem");
});

Environment Variable Mocking

import { test, vi } from "vitest";
import { configService } from "../services/config.service";

test("API key configuration", () => {
  // Mock environment variables
  vi.stubEnv("API_KEY", "test_key_12345");
  vi.stubEnv("API_URL", "https://test.api.com");
  
  const config = configService.load();
  
  expect(config.apiKey).toBe("test_key_12345");
  expect(config.apiUrl).toBe("https://test.api.com");
  
  // Restore environment variables
  vi.unstubAllEnvs();
});

Mocking Strategy

Layer-based Mocking

Mocking by test level:

Unit test: Mock all dependencies

test("Service unit test", async () => {
  // Mock both Repository and External API
  const mockRepo = { findById: vi.fn() };
  const mockApi = { fetch: vi.fn() };
  
  const service = new UserService(mockRepo, mockApi);
  await service.getUser(1);
});

Integration test: Mock only external dependencies

test("Service + Repository integration test", async () => {
  // Mock only External API, use real DB
  vi.mock("axios");
  
  const user = await userService.getUser(1);
});

E2E test: Minimize mocking

test("full flow test", async () => {
  // Only mock costly operations (payment, SMS, etc.)
  vi.spyOn(paymentService, "charge").mockResolvedValue({
    success: true,
  });
  
  const response = await request(app).post("/api/orders");
});

Mock Data Management

Manage test mock data in separate files:

// __fixtures__/users.ts
export const mockUsers = {
  admin: {
    id: 1,
    username: "admin",
    role: "admin",
    email: "admin@example.com",
  },
  user: {
    id: 2,
    username: "user",
    role: "user",
    email: "user@example.com",
  },
  guest: {
    id: 3,
    username: "guest",
    role: "guest",
    email: "guest@example.com",
  },
};

// __fixtures__/api-responses.ts
export const mockApiResponses = {
  weather: {
    success: {
      temperature: 25,
      condition: "Sunny",
    },
    error: {
      error: "API_ERROR",
      message: "Failed to fetch weather information",
    },
  },
  payment: {
    success: {
      success: true,
      transactionId: "txn_12345",
    },
    failure: {
      success: false,
      error: "INSUFFICIENT_FUNDS",
    },
  },
};

Usage:

import { mockUsers } from "./__fixtures__/users";
import { mockApiResponses } from "./__fixtures__/api-responses";

test("admin permission test", async () => {
  vi.mocked(userService.findById).mockResolvedValue(mockUsers.admin);
  
  const hasPermission = await authService.checkAdmin(1);
  expect(hasPermission).toBe(true);
});

test("weather API success", async () => {
  vi.mocked(axios.get).mockResolvedValue({
    data: mockApiResponses.weather.success,
  });
  
  const weather = await weatherService.get("Seoul");
  expect(weather.temperature).toBe(25);
});

Cautions

Cautions when mocking:

Avoid excessive mocking: Too many mocks can diverge from actual behavior. Only mock what’s necessary.

Clean up mocks: Not cleaning mocks in afterEach can affect other tests.

afterEach(() => {
  vi.restoreAllMocks();
  Naite.del("mock:*");
});

Match mock and real behavior: If mock behavior differs from the real service, tests may pass but fail in production.

Type safety: Specify types for mocks to prevent runtime errors.

// ❌ No type
vi.mocked(service.method).mockResolvedValue({});

// ✅ With type
vi.mocked(service.method).mockResolvedValue({
  id: 1,
  name: "test",
} as User);

Naite Mock scope: mock:* keys work globally, so always delete them at test end.

Next Steps

runWithMockContext

Testing with Mock Context

Database Mocking

Isolating DB tests with transactions

Naite Logging

Recording and tracking test logs

Bootstrap

Test environment initialization

​What is API Mocking?

​Types of Mocking

Vitest Mock

Naite Mock

Manual Mock

​Using Vitest Mock

​Module Mocking

​Function Spy

​Timer Mocking

​Date Mocking

​Using Naite Mock

​File System Mocking

​Creating Virtual Files

​Practical Example: File Upload Test

​How Naite Mock Works

​Implementing Manual Mock

​Creating Mock Class

​Using Mock

​Practical Examples

​External API Call Mocking

​SMS Sending Mocking

​File Upload Mocking

​Environment Variable Mocking

​Mocking Strategy

​Layer-based Mocking

​Mock Data Management

​Cautions

​Next Steps

runWithMockContext

Database Mocking

Naite Logging

Bootstrap

What is API Mocking?

Types of Mocking

Using Vitest Mock

Module Mocking

Function Spy

Timer Mocking

Date Mocking

Using Naite Mock

File System Mocking

Creating Virtual Files

Practical Example: File Upload Test

How Naite Mock Works

Implementing Manual Mock

Creating Mock Class

Using Mock

Practical Examples

External API Call Mocking

SMS Sending Mocking

File Upload Mocking

Environment Variable Mocking

Mocking Strategy

Layer-based Mocking

Mock Data Management

Cautions

Next Steps