How to test file system and I/O operations in Jest? How to mock the fs module? - 面试题

Testing file system and I/O operations in Jest requires using mocks to isolate file system dependencies:

1. Mock fs Module:

javascript
import fs from 'fs';
jest.mock('fs');

test('reads file content', () => {
  const mockContent = 'Hello, World!';
  fs.readFileSync.mockReturnValue(mockContent);
  
  const content = readFile('test.txt');
  expect(content).toBe(mockContent);
  expect(fs.readFileSync).toHaveBeenCalledWith('test.txt', 'utf8');
});

2. Testing File Writing:

javascript
test('writes file content', () => {
  fs.writeFileSync.mockReturnValue(undefined);
  
  writeFile('test.txt', 'content');
  expect(fs.writeFileSync).toHaveBeenCalledWith('test.txt', 'content', 'utf8');
});

3. Testing Async File Operations:

javascript
test('reads file asynchronously', async () => {
  const mockContent = 'Async content';
  fs.promises.readFile.mockResolvedValue(mockContent);
  
  const content = await readFileAsync('test.txt');
  expect(content).toBe(mockContent);
  expect(fs.promises.readFile).toHaveBeenCalledWith('test.txt', 'utf8');
});

4. Testing File Existence Check:

javascript
test('checks if file exists', () => {
  fs.existsSync.mockReturnValue(true);
  
  const exists = fileExists('test.txt');
  expect(exists).toBe(true);
  expect(fs.existsSync).toHaveBeenCalledWith('test.txt');
});

5. Testing Directory Operations:

javascript
test('creates directory', () => {
  fs.mkdirSync.mockReturnValue(undefined);
  
  createDirectory('new-dir');
  expect(fs.mkdirSync).toHaveBeenCalledWith('new-dir', { recursive: true });
});

6. Testing File Traversal:

javascript
test('lists directory contents', () => {
  const mockFiles = ['file1.txt', 'file2.txt'];
  fs.readdirSync.mockReturnValue(mockFiles);
  
  const files = listDirectory('test-dir');
  expect(files).toEqual(mockFiles);
  expect(fs.readdirSync).toHaveBeenCalledWith('test-dir');
});

7. Using Temporary Files for Testing:

javascript
import { tmpdir } from 'os';
import { join } from 'path';

test('writes and reads temporary file', () => {
  const tempFile = join(tmpdir(), 'test.txt');
  const content = 'Test content';
  
  fs.writeFileSync(tempFile, content);
  const readContent = fs.readFileSync(tempFile, 'utf8');
  
  expect(readContent).toBe(content);
  fs.unlinkSync(tempFile); // Cleanup
});

Best Practices:

Use mocks to isolate file system operations
Test both success and failure scenarios
Verify file paths and content parameters
Clean up temporary files and resources
Use jest.clearAllMocks() to clear call records
Consider using in-memory file systems for integration testing