Introduction to Mock-FS

The mock-fs module is a utility for creating mock file systems in Node.js, enabling you to test your code without actually creating, modifying, or deleting any files on your system. It’s perfect for unit tests where interacting with the real file system could lead to potential issues or inconsistencies in test results.

APIs Offered by Mock-Fs

Below are explanations and code snippets for some of the most useful mock-fs APIs.

Mocking a Simple File System

  const mock = require('mock-fs');

  mock({
    'path/to/file.txt': 'file content here',
    'path/to/empty-dir': {}
  });

  // Your test code here

  // Restore the real file system after tests
  mock.restore();

Mocking a File with Metadata

  mock({
    'some/file.txt': mock.file({
      content: 'file content here',
      mtime: new Date(1)
    })
  });

Mocking a Directory with Metadata

  mock({
    'some/dir': mock.directory({
      mode: 0555,
      items: {
        'file.txt': 'file content here'
      }
    })
  });

Mocking Symbolic Links

  mock({
    'path/to/file': 'file content here',
    'link': mock.symlink({
      path: 'path/to/file'
    })
  });

Creating Nested Directories

  mock({
    'path': {
      'to': {
        'nested': {
          'dir': {
            'file.txt': 'file content here'
          }
        }
      }
    }
  });

Mocking Binary Files

  mock({
    'binary/file': new Buffer.from([8, 6, 7, 5, 3, 0, 9])
  });

Combining Multiple File Types

  mock({
    'path': {
      'file.txt': 'file content here',
      'dir': {
        'another-file.txt': 'another content'
      }
    },
    'some/empty/dir': {}
  });

Example Application Using Mock-Fs

Here’s a simple application that demonstrates using several of the mock-fs APIs.

  const fs = require('fs');
  const mock = require('mock-fs');

  mock({
    'app': {
      'config.js': 'module.exports = { port: 3000 };',
      'index.js': `
        const fs = require('fs');
        const config = require('./config');

        fs.writeFileSync('log.txt', 'Server started on port ' + config.port);
        console.log('Server running on port ' + config.port);
      `
    }
  });

  // Simulating app start
  require('./app/index');

  // Verify the log file
  const logContent = fs.readFileSync('app/log.txt', 'utf-8');
  console.log(logContent);

  // Restore the real filesystem
  mock.restore();

Using mock-fs in this way allows us to test all file operations without touching the real file system, making our tests more reliable and easier to manage.

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