Mocking Node.js Path Separators: The Dependency Injection Solution

The Problem

Node.js path utilities like path.sep, path.join() are hardcoded to the current platform and readonly - you can't mock them for cross-platform testing:

// This fails - path.sep is readonly
path.sep = '\\'; // Error: Cannot redefine property

// This ignores your wishes - always uses current platform
path.join('a', 'b'); // Always 'a/b' on Linux, 'a\\b' on Windows

The Solution

Don't mock the global path - inject platform-specific implementations using path.win32 and path.posix:

// Instead of this brittle approach:
function createTempFile(name: string, separator: string) {
  return 'tmp' + separator + name; // Manual string manipulation
}

// Use dependency injection:
function createTempFile(name: string, pathImpl = path) {
  return pathImpl.join('tmp', name);
}

// Now you can test both platforms reliably:
createTempFile('data.json', path.win32)  // → tmp\data.json
createTempFile('data.json', path.posix)  // → tmp/data.json

Why This Works

Node.js provides path.win32 and path.posix as separate implementations. Instead of fighting the platform dependency, embrace it through clean dependency injection. Test Windows logic on Linux, test Unix logic on Windows - no mocking needed.