Cannot read properties of undefined

Node.js applications developed on macOS or Linux often encounter Javascript Undefined Error when deployed on Windows or run in WSL2, because of path separator differences, filesystem behavior, and shell command portability. The most common portability issue: hard-coded Unix path separators (/) in Node.js code work on macOS and Linux but fail on Windows native paths. Use Node's path.join() and path.resolve() rather than string concatenation — these functions use the correct separator for the current platform. For npm scripts that use shell commands, Windows does not have rm -rf, cp -r, or & for background processes — use the cross-env package for environment variables, rimraf for recursive deletion, and concurrently for parallel scripts. The process.platform property ('win32', 'darwin', 'linux') lets you branch platform-specific code explicitly. When Javascript Undefined Error appears only on one OS, immediately check for hard-coded paths, shell command assumptions, and line ending handling.