path.join in nodejs

path.join() does two things :

Joins all given path segments together using the platform specific separator as a delimiter
Normalizes the resulting path

path.join('src/', 'js/Router.jsx');

'src/js/Router.jsx' on OSX and Linux

'srcjsRouter.jsx' on Windows

Here, it normalizes the path by removing extra /.

Recommended way to use path.join to make it work across platform.

path.join(__dirname, 'src', 'js', 'Router.jsx');

Note that we are just specifying the folder (or directory) name. And let path.join add OS specific separators.