I am not quite sure since when npm has this arguments and why I missed that, but they are really great.
npm install something with
--save-dev, it will not only install it, it will also put it in the right place within your package.json file.
So, go ahead and try for example this one:
npm install express --save
And you will see it put
"express": "~3.4.4" at the end of the
dependencies block within your package.json. It will always use the ~ notation with the full version number, so you will not have problems reinstalling the same version again on different systems.
Or put the dependcy into the
npm install supertest --save-dev
If you write a new application and are trying out differnt npm packages, it really is a time safer.