$ sudo apt-get remove npm 

Uninstall npm including dependent package

If you would like to remove npm and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove npm 

Use Purging npm

If you use with purge options to npm package all the configuration and dependent packages will be removed.

$ sudo apt-get purge npm 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove npm 

See Also,