I recently kept running into issues with Node.js > 9 (Node.js 9.3.0 to be precise) and it not liking the latest version of npm (5.4, 5.5)
It turns out the simplest approach isn’t to downgrade Node.js or any of that stuff. With Node.js 9 there’s a patched version of npm. All I had to do was move the symlink npm that’s in /usr/local/bin and relink to /usr/bin/npm.
This is Debian Jessie; ymmv.