Recently, I was playing with Node.js 6. After downloading the install, I tried
node -v which works fine but
npm -v returned an error.
I was able to fix the issue by manually installing NPM using the line below in bash shell (If you don’t have babun or Git Bash, install just curl)
curl https://raw.githubusercontent.com/npm/npm/master/scripts/install.sh | sh
Hope that helps.