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.