source <(npm completion)
Enables tab-completion in all npm commands.
The synopsis above loads the completions into your current shell. Adding it to your ~/.bashrc or ~/.zshrc will make the completions available everywhere:
npm completion >> ~/.bashrc npm completion >> ~/.zshrc
You may of course also pipe the output of npm completion to a file such as /usr/local/etc/bash_completion.d/npm or /etc/bash_completion.d/npm if you have a system that will read that file for you.