If any user has npm installation issues either with or without prefix configured, then it might not be very easy to judge the problem on the user's machine, especially with npm prefix configuration.
Instead of asking user each time to run a series of commands on their machine to figure out the underlying problem, we can add those commands in the diagnostic tests and that should let us give a fair idea on what is going on user's machine with just the diagnostic test logs.
When the NPM updates are invoked, either during startup or by the user -
1) It verifies whether the package is installed on the machine using 'npm -g ls <package>' command.
1.1) If the above command fails for any reason, then invokes 'npm -p ls -g'
2) If the package is available on the machine, it figures out the installed version and latest available version using 'npm view <package> version' command.
Also, since PREFIX is one of those dark areas, we need to add the prefix configuration to the diagnostic logs.
1) Environment variable : Get the system property for NPM_CONFIG_PREFIX
2) Get the prefix key from the npm configuration : npm config get prefix
3) If only the environment variable is set, then the local prefix config value differs from the global one. That can lead to discrepancies in the installation of npm packages. So we should get the output for : 'sudo npm config get prefix'. However, since we don't hold user's password, this might not be a feasible value to store in the diagnostic tests.