The process.mainModule property provides an alternative way of retrieving
[require.main][]. The difference is that if the main module changes at
runtime, [require.main][] may still refer to the original main module in
modules that were required before the change occurred. Generally, it's
safe to assume that the two refer to the same module.
As with [require.main][], process.mainModule will be undefined if there
is no entry script.