Run The Currently Open Lua File In Neovim

The Command

You can execute an open .lau file buffer inside Neovim with the "source" command like this:

:so

The :help source entry lists the details of :[range]so[urce] [file] as:

Runs Ex commands or Lua code (".lua" files) from [file], or current buffer if no [file]. Triggers the SourcePre autocommand.
The command works with files that have a .lua extension
The command also works with non-saved buffers that are named with a .lua extension. For example, you can do :e tmp.lau, write some code and run it with :so, then delete the buffer with :bd!
The :so command is shorthand for :source. I like using the shorter version, but the longer one works the same
The command can also be run with % like :so % which uses the file path of the current buffer instead of the buffer itself. That means if you make changes to the file buffer without saving them you'll see different behaviour from :so and :so % until you save the file

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I copy/pasted the entry above. There's some more links and other stuff around the entry and I've never found skimming around docs to be time wasted