Ubuntu 16.04
$EDITOR is not set normally on Linux, you can use the alternatives system on Debian at least to get the default text editor. On Windows you can probably just launch the file as a child_process and the default program will open.

It is also not working for me under macOS using zsh.
There are alternatives to using $EDITOR https://github.com/lahmatiy/open-in-editor
With that said, prompting the user to set $EDITOR is probably the best option since then they'd be able to choose which they prefer.
@lo1tuma are you getting an error when using zsh? i was getting an error with the # character that was not being parsed as a comment (zsh: bad pattern: #)
i added set -k to my .zshrc and that fixed it.
Fixed in this PR by adding a fallback https://github.com/zeit/hyperterm/pull/517
This was fixed by #736 :boom:
Wait, I think I misunderstood the issue. It looks like #517 actually fixed it.
Looks like it should be fixed in the newest release. Please re-open if what I'm saying is false.
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It is also not working for me under macOS using zsh.