Success and error messages are printed to stdout when reading from a file

This makes it hard to use this tool as a drop-in replacement for prettier, for example when using Emacs editor integration https://github.com/prettier/prettier/tree/master/editors/emacs as it works using temporary files instead of --stdin.

I was able to fix this easily by using console.error instead of console.log in format-files.js, but couldn’t send a quick PR as it breaks more than half of the tests.

$ echo "import      'foo'" > file.js
$ prettier-eslint file.js > formatted.js

formatted.js contents:

import 'foo'

success formatting 1 file with prettier-eslint

[osener]

Stderr is not only for errors, but also for diagnostic messages. Outputting the data in stdout and printing the meta messages on stderr allows users to consume the formatted JS by reading stdout. Simplest example of this is prettier-eslint file.js > formatted.js.

This is outlined in POSIX standard as explained here: https://unix.stackexchange.com/questions/331611/do-progress-reports-logging-information-belong-on-stderr-or-stdout. The third answer also makes a good point that no diagnostic information should be outputted in case of successful execution (unless the user asked for it), so I think even if --summary-log is added, it should default to false.

[kentcdodds]

This has been released in the latest version 😃