I think it'd be better to use a reporter-like solution:

prettier --reporter my-reporter.js

// my-reporter.js

module.exports = function ({ filename, source, formatted }, { logger }) {
  // ...
}

(It looks like we can even rewrite --write and --list-different as reporters.)

Hi @ikatyang! 👋

Will a project owner have to add my-reporter.js to their code base? It will be unfortunate if so given that the idea of --check is to encourage usage of Prettier in CI while keeping things simple and accessible. The concept of a reporter make sense to me from the architectural point of view – it'd be great if Prettier could come with a couple of easily invokable reporters.

One more thought: implementing --check with its humane output requires printing a summary line before paths to files. This means that the reporter's signature would have to support that.

Optionally, we can park the concept of reporters till 2.0 as suggested by @lydell in #5520 (comment) and simply focus on giving users a simple working solution.

I’m wondering if --check should display a diff, as requested in #4612.

I meant that the reporter could be a sharable module just like eslint --format <my-format>. And of course there should be some built-in reporters.

Optionally, we can park the concept of reporters till 2.0 as suggested by @lydell in #5520 (comment) and simply focus on giving users a simple working solution.

I'm fine with either way as I think we probably need to rewrite our API/CLI in 2.0, and it does not look like it'll happen soon.

@j-f1 good point about the --diff – @ikatyang's reporter suggestion makes even more sense with it!

@ikatyang:
I'm fine with either way as I think we probably need to rewrite our API/CLI in 2.0

2.0 feels like a good time to reconsider the API design, so perhaps the introduction of custom / built-in reporters can be done later. Showing diffs fits the idea of reporters quite well, but I'm not sure that it should be enabled by default. I was recently helping to switch one rather large project into Prettier, which involved adding prettier-check to the pipeline. If we were shown Prettier diffs during the transition period, a message saying Forgot to run Prettier? would turn into a needle in a massive haystack 😅

The main use case for --check as I see it is in CI to avoid merging unformatted code into master. Making a simple variation of --list-different should add enough value for now IMO as it will propagate the right habit across the community. In 2.0, --check and --list-different can be merged due to a shift to the reporters API, but either way this won't be a painful change.

Let's wait for objections to what I wrote in the docs for a few more days and I'll go for implementing a minimal change, which will make this feature real 😉

I'm 👍 on the stuff described in the docs.

RFC 🕺 A few notes:

I had to change the proposed output from

Forgot to run Prettier? There are files without correct code style:
src/fileA.js
src/fileB.js

to

Checking formatting...
src/fileA.js
src/fileB.js
Code style issues found in the above file(s). Forgot to run Prettier?

This because the file list is constructed asynchronously and it would be challenging to "hold" it until all formatting is done in order to print the summary message above. The docs are updated accordingly. prettier-check, which inspired me, spawns Prettier using execa and this is what lets it gather the list before producing the final output. Streaming the list as it gets populated is potentially beneficial in large projects.

The implementation of --check is rather simple and I would even say boring. No massive refactoring or new abstractions, only minimal necessary changes to do what the new docs promise. Hope that this is fine, especially given a possibility of switching to reporters in 2.0.

Added tests are also quite boring – I just duplicated the ones for --list-different. New kinds of tests include:

• making sure that --check and --list-different cannot be used together
• making sure that -c works just like --check (and -l just like --list-different, for symmetry)

I noticed a tiny glitch in output with --list-different + unformatted differs when piped – the test was returning unformatted.jsunformatted.js. This is now fixed.

See below

Would it be reasonable to assign this feature to 1.16 milestone? 🤔 It'd be great if it was released together with the next batch of perks! ✌️

What do you think about displaying a line with the filename currently being formatted that gets deleted and rewritten if the file passes? This would give the user more feedback so it wouldn’t look like Prettier was frozen.

Good point @j-f1! I've implemented this. The filename currently being formatted is also shown in TTY for --list-different now, for symmetry with --check. This should not be a breaking change.

Just tried yarn add -D kachkaev/prettier#check-option in one project and replaced prettier-check with prettier --check. All worked as the new docs say! 🎉

We can probably remove if this helps with getting more reviews or with merging 😉

Thanks!

this is not released yet right?

it's a little confusing as it's already shown at https://prettier.io/docs/en/cli.html - I just wanted to set this up and found out that it's not working

That’s true, we should comment out the docs.

@daKmoR thanks for reporting this problem with the docs – it's truly confusing. There is a temporary fix in #5671 as well as a discussion of how to prevent the problem in future.