📎 Feature: --staged option to simplify the creation of git hook scripts

[castarco]

Description

Problem Statement

As of today, biome does not provide a direct mechanism to select (for linting, formatting or checking) only the files that are staged for a commit.

We have a "similar" flag, --changed, but its main purpose is to be used in CI, and allows us to check only the files that changed between 2 points in history (which can be explicit or implicit).

Many people think at first that --changed will also help them to select only the files that are staged, not being the case.

More advanced users have found workarounds to this issue ( #1198 (comment) ), but these involve having to write longer, more complex and slower scripts. Not everyone can come up with these solutions, even if they seem simple after we see them.

Others rely on tools such as lint-staged, but this involves adding more dependencies to the project (which in this case are much slower than biome), and opens up the possibility for more bugs caused by unexpected interactions between lint-staged and biome.

Proposal

After a discussion held in Discord ( https://discord.com/channels/1132231889290285117/1222450681919832134 ), we agreed that it would be a good idea to introduce a new --staged flag for the lint, format and check commands that will make biome take into account only the files that are staged.

Addendum

While --staged doesn't give us much when we are running "potentially destructive" commands such as format or check --apply (because we still need to independently obtain a list of staged files so we can re-add them to the "stage" before they are committed), it gives a clear advantage for readonly-mode commands such as lint without --apply.