Add --no- prefix support for boolean flag negation

[Rugvip]

Hello @privatenumber! 👋

I've been searching for good simple flag parsers with TypeScript support for adoption in the Backstage project, and cleye caught my... eye 🤦. We're looking at what a potential migration looks like (backstage/backstage#33061 if you're curious, WIP), and overall it's very smooth.

One diff that we spotted was the support for automatic --no- prefixes for boolean flags. It's not a huge deal and we might also implement our own wrapping and deprecation and get rid of support for it, but I wanted to check in here what your appetite for supporting it natively is.

I ended up with the implementation here as a minimal zero-config approach but of course with a bit more opinionation baked in, compared to for example Node.js' allowNegative option for util.parseArgs. I also tried an implementation in cleye directly with a preprocessing step, but that got ugly fast.

Let me know what you think, happy to try other approaches or skip altogether 🙏

[privatenumber]

Thanks for this and for the context on the Backstage migration — glad cleye is working well for you.

I like the idea of --no- prefix support, but I'd rather not have it on by default since it changes parsing behavior for everyone. I prototyped it as an opt-in option:

typeFlag({
    verbose: Boolean,
}, argv, { booleanNegation: true })

When enabled, --no-verbose rewrites to --verbose=false before parsing, so last-wins works naturally (--verbose --no-verbose → false, --no-verbose --verbose → true). Only applies to boolean flags — --no-count on a Number flag stays in unknownFlags.

If that sounds good I can push to this PR with the updated approach.

[Rugvip]

Love it! Indeed I had the same concern, you can push updates to this PR.

Would this manifest as a global option in cleye or a per-flag option? I don't have a strong opinion to be clear, just curious, either works.

[privatenumber]

Pushed the updated approach to this PR — rebased on beta and replaced the implementation with booleanNegation option.

For cleye I'll expose it as a global booleanFlagNegation option since it's global here too.

[privatenumber]

🎉 This PR is included in version 4.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[privatenumber]

🎉 This PR is included in version 5.0.0-beta.3 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

Your semantic-release bot 📦🚀