[Bug]: yarn@v4 install Rewrites package.json when it shouldn't, stripping blank lines & failing to execute postinstall when it should

[fbartho]

Self-service

• I'd be willing to implement a fix

Describe the bug

Originally raised as a discussion in 2021 it's definitively a bug, or it should be documented somewhere that yarn is always willing to rewrite your package.json, even if no logical changes occurred.

1. yarn install will call a script called postinstall on first install. This allows devs to hook in custom tooling.
2. But, if no dependency changes occurred (eg. changes to node_modules or other caches), it will skip calling postinstall -- presumably as an optimization.
3. 🐛 Executing yarn has a side effect of rewriting your package.json, it strips newlines, and has some other purpose.
4. The optimization from 2 and the unwanted side-effect from 3 means your custom tooling cannot guarantee to be executed to ensure your package.json is formatted the way your repo demands.

A user workaround exists by installing a custom plugin in .yarnrc.yml (endorsed on yarn's plugin page), and hooking your postinstall script into that. But that's a performance deoptimization, and gross. Why should yarn endorse a workaround to unwanted features?

To reproduce

.prettierignore

package.json

package.json:

{
	"scripts": {
		"postinstall": "touch postinstall-side-effect.log"
	},

	"packageManager": "yarn@4.6.0",

	"license": "UNLICENSED"
}

1. yarn install -- creates .pnp.cjs, .yarn/*, yarn.lock as well as postinstall-side-effect.log -- it also rewrites package.json stripping the blank lines.
2. reset the package.json to the above, and delete postinstall-side-effect.log
3. yarn install -- this rewrites package.json stripping the blank lines, and does nothing else

Expected behavior:

A. It should not rewrite my package.json. Blank lines are not illegal, no logical changes occurred.
B. I have a postinstall script, and I told it yarn install. It should execute my postinstall script.

Yarn should not introduce unwanted formatting side-effects. Yarn should respect standard script hooks.

Environment

System:
    OS: macOS 15.5
    CPU: (10) arm64 Apple M1 Max
  Binaries:
    Node: 23.6.0 - /private/var/folders/01/4rfqmhmn03s0vby9xw8cz6qr0000gp/T/xfs-33b4dbe9/node
    Yarn: 4.6.0 - /private/var/folders/01/4rfqmhmn03s0vby9xw8cz6qr0000gp/T/xfs-33b4dbe9/yarn
    npm: 10.9.2 - ~/.nvm/versions/node/v23.6.0/bin/npm

Additional context

No response

[arcanis]

Duplicate of #6656; I have a fix to change the serialization but it won't land until the next major.

B. I have a postinstall script, and I told it yarn install. It should execute my postinstall script.

It does as you noted, if the dependency tree changed. It's not a bug, and while I could imagine reconsidering that in the future I don't foresee it for the next major or so - and certainly not to just fix some formatting when you can just workaround it by disabling formatting on the package.json files (which isn't a significant issue since, has you noted, Yarn enforces the formatting anyway).

[fbartho]

@arcanis

1. The fact that one use-case for postinstall is formatting is an unfortunately “minor” use case. The more important fact is that postinstall is designed for side-effects to occur after the user asks yarn to ensure all the packages are installed. The fact that packages were already correctly installed is unobservable state to the user.

yarn should not & cannot assume what side-effects are happening in postinstall.

Other side effects include triggering codegen for build tooling, ensuring repository conventions are followed, verifying that repository external tooling versions are properly installed at the right version. (This postinstall is at the root of a monorepo, it is not in a published package)

2. It is not yarn’s responsibility to manage formatting in my package.json. Yarn doesn’t manage formatting in any other file in my repo. Indeed, it is preventing me from formatting my package.json correctly. Yarn doesn’t have the ability to understand the formatting rules in my repo. I wouldn’t expect it to do so, and this is not a feature request! I’m asking yarn not to do things outside of its scope.

yarn is a package dependency manager, not a formatter — or am I mistaken?

[fbartho]

(Thank you for tagging against the issue I had filed previously. I forgot that I had filed one that was only focused on the formatting issue)