webpack removes files specified in package.json sideEffects

Bug report

What is the current behavior?

Files specified in package.json sideEffects is removed

If the current behavior is a bug, please provide the steps to reproduce.

Minimum reproduction: https://stackblitz.com/edit/node-thpmrj?file=dist%2Fwebpack.js

Note that src/lib/package.json has sideEffects: ["./injectDevTools.js"], but injectDevTools.js does not show up in final result dist/webpack.js

Interesting findings while debugging: I added console logging in SideEffectsFlagPlugin.js, it returns true when checking ‘injectDevTools.js’ against the sideEffects flag

What is the expected behavior? Files specified in sideEffects should be kept. In the above example, injectDevTools.js should be included in final result dist/webpack.js

Other relevant information: webpack version: 5.70.0 Node.js version: 16.14.2 Operating System: Additional tools:

[vankop]

@amitbeck please create a small reproducible repo, I will take a look

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simple check for you will be set optimization.sideEffects=false to disable side effects “check” by webpack

[vankop]

webpack does not track transitive sideEffects ( in your case it technically should be since index.js depends on injectDevTools.js), so you should mark all files by yourself.

In other words sideEffects flag does not have transitive state, so in your case index.js is side effects free.