Add sideEffects property to package.json for our packages

[talldan]

Description
The sideEffects property indicates whether a package introduces any side effects. It can be specified as either false or as an array of files that have side effects.

By specifying this property, webpack is able to reduce the size of JavaScript bundles by eliminating unused imports:
https://webpack.js.org/guides/tree-shaking/

sideEffects is particularly relevant when implementing feature flags. Without it, features need to be flagged at every possible point. For further details, see this discussion: #13829 (comment)

How to implement this
Packages will need to be assessed individually. The biggest cause of side effects is likely to be registerStore. Any function call in the outer-most scope can result in a side-effect, particularly if there's no return value:

registerStore( //... );
registerBlock( // ... );

Mutation of global or imported variables is another example:

import store from '@wordpress/core-data';

modifyStore( store );
store.foo = bar;

window.shim = window.shim || function() {
    // ...
};

Another good indication is any import that doesn't define any new variables. The imported file will almost certainly have side-effects:

import 'something-with-side-effects';

Other ideas
It might be possible to use a linting rule to enforce this. A quick look at npm shows limited prior art, so a custom rule might be needed.

[gziolo]

@talldan should we enable sideEffects property in some of the packages manually as the first step?

[gziolo]

@talldan do you plan to continue work on this issue? We added this flag to many packages where it was straightforward. I would be fine with closing this issue if handling the rest is too complicated and thus unreliable in the long run.

[talldan]

@gziolo Sorry for missing your pings. I didn't realise I'd self-assigned this. I stopped working on the eslint rule for package side effects (#14135)—although it half-worked there were too types of side-effects it didn't catch. I should've unassigned myself when closing that.

I think you're right, having an open issue here doesn't add much value. I think it's fine to continue on a case by case basis for packages and close this. Thanks for handling those PRs 👍 .