Add side effects to package.json, help bundlers cut unused code with treeshake. Good for esm, modules, named imports.

ESM is how files and imports are organized, Side effects is how a bundler can remove obviously unused data from an assembly. Only both make maximum effect.

rollup https://rollupjs.org/configuration-options/#treeshake-modulesideeffects
webpack https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free
esbuild https://esbuild.github.io/plugins/#on-resolve-arguments

Even update not make effect yet, but make package modern, wellth and ready to future