Add sideEffects: false to react-router-dom package.json

[taylorc93]

Webpack 4.0 introduced support for a sideEffects flag in a project's package.json: https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free. This is awesome because we can now get Webpack's tree shaking to work as intended for most, if not all projects. 🎉

I've noticed that in my projects where I've used react-router-dom, the recommended import syntax (eg. import { Route } from 'react-router-dom') still pulls in the entire library. While react-router-dom and it's associated packages aren't that large, I'm a bit of a performance junkie 😃. All this PR does is add the sideEffects: false flag to the react-router-dom package. I created a simple test repo to demo the effects of this flag: https://github.com/taylorc93/react-router-test. I'm noticing ~20KB of minified JS removed from my bundle as a result of this change.

The only thing I'm slightly unsure of is whether there are, in fact, side effects. From what I can see, there aren't any, but I could definitely be missing something. Let me know what you think / if there's anything else you need from me to get this merged!

Side note: Love this project, I've been using it for years and it's worked fabulously. Keep up the great work!

[timdorr]

Can we add this to the other packages as well? I put it on Redux as well. It doesn't really hurt, since we don't modify globals.

[taylorc93]

@timdorr All set

[timdorr]

Thanks!