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Pros
Pro Provides tree shaking
Tree shaking can significantly reduce the size of your bundle by getting rid of unused code from the libraries you are using.
Pro Reactjs droped Webpack and is now using rollup
Pro Outputs almost any module format
AMD, CommonJS, ES2015, Globals and UMD are all supported.
Pro Easy to configure
Rollup is very easy ton configure through a JSON file.
Pro Supports plugins
Rollup can be expanded just like webpack and browserify through the use of plugins. Plugins can do things such as transpilation (e.g. with Babel), import data from non JS places (like JSON files), etc.
Cons
Con Relatively new
Rollup is still relatively new so it may be a bit harder to find information on how to do some things with it. In addition, that means that plugins can be a little scarce and it may be subject to minor changes as it matures.