rollup-watch

This module is used by the Rollup command line interface to enable automatic incremental rebuilds.

Install it to your project like so...

npm install --save-dev rollup-watch

...then invoke it by adding the --watch flag (or -w) to the command that starts Rollup. In this example, npm run dev will create your bundle then recreate it whenever its sources change:

// package.json
{
  // ...
  "scripts": {
    "build": "rollup -c",
    "dev": "rollup -c -w"
  }
}