bundle js modules using rollup

foo.js

export default function foo() {
    return "foo";
}

index.js

import foo from './foo';

console.log(foo());

package.json

{
  "name": "quick-spike",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "build": "rollup index.js -o build.js -f umd"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "rollup": "^0.41.4"
  }
}

put these files in a directory
run npm install
to build run npm build