example date-fns bundler

date-fns.js

import differenceInBusinessDays from 'date-fns/differenceInBusinessDays';
import parse from 'date-fns/parse';
import toDate from 'date-fns/toDate';

export default {
  differenceInBusinessDays,
  parse,
  toDate
};

package.json

{
  "dependencies": {
    "date-fns": "^2.15.0"
  },
  "devDependencies": {
    "rollup": "^2.23.1",
    "rollup-plugin-commonjs": "^10.1.0",
    "rollup-plugin-node-resolve": "^5.2.0",
    "rollup-plugin-terser": "^6.1.0"
  },
  "scripts": {
    "build": "rollup -c"
  }
}

rollup.config.js

import resolve from 'rollup-plugin-node-resolve';
import commonJS from 'rollup-plugin-commonjs';
import { terser } from 'rollup-plugin-terser';

export default {
  input: 'date-fns.js',
  output: {
    file: 'bundle.js',
    format: 'iife',
    name: 'dateFns'
  },
  plugins: [
    resolve(),
    commonJS({
      include: 'node_modules/**'
    }),
    terser()
  ],
  treeshake: true
};