Babel gentleman setup. Adds only the transforms and polyfills that are required by the browsers you support. Each file is a setup for a different use case.

babel.js

/*
Setup for using only Babel. It produces a file which require()s Babel helpers (ok for Node.js environment).

NPM dependencies (check the output file to know which of them are really required):
  core-js ^3
  @babel/runtime ^7.4
  
NPM dev-dependencies:
  @babel/core ^7.4
  @babel/plugin-transform-runtime ^7.4
  @babel/preset-env ^7.4
*/

const {writeFile} = require('fs');
const {promisify} = require('util');
const babel = require('@babel/core');

(async () => {
  const {code} = await babel.transformFileAsync('./src/app.js', {
    presets: [
      ['@babel/preset-env', {
        targets: {
          browsers: ['last 2 years', '> 1%'] // The browsers you support in the Browserlist format: https://github.com/browserslist/browserslist
        },
        useBuiltIns: 'usage',
        corejs: 3
      }]
    ],
    plugins: [
      ['@babel/plugin-transform-runtime', {
        regenerator: false // Both env and runtime plugins insert a regenerator runtime (and often different ones). The most reliable way to avoid it is to turn it off in one of the plugins.
      }]
    ],
    sourceMaps: 'inline' // Remove the line to turn off
  });
  
  await promisify(writeFile)('./dist/app.js', code);
})();

rollup-external-helpers.js

/*
Setup for using Babel with Rollup. It produces a file which require()s Babel helpers (ok for Node.js environment).

NPM dependencies (check the output file to know which of them are really required):
  core-js ^3
  @babel/runtime ^7.4

NPM dev-dependencies:
  @babel/core ^7.4
  @babel/plugin-transform-runtime ^7.4
  @babel/preset-env ^7.4
  rollup ^1
  rollup-plugin-babel ^4
  rollup-plugin-commonjs ^9
  rollup-plugin-node-resolve ^3
*/

const rollup = require('rollup');
const rollupBabel = require('rollup-plugin-babel');
const rollupCommonJS = require('rollup-plugin-commonjs');
const rollupNodeResolve = require('rollup-plugin-node-resolve');

(async () => {
  const bundle = await rollup.rollup({
    input: './src/app.js',
    plugins: [
      // Adds a possibility to import packages from node_modules and files with an extension != .js
      rollupNodeResolve({
        browser: true,
        extensions: ['.js', '.json']
      }),
      
      // Transpiles the future JS
      rollupBabel({
        exclude: 'node_modules/**',
        runtimeHelpers: true,
        presets: [
          ['@babel/preset-env', {
            targets: {
              browsers: ['last 2 years', '> 1%'] // The browsers you support in the Browserlist format: https://github.com/browserslist/browserslist
            },
            useBuiltIns: 'usage',
            corejs: 3
          }]
        ],
        plugins: [
          ['@babel/plugin-transform-runtime', {
            regenerator: false // Both env and runtime plugins insert a regenerator runtime (and often different ones). The most reliable way to avoid it is to turn it off in one of the plugins.
          }]
        ]
      }),
      
      // Add a posibility to import packages written in CommonJS style
      // Must come after the Babel plugin to avoid the problem: https://github.com/rollup/rollup-plugin-commonjs/issues/247#issuecomment-370677203
      rollupCommonJS()
    ],
    external: module => ['core-js', 'regenerator-runtime'].some(externalModule => {
      return module === externalModule || module.startsWith(`${externalModule}/`);
    })
  });
  
  await bundle.write({
    format: 'cjs',
    file: './dist/app.js',
    sourcemap: 'inline' // Remove the line to turn off
  });
})();

rollup.js

/*
Setup for using Babel with Rollup. It produces a file which is ready to use in a browser.

NPM dev-dependencies:
  @babel/core ^7.4
  @babel/plugin-transform-runtime ^7.4
  core-js ^3
  @babel/preset-env ^7.4
  @babel/runtime ^7.4
  rollup ^1
  rollup-plugin-babel ^4
  rollup-plugin-commonjs ^9
  rollup-plugin-node-resolve ^3
*/

const rollup = require('rollup');
const rollupBabel = require('rollup-plugin-babel');
const rollupCommonJS = require('rollup-plugin-commonjs');
const rollupNodeResolve = require('rollup-plugin-node-resolve');

(async () => {
  const bundle = await rollup.rollup({
    input: './src/app.js',
    plugins: [
      // Adds a possibility to import packages from node_modules and files with an extension != .js
      rollupNodeResolve({
        browser: true,
        extensions: ['.js', '.json']
      }),
      
      // Transpiles the future JS
      rollupBabel({
        exclude: 'node_modules/**',
        runtimeHelpers: true,
        presets: [
          ['@babel/preset-env', {
            targets: {
              browsers: ['last 2 years', '> 1%'] // The browsers you support in the Browserlist format: https://github.com/browserslist/browserslist
            },
            useBuiltIns: 'usage',
            corejs: 3
          }]
        ],
        plugins: [
          // If the transform-runtime plugin is not used, the Babel runtime code won't be shared between your code and the code from node_modules
          ['@babel/plugin-transform-runtime', {
            regenerator: false // Both env and runtime plugins insert a regenerator runtime (and often different ones). It leads to an increased bundle size. The most reliable way to avoid it is to turn it off in one of the plugins.
          }]
        ]
      }),
      
      // Add a posibility to import packages written in CommonJS style
      // Must come after the Babel plugin to avoid the problem: https://github.com/rollup/rollup-plugin-commonjs/issues/247#issuecomment-370677203
      rollupCommonJS()
    ]
  });
  
  await bundle.write({
    format: 'umd',
    file: './dist/app.js',
    name: 'MyApp',
    sourcemap: 'inline' // Remove the line to turn off
  });
})();