build.js

const rollup = require( 'rollup' );
const babel = require( 'rollup-plugin-babel' );
const fs = require( 'fs' );

const writeFile = ( fileName, data ) => {

  fs.writeFileSync( fileName, data, 'utf8' );

};


const inputFile = 'src/index.js';
const outputFile = 'build/index.js';

const defaultPlugins = [
  // nodeResolve(),
  // commonjs(),
  babel( {
    compact: false,
    exclude: ['node_modules/**', 'src/shaders/**'],
    babelrc: false,
    // plugins: [
    //   'external-helpers',
    // ],
    presets: [
      ['env',
        {
          modules: false,
          targets: {
            browsers: [ 'last 2 versions', '> 5%' ],
          },
        } ],
    ],
  } ),
];

// rollup inputOptions
const inputOptions = () => {

  return {
    input: inputFile,
    plugins: defaultPlugins,
    perf: true,
  };
};

// rollup outputOptions
const outputOptions = () => {
  return {
    file: outputFile,
    format: 'iife',
    name: 'output',
  };
};

// stderr to stderr to keep `rollup main.js > bundle.js` from breaking
const stderr = console.error.bind( console );

async function build( inputOpts, outputOpts ) {

  // create a bundle
  const bundle = await rollup.rollup( inputOpts );

  // generate code and a sourcemap
  const { code, map } = await bundle.generate( outputOpts );

  console.log( 'Writing file: ' + outputOpts.file );
  writeFile( outputOpts.file, code );

  console.log( 'Generated bundle: ', bundle.getTimings()[ '# GENERATE' ] );

}

const inputOpts = inputOptions( inputFile );
const outputOpts = outputOptions( inputFile );

build( inputOpts, outputOpts );