Gulp javascript task — rollup version

gulpfile.js

const gulp = require('gulp');

gulp.task('js', async () => {
  const rollup = require('rollup');
  const replace = require('rollup-plugin-replace');
  const postcss = require('rollup-plugin-postcss');
  const resolve = require('rollup-plugin-node-resolve');
  const babel = require('rollup-plugin-babel');
  const { terser } = require('rollup-plugin-terser');
  const commonjs = require('rollup-plugin-commonjs');

  const options = {
    input: ['./src/index.js'],
    output: {
      dir: './dist/js/',
      entryFileNames: '[name].js',
      sourcemap: false,
      format: 'system',
      // name: config.params.name,
      // amd, cjs, system, esm, iife, umd
    },
    plugins: [
      replace({
        'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV),
      }),

      resolve({
        browser: true,
      }),

      commonjs(),
      postcss(),

      babel({
        exclude: 'node_modules/**',
        presets: [
          [
            '@babel/env',
            {
              useBuiltIns: 'usage',
              corejs: '2',
              modules: false,
              targets: '>1%',
            },
          ],
        ],
        plugins: ['@babel/plugin-syntax-dynamic-import'],
      }),

      terser(),
    ],
  };

  const bundle = await rollup.rollup(options);
  await bundle.generate(options);

  const { output } = await bundle.write(options);
});

html

<script src="https://cdnjs.cloudflare.com/ajax/libs/systemjs/3.1.6/system.min.js"></script>

<script>
  System.import('/assets/js/index.js');
</script>

package.json

{
  "devDependencies": {
    "gulp": "^4.0.2",

    "@babel/core": "^7.4.5",
    "@babel/plugin-syntax-dynamic-import": "^7.2.0",
    "@babel/preset-env": "^7.4.5",

    "rollup": "^1.16.2",
    "rollup-plugin-babel": "^4.3.3",
    "rollup-plugin-commonjs": "^10.0.1",
    "rollup-plugin-node-resolve": "^5.1.0",
    "rollup-plugin-postcss": "^2.0.3",
    "rollup-plugin-replace": "^2.2.0",
    "rollup-plugin-terser": "^5.0.0"
  }
}